THE G-YP:SVM-· -ANHYIJRI;I'E EQUILIBRIUM' AT' ONK Aa'MOSPHERR )>RESSUREi

.... ~Awtilli:'!Qi·'.A.,. ·B.Ji@IT,:j.•Dep.ar/n~r~{ of Geci!qgy, _Tlzi JbTJ:tiS'

Hbp_k'i~-s~ fh~{versity1 BI.Ut.i~o;e, .Ma:ryja71d ··

~gRACT

J:hf!· equj.liJ:>n!jm t:«n;'Qera;tut~#r th!i ~eacfiop., CaS0~:2Hz(}=C~O~+~I.f20t!o~: sccin): ha.S been dl':termi.tied as a ftmctipn: of fictivity_of Hi). (dn~o): ofllie :wlution: _S:yntheti~ $Y})sW.'Il, arid ifnhydrite o)'.! li f Iitixtilres were' sti~reU in solutlori~. 6f known 12H1o (caJcul;t~eQ from:vapor pres8ui-e data for the Na.~$04 aml:H~04 sulution.S};_ at constimt_ tei:I':tpeiatute for ?-S.IIlu¢h as 1~:nwn~.-J::lie.1evEJ~bJe·equi_tiE.iii)..rq. Vf'as_appwached fro.m goth sides ind is. g~~~d by: _<lH~o,= 9,,9,pQ af ~~0,:0,84;~ it~ :)9°, 0,7'10 ·a.t_ 4}°C. J.'J:oYid~ the solid~. <Jo.'not . cfumge in <;oinp9sttio~, the: eql).ilibrium at co!)~t~p.t P and f i~ a func.tiop of aHiY o)i_ly and_ is . indej}endent of the constituents ill sol~t!oll:: Extrapoi~tion to ·the. boun:~ioi syst(!m CaSO~.-:: H~ (aHill ~ H>OO) yields 58"-:t.z:~. This is withhlthetrnl)d~c talculatloos (46° ± 22°C) but highel.' than ~Ol'\lbiJity measurements (38° tq -f2~C).· T;h~:.U~'\V data indicate t:hit in sea-. 'Watei ~.turate-4. ~ith halite 34d gyps-um !?l)ould qehydrate ·a9_o:ve 1S°C. The scarcity of an-_• hydri~~m, h~o'd,ern ~vap9-jit~ dep6sits 1s p~ed.kted. by th!,! present re5'!-lfts. The available data on thete~p"?iature-saibity- ci:mdltians· ~der whiCh anhydrite anci g)rpsuin. e:xlst l.n the R,e..: cent supratidal :Bat.sedin;tents of the Trucial Co<!Sl:, Persian Gulf; are co~patibie w1ili the, present experimental dat~. . . · . . ···: .. . . · ....

t&TR.OPU¢trbN

.. The ~tabjlity: rel,ations of gypsum (CaSO~· 2:1:i20} and' a]lhy:dritt!, (<::;a.SOJ. are··of. considerable interest·beeause mostp.atural :marine:evapp- .

. rite depnsits· con:si~t·e~·en:tially-of gyp~11J!i and/or anhydrite interbedded With dolomite~ liiiie.s-t:6n:e. an.d .dasti~- sed:iment$- (e:.g~, $tewart_1963, Table i8). In- the~ binary ~ystem CaS04~H2.0~ ihe=-reaction. Ca~04 • 2H2.0~CaS04 -+2 H~O(liCt;>:hasbeen ~tudied experimentally at one_;ttmospliere pressure_ by _van't Hoff. ··_et al. (i903)~. Part~idge .and White (1929}, Toriu:rn.i and ·

. ·Ha~a (1934),_jr1p, (1934), PQ['hja,k (~93M :LPcJ IXAmr et al U9SS). Kelley, · Sog_th~rd: a:n4-AP.d~r~ol! ( i 94i) · ·w~i~ur~.d ·the t~erl"Q.Q_c;ii:~nii'¢:i.f properties of the, solid. phases--of the sy_s.tf~m: at a. tm.9splieric press~:rei ·;:tnd ft9!4 -i:~~~e data:. cakulated· an· equilih~·ium:· temperature:· for the. gypsuni-a:i:iliydfik' tr.~nsition. M{trS~L(i9$2}, MacDon·a:td {1953},- Z~n (196.5) and- Hardie: .(;(96_~; pp,'._-:ts~~o) ~-~fciii~tf~' th.e ~tte~t p_~ P.~es~lJ:~e o~:. fu.e_-.re~~-tip_n. . i':he-.. effect. of' ·salt ~oJutions 0]1.'-th.~.- gyp~urn-~n:hydPte ~qu!libri~m. if

__ a.t.rnospil'eric pr.e5sure: has, been consid~rid}h S~me d~taif by S~Veral V{Or:kc:­

_erS1 alf of whom,.have verified ~hat the _transition·tetnperaturej$-lo.wered w.ith 'intt-~8$ifig: sa:l)n.lw .. Tb:~ ·$y~tem Ca,SO:"N1CI-H~Oji~ been inyes­ifga.~e4 -~~p~r)iPentall)/by van.'t jtoff--c.f, g:t (19.0J),- D',Ans .i!.~ til (l955), Madgi:r;l aii~r-s·w-a.les- (_195.6).; Bock (i9.6.1fand: Zen: .(~_965).. ~1:icDog!:~.ld

l Thl$'WP~~ iS::f;1k~ri f);gm a-.J?b~D .. ~~sseitittion s~l:)mifted'by L,. A::Hard1(< to· _ihe: ;De'-· patt~p.enl:-o{Geoiogy;_:Th~ J.:/ii.ns B.;opkins Univ~s£i:y, Baitim,.Q~, Marylan4~:

171

.. . -·

172

. (1953)·· :cakulited .die. effect. o£ :NJ¢I :?.9.1'\.ttiptls {ln_ th.¢' .~qul.Jib).}vm t(!gi-. petat~re· .. ·t'h{ t,riii;isi'tim;i: In: s¢~ w~t~t:~~·f!~~P:· .$t~Jli~9. 'by TQrhtll1i ~t; qJ (i938) and:P9:mjak (i94or Other pertinendaboratorY's6J,cljes~are those of;aUr and.~Wills· (i938j "and 'COtiley''and Burtdy'(i958)'in' the· system!' . CaS04-Na2S04.-ff:~Oj D'Aris .and;;·'Htifer-< {i9J7)i iii: th~ ~ystem Ca_SQ4- · 'Ha.P04-:-H'!O~· and Ostroff..· (i9Q4):,. w119.= ~oJiverte4. gypslibi tq an~yiJ.rite j.p. · N a(::l~ MgGb ~{)ll),_iigns.- . . . . ·· . .. .. . . .

. 'B.Qr: fu~ ~o;t;-pait the =resulfs pf:t:9-~·_different. workers a.re: ift-_,poor .~gteement.:.For:this~Tea.son; _anci::betaru>e the, :piethods used ·we(e)1tg~ly irtd!rect (e.g~ $biubility-·ti,nd dilii.t9.n1eter· i.ri.eas'Qren1-~Ti..t$,, t~erb1odyp·~mk calCulation)''-. a:· re:-e,'ra:ro"ln~.tiQn··9I: th~ problem, using 1:1.iliffererit iaborar. t()ry appro~ch,: ~eellied. des1ra;p)¢. . . . . .

The cQ)lversion of gypsuni. to :auhydritet, and·a.nhyorite: to g:x-:psum;_ W~'S' studied. at-.abnosph~rk pressure as· a']t(J1cti6n .. of., temp~J;a~ut:e; ·~ud :idi:vity·of·H~Q·· (li'n:iO)- Fotfhe ·ie~ct:ipn · · ·· · · · · . .

,(;aSO~· 2:f;I~O(B) ~-CiS04(a) + ~H.zO(liqui:d. i~ ~o:itttion) .. .. :. -- . ..:.:.. :.

~he. ~quilibrittm~ constl,!;nt may l:le.d~fi:ned ih ~hm5 o~ a:cti~ties, as fdliows:

If the.standa:rd-states of the cortrponents of.tbej:eaGtion ate·.¢ons{i;lered to~ be. pure= H2.0i liquid .. y.tate:t,. pu.i:e: (;iystaUin~ CaS04 ·and pili:!!. _q:ys.t.~lline · CaSO~.·.ZH20: at ·~me atm~·spb,j!re: tot~.p:r~ssur~.:·a.nd at the· tem:pera:tme. of.--reaction, 'the· equilibri1mi constan:t simplifies to .

· (K~)T.~~.:~ a~~=

It.folibws th~t the· dehy~i:-~tion_;fgyp~ilm tq a.P:hytlri.te;·~:U~tmospheric: p:rf:S~:m:~/i$_·;a.Jimotion of tempeiature··.and activity of H~O·only. There~; tore, proyided the. solidS "do nbt change in' composition, the'' equilibrium:. ishidependent of the· componei;lt~j]:i't:he CC)·,-~xistill.g:.s<:ilutigti: ·

The. ~ti\ii.ty. of ·11~0 6f the iol~tio:n~.:co-~~stlilg_vrlth; gy--QsJJ.W: or an])y­di;ite .. ~as::y~l-{ed ~Y i!!ldlug·Na2~0~ or;.H2SO~~ ·these.$er¢ chosen· .. because· the: gypsum~:anhyth:{te .c~nye.tsiou.:~~tes= were found) to. he: .r~iatitely· ptpi·t:); k. sacUi:m,l iulfa.te. ~r sliltti.rib= ~dd soiuiions. "in_. the .sys.t~m CaSd4- · N3::rSO~-H2.0 n:eith~r gypsUl:J.l: .. 'P.-~1"':· :inKJ4t{i:~·:caJi.=. c·Q.-e~:;t .vrlth. a;ffo.l.~.£d<rn oLNa:~~04, c(fti~~M.niti(i~' gt~It:~:~; t$,(:l,:q. tll:l{ fixed. by Uie: Q.:IJ:_e'.!'!:Un.Q_spheie · i~<ithe.pnally 4lv~nant a.S~ertibl~g~:. gypsum< (or ai~hydriteftglauberite (CaS04·;NazS04)+so~utiorr+vapor.' The !1-JI:~o of'tne:·invatiitnf.. solritiotis varies ':frorr1.cal:iout: Oo90 at'·Z5qC to, about '0:'96 'at- '7();°C. This: limitS' 'th~:

:-·:::.~ • .·.. . .. : .-=:· . .... . . . o.; ••

173

:study·to· ver}~ dilute solutionsonly1 a.con:ddetabiadis~dva~tagepecause J4.ariy natl).ra} ~::~dtim.- s:ttlfafe depo:Si.ts milst have. ~otmed in hfines with a~~ivi.ties of B~O at ieast ·as low· ~s 0~ is, as 'det~.ru~d. QY the a~se;mblage

_gypsum: (o{ anhy·drite) +halite+solution+va.por ·in the. ''haplq~ev:ap:. · oi:ite;;· (Zen, Ef65~. p .. · i25) -sy~i:eni cas04~Na.Cl~HiO~-., In the system Ca.f:rbt·H~S04:-H~(}; however, the stabillfy :fi~id~ 'of ·g-y_psuin:· and anhydnte ¥C ~ot limited .by d~mble.:~alt. formation. 'fb\l.S, t~~:-;tea~tion Iilay be ·studied ill H2S01.solutrons~which have' a range of ad~vity~ of H20 coTI;i.-.. pa:rable to-that found iii natu:ral waters, ·that 1s, fto1.n'iiear Loo:to_ libout .o.so.

. 'EXPERn!ENTAL METHODS

. Stari·ing ,J.lfa.ier1·als. The solid starting materi.a.Js, were: artifi.Cial·¢iSOot • 2H~O,. CaS04 (atw)"drons)· a)ld Na2$0~ (anhyd~o~s) ot reagent gr~de~ .·

The. C'a:S0.-·2I:Ii)· (Baker Analyzed, Lots N9.·. ;2,5Q9Z ~p.d :25286) ·wis ~e.-grained but variable~s!zed material (();1 IlllP,'. tq less··.thp,n ·o.cn P,1l)l),

whiCh showed the, tharacterlstic 1norphology~ ·of gypsum- ·e1,1hedt~. The X-ray di.ffractien patte:tn ·was indistinguishable from th;it of natural gypsurp, and the ~ateriai wa~ used as ~uch with no futther· treatment. The CaS04 (Baker Analyzed ;;tnhydrm,Is [sic] Lot ·No. 90128} yielded an X-ray. patt.ern consistent: with hassanite, C~S04 ·1~B2.b (n;50.5), When this:materiahvas heate:(f at 450_0(>'to_.550~C for 2 to- 5 days, a very fioe­graine_d.-powder ·was pro=duced which gave: an excellent anhydrite .X-ray patte;rli;.·In· tlie .experiirl.ental ru1JB with anhy~rite as.~ ~t.a,rting material,

· the heat· treated CaSO{.was·used. Zen:· (19657 ·p .. 15_i).fqund th.aU:trtiflcial · · anhydrite, prepared by' dehydrating gypsUIIi overnight=;,it :J00°C, :r_eadi~y rev~.tte'd. to gy-psun'ls~lien brought into co~t11ct with. water; he tlu~refo:r.e considered s{i:ch anhydrite pns'uitable as a starting In~t¢rial. In- the pres­ent investigation no, $Uch .rehyd.rati~n of artifidai anhydrite o~curred~ even lll' stll:red: nws_,_(if: 6.mo:q.ths_tiu~a:t}9.~'---un.~r. ~on_dition~ where !in~ hydrite.yva.s con&id6-ed· to be stable. 'Ci-itical experiments h~'\Vev.er:~ wer~ r~peat_e,d lf~:ing n:?-f:iir.a~ gy"Psutn a.n6/o.r anhydrite! The. gypsum was large· . . d~a,r s.ele:nlt.e p1~te~ £mpi.Mo.n:tm.~:~;~r¢.,= P~d$, a,IJ,d t.h~ t\ihY4rite ~as mas-· sive fi.ne-g:rained ·matedal from Richmond Co., Nova S~otia (WjJ.liams Collection; The Johns Hopkihs University)-. _

The :N?2SOi.(Bg,ker An~lyzed; a~).iydrou.s,_LofS.No: 2.~581 a;nd:-22088) g~ve a: sharp_ thenardfte X-ray pattern a:o.d: w'as Q.s~d without :fur.ther~-re:-finetnent.,_. · ·

The. s\llfuri):;. 'a,dcf solutions .. #ere ·prepared by ·mi:U~ing ~~~~;r A.nii-iy_:zed 95 ·percent R_2q0~ ·WJ.th ~dcruble::d:istilled water: to the· requll:ed ·~;onc.en,tia.:.. tfons'; TbJi"eiacfcoiiceritratio:q.· in· weight percent H'2SO?- :was: then· deter-

11\ined by· titratiOn of carefully ·weighed i.lliquots of eadi'solution apainst· lN· NaOH solutim:{ (CO~ ftee) usmg: nieth;d orang_e as au indicator .{Wekhei-,: i962,_ P' 540.). the.· :ce$ult!?· were checked. against t;he: fi"2SO;i. ccincentr~tion 9-et~~ed: by spe~:!fic grayity measurefitents, . using, Jhe ca.libr~tio1;1 cur~e ofHo9gplqn.(i9SJ, p. 1894).

Exper·i;ne.rr:t.al proced-ure

(a) Static Metho,d,: At. tlie, st~rt .of. th~ g;tu~:lr q. tedinique .ii'milat. tli' that li?-¢.d in. hydrothe:rm~ w-ork was' employed. Fineiy gi:,ORud wixtu:res' ·of anhydrite; or gypsum, .and thenardite were accutately 'weighe:d. >"Vlth the t~quired amou.nt of distilled water, into .py!'ex glass t~bes (7 X 60 nim)_

·which were. then seale.&.usihg. anox)r::.a:cetylene·torch.-to~s of distilled water was success~u.Uy '~:Void~ci. du~in-g· the. sealing· proce~s by wrapping the fuhes j,u wet:fil_ter pap~r~ Tbe.sealed:tube~ werethe~::toblly bmnersed: in _thermostatically ~ontroiled._watet-baths. At the end 6f ·the run perlods, which varied ftbli'i se\rentl.days to: ;many :hi.qnt.h.s,; th"e tt!l.:>~.s.·w.~.re br()keil . . opeh ruld th_e i{qlid Pr:oducts $eparated·f~o:rn the·: ~olution,on :·abl)PJ:hent pape.(. Samples_ Were imm~diq.telY. examined; b.Qth 1lhdei: the J.nicroscop:e aP.d: by X~ray ·diffraction·. Although· by this teclinique.many runs· ·e:an. be C<);rried ·out simultaneously; it has. the. obvious clisadv:antag~: that the s.olution vohunes are too>small.ft,ir a.n~lysis. UnfQttunateiy, th~ in~th(>d' .i;ir.oved i:o have 'an eyen gr.eater: dis_a.dva~tage; in n~any ru.~s eq!Jilib~h@ w~ not. at.tilined. even. alter. periods of-many months. ·When it was a:p-. parent that some form of agita,titmwas-required·toprmnote:\he reactions;, tb~ s~atic m~thod· wa:s: abandoned. Ho>,vever;, it. was poss·ibl!:!. t.o salvage enough ~igmncant.

0

iclormatign, to:V{arran..t; !l.i$.Q.I~~~9~1;' ~n~, .. c.mnpa,risol). w1th the r~~ui~$. of~l,il!~.qt~~-~d olJ.t with. tbe dynab:ijt_pl_etb..od.(c,on.~iqu,QJJs· stir:tii1g qf ~h.e ~a,rge) 1 'w·hich ·w4.s· used through. the· resf of the sfudy.

(b)-. Dynamic Method,.App"roxii:nately 200g.of starti'ng n1a terials were weighed; C± o·.l %tin to a.2so:m.LEdenh1eyer fiask{n~ted.-~~ttJl.·a_mertu:ry., . in-glass·~]r-tight ~~al;~h.iO.~gli w'hidl .. the.soiut{m;·_;,..~a:~-~ti~te.d.~u:fli~.i.~~tiy hi keep alHJ:te s.olid·,mater'i.ai in. ~o_n!\tant agitation::the ch.arged,xC.acti9;n :ve~:w!!l~:we,re T1.11:m~r~ed_i:rr witer· baths. of c~p~tity 30.liiers1 'then:h~statj­cally controlled:· to: ±o~ i ~c~ . . . . · A ma;xjhmm \"ariatl.on of ±:,Q.,5~.C .was o.~~~i:\ti;d Qvet· ?> perio·~ o{'_rdi!.

months. A:titi.tervii!>, one· mUiilite:F:o£ \.h.e· suspension-charge{! solution.-\¥as. with;_

drawn: \vith a ·pipette. an:d.r~pidly' p;~.ssu:re~filtei'~d-thrbtigh a ·Buchner funnel: this removed alruost.aH the's.olution. Th~,--soJid 111ateri_al wa~:.ifu­:tnedi~tely \vashed. s·~ye,ra;tti!Il~$·. w.ith>a~~:;{ori~ .~D.d: ·.fl.iro-d,r~~d: A p~r:tion 'of the S~mpk ':yas :exWutnf:';~t under :tbe petr.ggraphl.~· roi~J;OSCOP~i .the _1:~,-·

n).ainder WQ.S. hand~grotind uiide{ acetone ;tnd <'!i ·smear inoliht-prep;:tred. fo~ x-ray diffraction> ·· .

Deiernt .. i"nati6ns oj ad1vity ofH·;,O.'i~r;, sv.luiion-s. Th~ act1v.ity o$ a,rtyc coli~ stitu~Jit ·of a :s.olution is given by the- ratio of the·f:iJ:gacities: · · ···

'At one atmosphere totafp;ressure1 water vapor· may be rega:rd¢d -~s mi ide~l ga? ·sq tli~.t the ftJg~tities may be safely replaced. by the p;:J.rt1al pressu:r:es of HiO;, ·

aH2o. == jYg;_o/PtQ

TJie standard st~te is taken as pure liquid: water; .. at one ·atDJ.osphere pressure anc!. ?,.t_ the te:mperahrre of reaction~ for which. the activity is unity.

The $olubilities of gypsum .and anhydrite. tn sodium s:ulhte. il.rtd su1-· fui:ic: acid soiU:tions are ,yery low (less th~rt 0.2). pel.·cent by: weig~t}·. Ther~fore, the vapor pt.e~swes of t}:lese solutions ~::~:tura(ed wit~ CaS04 are given, within experimental mep.surement, By, th!:! vapor pressures of ilie "CaS04-jrr;e, solutions',

For the sodium sulfate solutions ·the. vapor. pr:ef>.$.Ure q~tfl.· gi\ren ih Ill-· t~r:i:iatioriaf C~:-j tical Tables (III£ p .. 3-71). were, used to calculate aa~o- 'X he

. ~Hi) ·of. ea,.dl.sol~tion .was.computM at th~. teinperature of each experi:.. men.t. At the. end o.f a run the total dissol~ed solids~c.ont.ent of the equi~ lib;ium ~olution was d~terWlned ... ·This y~i~e~ w~s ~~~p1;1red with the

.. ~~_art~ng Na2SO~ content to, ptovid~ a·checli:. ol)., the assigned aH2o yalue. The·:a.c'tivities of ·H~d of the suifur-i"c. acid soiutions were taken. from H~we.d -~nd o,~en (t9.s8i. p·: .. ·s·i4). Th~$e:"Qa.t~,. ~~produced iil Table J, show, that within ·the ran&e of- H2S04 concentrat{qn us~.d iii _the study; a-H~ values. d"eterr):iined by. EM.F. ·m:easuremerits.· are in excelh;Jit agtee~ ment.. with fho~e calcU,l~ted· fro:rir vapot- pressute'nieasurements: Tbe .ini­tial. gypsum and (.or): anliydnt:e. :~odt,:tb,i:t~<i qrily about 3 p~:(cent ·pf the total c)large. Cons~ql!.eut(y tl.l~ B:·zSO~ ~o.ru.;!enti~ti.on· of the·solutioll, and hence ·the activity .of .Hi>,, ~a~ ;not, stgn}fie~_ntly chap.ge4 by· the· H~O released ot .absorbed:: hy, the:: gypsmn-:anhydtite' conversion .. The HiS04 content o.£: i:11.e s6lutic;~: was ·:che:c).Ced·. by titration ·.1it the completion of each ru.rt. ·

. Th.e aB:2o:Q:f the nii~.SqlJJtihn ik¢a~h qftW.(}.tilll.S '(one _in H2S04 ai14 one l.n Na2$04) was ineasu.red directly us}ng- ~w ji20,._se.Ij.sing- apparatus (H~rdie-, 196Sa; p. 25-2};' the>.v:alues ·.did .. no1Ulif£er-.m~~A¥ra,bly' ~rpn_1 _the acti\rit1es of: H20 deter:rtiio.OO,Jn the CaSO~~free s.ohttion_s.-. .

.~

TA:BLE·i, :Acrtvrt-1.! OF:I:f~O 1.:."'1 AQUEOUS s{;~Fu'Ric Aero Sor.:u'l'IoNS . ·(Alter: Hairr~d ~~d. Owen, 1958, p·. 57 4) •

..... ___ .-.~.-~-:-···

Wt. :% .

.. n~so. emf

· ~.9; · -~r_--.·. -~·~;;~~~ .-··>o-.~62~- · ·l · -~~:~~~;·· 1.2..,8.~- J!-:Q3.?.1 '; ;()}93.§~ 0,9~;?, ~6.4,o 0.9i36 0.91i9 0.9i5s. 2

3 22.73 ·J~>a:So6. o.ssi4 o:.ss<,i8 4 28.1& 0.1175 0. 7i95 . 0. 7850 5 32.90: 0·.6980 0.7030 0.1086

l. ·®"C'

_ .. ,' z:. : '0.9fJ~

,0.9~80 l n.86o2 ; .(}.7950

0:7229 -0.6$05 o:ss1s

& .,. 3T.os • ._o.6ioo ._o~62sz o.62S&

-~·~.-::"':':·.~-""'~---,-. .. ~-.:-:-?c:-... ~=-'-=--:-40..,..,.,...,7~1...........;[ o _ 5453. .. o. 5497. · . --~. ~~os__ ......

DESCRIPTION AND--PROPERTIES OF_.~ SoLID PIIASEs

Th~ gyps~ ~ynth~!zed ·by hydration.. ol.a.nhydrlte _(hereinafter re-. ferred.: to'~ a? ~'S1;n~h.ettc gy-p~tim; 1) c6:iiiTI1ol;llJ ro1Isis~s. ()f thm Pl~:ttes. f.l~tt~!i~d p;:trall~l· t() (Q.lO),- w~t4 th~ cl;l_a.rac.t~rfstic ~on~cli:O.k o:utlin~ (Fig. 1)-. Between crosse<fnicols the thin p1ates·,show first~oider white or grey iriterference colors· and oblique extinction. In' rUJ1S where gypsum was conve:d:ed to anhydrite, the :first s_tage of the pr'ocess ·w.as recrystai-

::tizat~on .o~ t1l.e fi._~e:-ir~fu;e~: ~rtl:fidhl g:YJ?surri used ·as' a ~tarting: p1aterl.al to ~(>.ani~r l:)laded. cry~tals_; Thi~ recrysta~ed gyp~um~ seemingly of. a mo_r~

'Firi.: LPhq_f.omi~pgraph_ of synth~ii.~ gypsUID!.,J?rep~~;ed .froJJ;J. aJ;i,.hya.cit.~ a( 1i:)0 (j:'9-ruf· 1.J4mos:PJ1~£:e.iil·zz%.H2SPi·wrution; ,;(i_i:.h seec{ing;·.fu SG days (Run.L.S 2.3) i A:vera:M·kD-gili; of ~tab o:2 t9 0.:) rom.. . : ' . . . . .,_ ...

GYI'S.TJlJ--A.NHYDRITE EQ.tiiilJJRiUM .. ~. •'• . -

J71

stable habit tb,an)he ~;nigi]J..pl s:inalle;r-c.rystaJ.s,:;vv:as morphologically'and optically .indislinguishable· from the syntheti~ gyps:u~n'., · ..

The optitai p~operties of the s:ynth~tic gyp~Um_ . were no( s1gp}ficau tly · diffefent hom thos~ of the- Jiatin-a1 mitertaL · . \Vuosf~r (i936}ha.S d~t~rm.ihed." th~ q.jistal struttiii~ of gypsum .. lli th~

afi plane: two sh~ets- of so4 teti:ahedra. are, bQ:und by ca. atoms w.it.hin. th~m. Between these sheets lie layers of·water; molecui~. the, Ca. atoms ·ate: linked td 6" oxygens of. so4 teb:ahedra. aud to 2 W:;:tt~r D?,ol~ctJ.le?­W~t~r-. :n:ioletmes· thus ocqipy ll:nportaiif sttuctural po·sitions and ~ve;ri.. partial dehydration must r~su1tk ·the destrP.ttio:ri. of the gyi?suin sb:uc-tur.e.. . ·

U~it cell._paraJJJ.et~ts. ol the synthetit gypsum wer.e determined by. X~ray pQwd~f. dlff_i-actf!}n methods,•Vitli either silieoJJ. o~· qua:t:tz asint¢i:.na.J . . standard;_ am( ~Q"Wl~ to. "Qe in e.xceilent ·agre.e:ment wi_tii .. -t'hose gi"ven !>Y

De~r et-"al. (1962)"for natural gypsum. Anhydrite· synthesized from: gypsum appeared Ulider the micros tope

as-~ mass of "inlnute; birefcing~nt grains;"ihdividuaJ·cr;tstal outline~ iV-e~e b~rely "distfuguishable ~dei- hl.gh power·, and refr3:di~e index measure:.... men_ts we're unreliable. . . . . . ..

The pr~sence of s.ynthetic anhydrite could be readily d~tected ln. the. re·~cti~n _flask by inspection: a :fine ·white mas~ stay~d _jn suspension long after th_y sth-ter was stopped,, In co:q:ttast)' fhe well-crystallized syn.thetic and seed g:yps:u.m ·settle~ -very'rapidly; l~avi:i:lg, in the absence of anhy-"drite, :a ·reroar~ably clear· solu.tiori.. . . - .

Iil one run (LS 51, Table 2), anhydrite grami.as'la.rge·as 0.:5.nirn'tt.,¢ro~s ·we.re·· s'ynth~h.ed 'from gypsum :in 22.·pe~cenf su:1fu~ic:·acid solution at ~0°¢'. th.ey shovved a. ~tubby pr{smatic: to equant shape with very high jnterferl;!~ce _colo~;; and_pa:r~Iiei extinction. ·· . · -

The cell Ifaran1eters ofsyrithetic anhydrtte_wendn exceUell.t ig:teement with those given l:!Y Swanson. efal.: (1"955), . . .. .

B;:tssari.it~:-(caso{·n:S:'l.o,-n~o,5) was ~nc'oun.feted .ofiiy in static runs 'jn w piCii, N a!; I h~d be~n. ~tl~t!d. to· the <;:h~rges.: it was ,ldent_i.fi:ed .by.:X -ray diffractioJi te.chpiques only, Using the·d.ata of EC>sl1j?.-k (t.Q-~8~ p. 253)_. _No ·distinction was. made betweel':l::~alcium sulfate .henrliiydrate (CaS0~-1/2H~O) ·ap:d:· solttbl~<anhyddte' {CaS04): hecause there, i~ c;on~1dex::~,PJe :®.<iertab:).ty ~·to the'relationshfp Qe.b¥een these- two· phases· (peer; :Howie.

aiufZussm~n-. 1962, v. s·, PP·· 201~208). .

;EXPERL"Ii!ENT . .u- REsULis

R:e.Ji!-lts·:of dynl:i1iiic; r?J.-t,tts. :the. data us-e·d -fo.'d!'!@~~ the position of th.e ·a-H~&t~xp:pe:J:i;tture, e_guili]:)ri~w t'urye, :for: tii~--~eaction. ·

i78

. are gi\r~:n.: rn:·t~b~e· z; a,n.cf plotted: ii1."fftgwe· .2.·- .. • ~:P<'lt~xmill.ation qf tht<' curve rest.ed.on._ i:he,-abill:ty to. co;n'~~~t.t g:Vps\Dl} .~o •.

~hy_drite~. q,nd 11?~e vet sa, anif.o11 the- a,hil,ity: t9--i(:!xe:r~·f! .. tti~ C.9nY~rsl!:m · ~'{hen ~ftlier one.pftiie p~ram~te:r~; Y(~'varied_ .

.. A J:iln .in. W hji:h .. n 9. .cl1C1ng e: oe~urr~d jh the·· s.tar:ting ·ph'.ase;" evert· after a , r~attioP: tiin~ Qf many months; was. n:of'considered ·~-J.idence of stability·.· pf that pliase,·aithough lli.mahy eases:·such data: ·provi¢-:ecl, c9~1l.rJ».ano~ o(.rever;ed'rutis. Take~~ by:its~if; a.rliQ~hi:'w.h.i¢!Irto .. I:ea~d()l1·~_ccur~ ,P,at. best illcondusive ~ilf:e' nieta~fable per~istence ofsta~i:ihg ph~rses is:-~om, m(mly···ep_ho\D;l'te~e(;t f~ experiinentai .studies. ofmineral' equilib:ria. Xn ~he

. pres~pt_studx:-this was.pattkula~ly tr1}eof· the t~hydration qfarihyd~~te in. sulfuric· ·acid sohitio!'is, and. 'induce(t fiuil~~-a:iiQn. l)y -~~~4iJig ·-proved necessa,ry. _Anhydri-te· rep1ai~~d~ u~'cnan,ge~--in: m.o~tunseed~d rur;_s· for v.p to. e~ht .tn.o;nths ;. t)le ;tddition: of seeds~ q(_gyps:mn. :promoted .. relatively rapidicqp:yi;!r!:!1l:m: ... oHhe'a,nhydrile (compare-·runs·LS .5' alidT.S14 a.r.35°C, jab}e; 2); Jn:tb~ system· CaS04-H20, the dehydratioii~of gypsur.fitq aj}py~ ,:Idte• has. beetf shown: to be incredibiy slow (t.g. ruh A--G~ ( 70°C;:··T~hi~ '2) l::iut,_ according to Pqsnjak (1'Q3S·, p·. '462) seeds 'o( anhydiite {:i.o initiate the'reaetiQIL1 Considerable doupt: has been-thrown Qn the.determination of st~bilitY byexp~rime~iS In which. ~~·eel-s h~ve· JJee~_add~io }he i:harge ~.ep~-1f~e Inet.as.t<!-bi~· grovyth ot' a phase)r,o:ri:l sollitionqn:~e~gs.~ qf. i\!3. qwn lrlnd is known to occur (:ir)t£~- e_t- al., 1_9s:8; )? ... 8j)". :RO.~eyer;. i.t. :4~§ n~vt;r b-een _'demo.risttat¢!1 t:hat ~e~q_ipg: w9:u(d p~~;nl}oti.· th~ d:fsappe3rari,c~--of: a stahl~ p]ia.~e:. a_nq gmwth ot' aiess ;ta bk on¢: Ip:-'the present study;·· cl1arges ~d:P,.ta}b,i"Qgd!quafparts by \veight of gytYstlhl and :inliydrite &ere used~ The PJ:90f of'stabiMy :in these, seeded ru~s-·wasj~rowdi ()f"Orie~p}ia,s~.arJ.9-. disappeai.:ance: of. th~ other. :WheP, tli-e, e~t.ent. of reactio:r: ep;ce~d,ed ~b_ollt 1 ·to: io ·percent,.'·rii~ta;:;ta.hi~ pre~jp:ft~:tioU: of either. ph,as~' on se.ed$.-could b;e' ruled out .. .-Tb!s follows from a consideration. of' .-t;h·e: solubilities'. of. gyp~~~;t~~d. ~:nhy.drlt~ i~~,Na2SOi and_H;iSOl_-~oi~tion;: (m;xiliiuni: about =~(f3-percent Ca;so~_~y :weight}, th~m~.$.of solutim;~.'k ~J;le,· rea.~tiqn.;Y~$~~r . (abcruf;-~·200-'~_5o··· g). ~~d. til~' pia'$s o'£ ~xces.s. s.tq,rt~ng·~~Qlid,s: (alwut ·. iQ _g}~ The· ext~P:t··ofrea2t~p.n.wiiS:·.~~ug:OO by.microse<:>P..e and-X::-ray exanlln'ati~n o~ ~, ~ede~. Qf ~a!tiple$ takeU: front the.:reactiori.' vessel. af:.interva.Is·c ::F.roni the x..:r:ay· iliff:iadion-paA:terns, the: relative int~nsitie~ '(?.f.-the. o·zo:peak of aJiliydcit~ (3.:499 A)' and the.'i-40 peak _b(g).1l~uin (3,6~5 '} .. ) weie mea-·sured, The amount o£ gyPS'Q.I\1 ip. ~ach s~m.pl~ w~:~ ~ead from a caiib~a-tion

~-1-"l:h~:was :llorsubstantmted in tfi·e present wo~k,- perhap;;·b.ecam;e..the runs were not-of su:Jticle~t drirn.tkm. However,. it w~l? -6>tmd th!I.J:. .ihe. p~eocc of 4m,J::':·~at.er. (an'2o= '1,00,, pft.=12A), with n.o.seecling, markedlymcreased tl\~ .. dehydratioil ~~e.Fhere a 5e!!~~ run in clistiUed H:.O s?o-_w~iino ~eactlon (~otp.J?a~e'AG j a;~d AG s,_tooc, Tabte 2j •. .

GYPSUiK-AN.HYDRlTE.EQUILIBRIUkf 179

t.A,B!-E z; EXPERIMENTAL ])ATA FOR_ GYI'SW>'!.AND. Alm-rn iUTE STABILI:r¥ AS_ A FiJNCl'lON DF

1\~;riV',I:lY ():E' ,H~O- AND. Th:MP:ER.~TURE AT An.rosPHERIC PRESsURE. REsur.,Ts OF-DYNAMIC RuNS- Omx-

AGi A.qs_ L!(9.

L'OZ· ~X25 LS30-~_

LX49-2-LS20 LS22 LS3o:l LSS LX49-1' LX41H TS5'2-LSil Lii'lB At;;-3 ):.St LS16 LX17.

t.XH

LX4S LSSl Lg32~2

LS2' r.&i L$f5. LX15 £X16 LS50 LS-i-:2 AG4-LS9 LX12

. ·.~X1,8 LX19-

_):.S23-

'.LS33-1

\

r

I I ;j­i l i ! i l

?Sg, 5a 10og­_1,00g

g;:gt l(!Og. 9lJg._~oa•· 100g: SOg, SOa $0g, 50a silg, ~oa>. lOOg' 100g.' ll.gi looi: SOg, Soa. 50g, soa: ~Sg;,S!!- j· iQOg· j 50g,5Ga l

t.2._3 18.9

1s:o

lS-.0_ 19._3

9,:,43

:t.b~ 9\43 -9.43-<;l.56

2:i.U. 4.03 9.56

l_OP.%~0 !}.56 9.56

ioog llts:o 1oog is,o

I 1 I a,gl i_ 20;9 · lOOg f.

1 !~20a~- r 12;24 22·.24

9.56 9._56 9.56 J ~Og,_.~Oa I_

i~Og 15.0-· 1003, 15.0 lOOg 22.24

23-.I'Hi lJXl%-IL.O

9.5!)

100g 9Sg,Sa,_

s~soa. ·1 10901:. • 15.0:-

lOOg : ~,-_ 21.0 lOOg 'l2 .;l

50g, 50a. __ f· 22.22 ·.22~ ll sag. soa.•. t ~ . l·

g-gypsUlli a=-anhydrite gJ..,.:g~iiberite tb.-th~dite mr-i:rrirabilite n;c.2n; ·dctedable.change, >l:.::.:;mtnrnl gypsi.mi and ~iiydnte­( -).::_trate- · - -

0.?6-~ 0 .• 942 6,961 !L~,SS. !).985_ 0.961 Q,961 o,96o <L9s:t o:9.{i 0,866 0,985 0.960 _1.0.00 0.960' o·,p6o -o_;'/53 o:9s3

O.~?.io ii.s63 ll.S63 o;9~9-0/'.59 0.9.59 0;952 0.952'

Oi862 0-:846 LOOO o:9s!.l o:9s;t b,?i6 0,922_ 0;?~~ CL86_0

50° so•-

~0

45~--

4;0"·

j400 I ~o·

40°

iw

t. 40° 40"

19 46

~47-

26 g

43 4-C\ 42 51. 51 35 5o

-99 99 42

-35 ?78 262

.45 155 47. 94

319

211 62

2'11 55

;rss­ss 88

100a:_

gi_ a. 45g, ssa. 94i;&l ~4g; 6a lQOg. .99g, lOa, ssg; 4~a 'llliJg a,gl 1.00~ 95g,5a 95g;5a !!5g,S!l-100i(

95g, 5a :J:

.l. lOOa

_H)Og 95g,_5a·

I, 9Sg,5a.

-.·. a,.gl, ·rg) , sog: si:la

J, 60g,-40a, H)Og too11 lOOg lQOg

i 70g, 30:i . -I· 93g, 1a. i .6Sg,353: I l

l l

l. r 1

1p!)g 1\)0g 9.lg17a· 100g 'too& 95g,5a.: 7sg, 2sa.

-~-:-ta·

g~a;

g:-+a-. a4g a-.g

g .......

n,c. ll.C.

g-+a

a->g a~g

n,c.

n.c. !lYg

-D.C.

g-+a(?) ~-:-+&(?) g-'a._ g--'>a. __

rr,-ll,_. il,.i:., ·a_;g

n.c. a.~~

g.....;;.a.·

g':'a .1'-,.,..,g;· .a-+_g: 11:-+g D.C~,

}SO J;AWRENCE 4>l!i(RDI_E

-: .. _\ ~~::· ... ;::,,J ~'lf· .1,. ~~ ;:;; ... ~~ I' #Mi

= --·--~ L- .. :~~ :Na.SO* .&so, ! ;..,:.,.··~··'~+,_,...,......,;,.....,Ji~~~--J.:-~~-_,_,,..,... LS3-1 IQOg. 13.60 - o-m -~'; ".ioo ~3 T -,~o(l& n.c.:

LS3-2 100g 2~.69 ci.~_4s .fO" ~il :iog;soa ~~a LSW ·sog', 56a 23.6!} o:(!44: ~f-5" ·3;; '?Sg; 5\' a-::-"-g

LS3Z-f .:soii,soa.•c ,22,24- o.SS9 3.S0 · 51 sog;-~o'" a:-1g_

LS4-1 joog ,; ~3.60 o·,s.i3 35"· 21 lOOg · n.c;· :rss:. )oiia. l'· ~-.66 o,i!43 J5°· 232 1ooa V?t~ -i ~9~r;~o11 23.:~ ~/8_43 15° 83 94g;6a,

:1:;:;;c4.0 j g, tli I ~;4.5, 0:90_7 ~'~-·-~ l 195: :~,~1 LS4.-3 !1

1

, .S.Og, saa· 23 ~6o- Q.a~z· . \( -~~ 1 toog Ls6·. -i.OOa. 23.60, 0:842! 5<? 11}7 }O.g~ . LSlf t 10og ·28.61. 0.17.t ',3()1'- 67 !,l_Qg(!J:>a:· LYs' 1ooa. 1~,-.3_ o._%2~ 2s" \~ &gg, 7Q!' l LYG. fOOa 15.s osi:i 25° 1&9 ®i, 20a ~ LSlO _5Gg,50a .i.J:_G(j· QJ4{i: W 55 "l~Ok l LS12 iWg: ~~;~i_ 0; 1J1 2S:'· 61 40g;6ba·

i;~7 ~~~~sOa 1!: Hi•1< ~~r.~r ~:;!!, f2o0"_~- l ~~-. , lj, ~~a LG2-l -1003. Jjj:,":J: 0. 943' \ a, g, mr ts"i3 1ooa 1•

1 2·a·.6X: <!;768' I'. 200 ti 67 45g;ssa._ ~~~..,.;.J~~--__::_-,-___ ___! ___ ---!-!.,.,.,e-_______ . '

0: !}.900 '(\! .

.:r:' IJ...:. .0'-'

~:

b 0.800 > ·r::-. 0 '<:{ 0.750

A A·J:I. ·:,..:.. G-/ }'' l~.'Gy·- Art : - ·

H~_.so,. -~- An+~y_-"' ,G-~ . v

,0 An-.,Gy- An

rs· An.-:o:~Y: o·Gv-- AI'\:

.. An•G.(-:- ·f;_t)" t!*f

nx .. ;1.'-+g

.n..c.· ~~ p,.¢:-. g..._...a: !'i:-:?g ~->g_

a:g­g4ii. g-7ta·

1!~. ~->g

~~? ··.:,

'0 G'J -~,A~ _} t_{;i;ew"ofer.•. '' :_'

JEMPERAJLIRE ,;f. :ifrG;_ 2. :The_ stability o:f g-tpsill_ll_?-nd· anhydnte:~¢temlked expcihueg_ta.\JY ~ a_-fqn~,'

~ibn;· 9f temperit:ure· arid littlV:!tyLttt '::Bif:it atinb$'hte_rl~ pressme:., Q,nl{~)l!Is; ~r{whicn: a: C<inversionwas: a.clllev'OO aie plotted. . . . .

GYP SUM:_A.NiiYDRITE EQU!LIBRIU1! 181

t::urve· of I(aJinyd~.ite)/I(gypsum) ~g~1ms_t 'vJ"eiiht perq;.nt _gyp~Il1- "\Vit4-a reprodu-cibility of l)ett~f than 2 ·percent• .

,13ecaus~·:q{ tP.e tf:rge lhnit1 most of the runs :Were stopped·before-eom-. pl~te: ~onver~on of' one :phase to ·the' 'Other· h~d: qc·cui.fed·. rl1ose runs' "\v:hich wenfallowe:d to. i:~acf completelY"! pro"videP: ·D1a:ter_f~l for- optl.Ca). and X-ray ·studies·. . . . .

Figut~.i shovfs thiit fllB.e:xperiin.ental data, irr.gene~·al~ are consistent. HoWEWe'r; some exceptions ·must Oe'il.Ottd. At 50"C; in':lS petcerit ~oiiillhl suJfate ~olution (aH2q-~(f9~3), anEydr~t~ $tfl:+t~d gtowhlg itt: the e;Kpense ot gypsum after 'lihout· 9Q,davS. rea,c.dori time· (rim: LX: li; Tible 2).

'TABLE 3 • .AcTIVlTY.ml' ~O-AND ThMPERATtrRE TltAT ITEFINE..TiiE.EQUI:L:urR. .. ""lJM."

· .Gns~r=A.Nfuiitri+2 ILOn;,.'"'~n. A.:r.A';rMos;p~~c :PR'EssuRE: · ·. =.

- ·- ... --~· ... . : .. ·.:-:··. :· . . · ... · .

Temperature-°C(±2!') r··

·sso." so" 45? :39q 35" 30° 230 18° .i2~

1.000 0.960 0.915 Q.sso· 0.84~ o.sis 0.800 .0.770 . (1,750. ·0.725

···:·.::.::-·····:·:·· '• ·-.····· .·.

~.§.

Remarks

extrapolate(}, "iii~~ . interpolated mterP.olated mea~iired interpolated' ~n.t_e.rp~Iated measured extrapoJated . ex~polated

·=-··.

Ifpwever, tb.e reaction app·arentiy stopped. {"!). a;s n.<l" furth~t' gro~- 9f ~nhydrit~ otcmred in 10 ni~ntM.~ U.~.4~~-_tpe, same ~9nditihp.r;;, ?-niiydri.te as·. a starting· phase··. r~inain~d unaltered after 5 months'· (ruu LX·'17-): With; suiflfric· add: of about-the same activity.-ofH2.0,. anhyd'rite·was con­y~_rted to g)'})SUPl. at 509C in a:. seeded ru.ri:: ~thiri a- inonth (~lts 16), while·gypsuni. was. unchanged hi·an. ll.llse~:deo_ r~ri o£9" montl.Is"(run ;L$1).­The. anbmaly rehiifus 'i.tilexplah1¢d ... An inco~~istent· resuit was also_.o.b­ta1ned af 55ciC' and {.i.H:!o.=;=:0.96.,-·In. an unseede.d_. ru~; gypslttU :w.as:,con~ · \re:rt¢-~ to-a@ydr.lte:&.s-nlfuric acid solution.(h.m._LS 8, about-SO p:ercent ~ea2tiol))·; }n seed.ea· ~uns .both synthetic·, atid· :na,tut!lt anhydtit~ 'w~;r({· transiorme~ into gypsum (tuns .LS 22 q,nd. LS 3.0~-i). .. ·Tl;tese resuits are ta~ to indita.te"·tlfat the rU.p.s an~· very'-·cl(j$e· b:r":t:h~ ·equilibrium curve.

Wh'li. tJie 'e'x:{:ep~:lon of J;\l..li LX ·11, then·;. the results ·obtained using sui~ flltic add soiuti~J1~, a;re· p.~fectiy consist~nt With thos·e obtai.ned:"usip.g $ogium ~ulf3,t.e s.ot~tions. · -- .

l'he a~():.T: values·~wh.kh -de:fi,ne "the eituilibriull1 -~u.~e are ~hr~n ·fu Table.-3.. · · ·

-.

182 LAWRElfCE k.lf.ARDLE/

Results.' of .static. r-u1ts. The: results .ofexperinie:n:ts mi-:·the: gypsum-~PhY.­.dtite·. reii'dio:n tarried" oul under: static ;<>onditions ·are- not:included in Eigp,i:e:Z_·or T:;tble 2 bee~use- the metho(i: wa:s'$ooil i;ibandoned irdavo..ri'of az1t~ted··rlll1S, A 4ise:u~s~o~-~f ·th~- dath; h()_we_V'er; .is~at'ranted.,All th~ ~ta~k runs werf;!. rn?-4~ with. su~de~t ·:t-ta2S04 to prouuce, gi~u)Jeri'te· .(C~SOi:· Na~SO~) ·as an __ additional phase .. Synthetic an.hydrite-.:wa.s;' t]le ~t~r:ting: ~-oli4~ a,It!tougli 1,n_ a few tun~- 4atur·a1_ a11.hydrite .. or; synilietic _gypsum w~e- l.ls_eQ. i~sfead. See&;we;re)iQt .?ed~~d, to a,~y of ti:if;; ¢harge·s~ R.eaC.tion· times vai:ie(i"fi:om· a few, d~ys to oyer' 12 months., The resPiis.

··were: ri:iost unexpected:· anhyclrite w-aS found to i:.-.ehydrate to. gypsurr( at tempetatur.es·a.S high as-''TS°C. The reverse reaction, the dehydi-atiori·'of g}:{>.SUJ.1?. t<? aJ;ih);dfite, ·v{<J.$, ll.ever achieyed;_ ~yeral·po$$ible eXplanations _come- .to UiiUd., .Firsf, _.the propert.ies· ·o(th~ $.yn.th_eti't:: a11hydrous CaSQ4_ may be different from. those of n~.tuml anhydrite. However~- thl!. :r:_esult:s_ _of th·e--dynaplic runs indiCaie tha£ the d:iffhencest. if any, are not signi:fi­!:'fi::Ut, Ano.ther_1)9$si.bility i~ solid solution ·between gypsum_ and sodium s:ulfate. Unfort~,na~~ly, thiJ> could_ n9.t b~ c)wcked by· chemical. a1t~lysis iiue to· sepa:pl,tion _di.flicv,ities .. However; no significant ch~nge in cell '.diinensions of-the gypsum was observed; which suggests.-little .0.~ I1o.sub-stitution- of Na+ for Ca++ in ·the gypsum structure. Indeed, a Q.ire;~t. substitution iS impossible smc~. it -yvould 'create a charge imbaiance, The subs.titu.tl.on Qf .N:a+ for ca.~-perhaps.: could be 4chieveQ. if a~cdmpa:nied

_by _an_ H~O.~..:. for. SO<\= substitution._ Chemical analyses-. of natt~ral gyp­sum and_ anhydrite· (Stewari, 1963~ p~ 33; ·Deer,_ ,Hilw1e· and· Zus.sman, ·1962; p ..• 206-. an:(f p.: 221) show no e-9-idenc!'!. of· tlus:' sodium only oJ;~~ws in trace amounts !fat a.u:~ . . . :.. .. .. . ~ ·-· ..... ~. ·•. . ...... . . .. .. .. .. . . . . .

a tJ;ti.rd :P9~.s~l:lUity. ~~ i:~I~t!=d. t.9' tl;!.~, ~,w._eJj:[q:{lt~;~al ~e~hpd~ -~n t~e -p;repa-ration.-O.f the,cbarges,' water was. a-dded to)J..-{)olid mi¥.ol a_tiliydfiie· +-thernardi k Local-high ·contentrations'of: sodium ·$ulfate soiuti.oo. ce_r.., tairily-'~ri.ste,d, anq/ptQq~RlY p_etsis!e'd;)n:; the initial stages: of the.run:s., ·c.ql1i'ey· aii~ . .:BmtdY-X19.S.8:) _ ~ila _ I:I~r4~e- (19(!$·, P~ .-1~6) ha:v~ shdwit that f:~hy~_dt~_ r,eact'~; ye~y- r~pi4ly- w,lth: f;P»~-e~trat~d _N~2SOi soJ~~ioifs,'f:o. Jo:rm Ca-N a- -double s:ulfates .. These. dotible::salts. ar~ :unst~hl~ W, djll,.lte. :Na~SO~ ~ol~tions ·{I!~tdie 1 :~.965,_p; 136';:)Iill amt Wills~ 19.38;_ ~__1652). ;a!J-d. ·i{etompose to g-)>psu_m_and/dt·fifa:\Jbetite._.In distilled_w~ater they:}mmedi:.,. ·:~~-~ly deco~np.Q~:~- to·tJ.:.Ps).U:n+ Na2Sb.~- sqhiti_on 9-J;'aU:.temp·eraiutes. up. to· 100°C, IUf!> p~s~ible, th~r,eto:re; that w:. the static ruU§. ~a);ly,£Q.rma,tiorr of .dot~.blf:~_salts occur:red in. the r:e'gion;s. of lP!='al_ .high;:t'fa2S04 'con~entq.-tion.. This 'reaCtion removed. ilnhydrife from the sy!>tem, With time·;:- diff\~!'!iQn . . kq to• iJ,-unifor'llliy t9.i1cl'!ntrah~d:.~o_lution too, loW: iir Na2S04 content for C!onbk~-~i'iit .,9ta:OUlty: ;Det9mpositi'6n foll~fw:i~,, _givi11g. gypsurri+g~au~

: bedte as products.· Then, with prolonged time the ~-yps\l.rri sli6_ulq coii~e:rt ·

to anhydrife. This.iS a,'-piau~ihle explanation because _in aU :tuns (st~tk oi stirred} iri whidi the .starting mihydrit~ wa~ ~add,tid to pre-ini,..:ed.;. uni-­

: formly cj)ncenhated so.dii1m sulfcl:te· S()lution, nq.- a.nom!).lbm· fonnation ol gypsui:tr was ob~eryed. . .. . Th~ work; of: Ccmiey•,an,_~- B~nciy (1958} is:pertinenLhete since_ th~y··

proposed ess-en,t}any·t~f~tnech:anis~1 for the tonv.et_sif)n_·Q£ ·~nihydrite to gypsum: jn -~llit·sohiii:6ns. They: suggested that ih~ reac.tionfor the con:­y¢rsil)n with activator soluti()ll~' such 'as_ sodlUm,o.r. potassium _sulfate is~

. . ';ttl'.. . . .. .. . .. ·- f!.o·:: CaS04 -'---:f· Ca+-1:: + SOi. --_· -_- ·. CaSOJ: 2H20

. ~t<): . . . ,,

arid is primarily dependent upo)i tempe.rat:ure:and:~:-con·centrat1on: .How~ ·ev_er, they achl.eved_.th_e· c'Q:Q.v:ei~io:n. oniy by \vashihg-lhe reaction:p:rod~cts with water:~· Tbjs ih f~c.t._re.ail-y· Q:Qly ·demonstra:te_s· t_h,at · a;Ohydrite _will react rapidly at low te;Qlp_eratiues and higi;t ~!kalt.s:ulfate t;qn~enttations _(see Co:nley and B'L.Uidy; ·1958; F'igs;: _1- ~.n.(L2)·to--iotrn double-salts which .de~ompose in wate:r' to' give gyp~u~:n and salt solti.t1on. It certainly does not_ prove, as they. ,rtiaipt'ained, ill 'the·.a:bsfni.ci:-·of their paper (i:>. 57), that "i::onfra:i:y to -recent h:ypothesis 9fgypsun:i: dehydration by concen­trated salt solutions, double salt~ alid/ or ·gypsllJD,'are. stCJ.'Qle phases below a temperature of 4Z0 C:": · --

Commen-ts o1r, tlu~ tizet:_hq,_ft,_is11~. qf deltyi!rati01i of gypsu,'fn: .to q/nhydrite. Thre.e differe.nt m.et;:han,is:rns: by which gypsum· in ·~:o:r;ti:ac·t with an aqueous me-di:nm cocld ·dehydrate to anhyd-rite appear<_ppssl.ble: ·

(1) a_ solu.tion..:precipita:tiop_ pro~~s. (2) direct .dehydr_ati~n to anhydrite--(loss of.stru-eiura;i. W!l-t~r}.- .. (3) step-wise dehydratloil through the inte'r;rl1e_9-ia.te hydrate) bassari!te:

·TB..e I?r:~sen.t-e:xpeiimental resUlts thro~ ~qme)ight, albeit·ve1:y- diffuse, ·op:- th~. problem~:

·•In a few oftheri.t.Ii~ip._w_hkh anhydrite w-as p.toducf?cl.'f~om gyp~u.m;-.. 1:!.

rind, presumed· from X rays i:obe anhydi.:i te, :Wa~ ob~:"~rved on, the s:t~.rface of~ and. alonf(cieavage:·~rack~ in, gyl)sill~H::rystal$'(~1g: 3). It·is·conceiv-

:!rl!{~~~~~!t~i~~i~li ·i;:ryst~l ·surfaces where: HiO niay be- transferred· to. the. solutio~ phase-. The ef!ect -oYseeding on i::he dellydratron..r?-te could n_dt b.e gauged viith ~riy- tertairity be~a~se .to.~ few dU:plicat~_ runs were _mad~. How.ever; ut'

0~ ... (\1 6:270.

:::r:::. tn -~ <{ 0~250. ;·~

0:: .t!i.

0 0

(XI 50

.. ,. " :J.

9 pos<JjnK!l~~B); ~ posnio~ 093fl):

D: tiurett a iillefl!t902l.: a ~;{ni937r . a ;n~_cri~.Tobie~ !19zi:!}' · .A·B-ac:i.: (1\)GIJ

:;j, Hiii a Wills [t9o8): 4tZen n$&5),' bRoiJ[le~trojlch i¢8~1 ,: c~tculO.ted · . C> Morigoac .!IB7.ii:l' li ·aoc~ lt961l_

Q,I~O t-.~·'--h' ,..,....~•..,.......L,i",_J;....,.::-!::P...~~~-:z..7 ,......c..._.:~--1 ;....·· .:.-Ji.....=.-.c;.:...:,+.....:......~'-:-'-"'"'-.. ... I() 20 . ···~·a·: . 40. .. so· ·so. 70

TEMP~F{A~UR_E., oc f'w, 4, T:h~ st;~lybiJity: ~~l<~.ti.Pns·qf g.J-.psum and: !!,nliydrlte in the syst~m ¢~sd~~H20:~,, ~ foottloii Of ,t~inpe~atur~· at aiDJ.OSp.herlc p:ressure:: a·: Ctnn21tatimi ofpte.VlOUS \ypfj{_~,

splubility da,ta therefore. must he .-regarded fl.$f.::yieldirig :rninhnum values only. Tlle argu±ii~lit· is ,crudal, fb~ it: the pofuts. plod:~. iii F.igutf! 4 ·-for ~!-uhydrite ~re .ihd.eed tn.inintum. values. '-then, fhe .t:ransition·,pqin.t}J.\ust )1e· at some,: temperatu.re: ~boY!!' ad~a.st 44~c, i~n'.s_. (19.65) dat~ being:t~ken: as the, upp_er::liroit of :gypsum:&d\1-hility. H:ilf(19J4~ ·19J7) di<f.recQgg1_z~ the.necessity·-for· approaching; a·'solvbiUty pp.X:V~:·,fr~nn··both: sides, and·, iQ.de~d1 l'eP,orted:h:i~ ·inhydrite -v~alues as:ob:tawed fr.om.bqtJi :il}:l~~rs:atu~ rated' ..and: supers.~tu~-a:ted· ~-oluti9ns. T.h.e, ·data; howeve.r;:. ·we-re e.xtr!lp..,: olated:fi'om.soluhilities ~eapur~d-in'potassiuni·sulfate solutions:be.cau~e: he was ~ah:te t9 athieve sup~rsatUrat1on. with tesped: to anhydrite: using· p~e wa.,t~,r:a,t t~mperat~:e.s b·eil:nv.:·6~.P'n. At fui~ and. b..igh~~:'~empet<i.ture~·· t:~· ap.pa:I,'~P,t!Y. w'~~ ~lj.c~~s.¥u1, b:.Qf, u:rifortupately,: he~·~eitb;er: d~ssrihed-, the pio-~dure nor··xepqrted,, ~djJ;~ct.predpitation· ofanhydrite·f.roll.i sohi~­tio:r:L · ·

-,

GYPSUl-.f-ANHYDRH'.E EQUILTBRIUM 187

, A similar ,-crit~cism: applies· • to the livaiial:>le gypsufu. an.d· anhydrite solubility me?sUrements. made in salt pr: ac::id solutions,· arid· probably explains wiiy :the results· of-c;liff-erent workers are in such poor agreeroeP:t. Ci:impatison 6t the resultS oi thes~ .workers _is. most easily made by com~. · :P,lJ.tingthe<actiVity ·ot .. tr,o· oft he soliitioiis reported to he: Ui-·equilibrium wi'th gypsum+anhydnte. Bec-ause' the CaS04. content of the$e stihitiohs

-.. . .

T .mu:- 4.- THE EFF.EGT oJ? SALT SoLouoNs. oN :tl:l:R GYJ'SUM.-Aim¥mu:m ....... -. . . .. - . . . ..... .: . --_ ..... . J,RANsrrro.N:: A Co~AR1SON oF P.REvrqus WgRK

· investigator

van't: Roff et ai. (i9o3)­D1Ans et al. {1955) Bpek _(1961) ~il.ds~ & Swa}~ ({9$6): w.At!S &.. l:f~fer (1937) .. Taperova (1940) D'Ans e~ ai. (1955). :0' Ansel ai: _(19$~) p~~\> 4 ~~,, .. (1Q.~5)

li .. ·- 'Trans, 'temp~ oc

1 . :il!:trapoli!-ted "t!Y, the P..r.esep.(a-gthor.

.7~.4

.922

.8¢ _.BM -. 790 &4:5 .881 .l~3S .751

~- The riisultS' of tlie5e 't~o s~udies were b.~en-Jrom Seidell (1958; pp., _665-6_61) •.

i? very low, ·;:~.ctj,vity of .H2Q may be obtamed with consid:erahle· accuracy from th,~ Y<:l.ttr v~ppr P.:re.~~lJ.re? ·pf ~he: CaSO.~-free s~lt or acid soiutions. Th~-vapor· pr~ssur~,data.fo_rthe; NaCf a~d· N~;S:o4 eqi.illibriutn:solutions were:takeh. from the Interna.tionai Critical T.a.bies (1928~ III; pp_; 370~--371) and those of.se;;i,.water. hon1. Aron~ and K:ientZler- (1QS4). Fot·ti;te· ph()spllorii:: ·ad~ solUti()~s ·the· vapor pressure value:? of. )~~abllik;ov and za.gwosdk,in ·(1935fwete use4. Th~ ie~lts: are su.rnmarized iri :Table:-4, and shown· grapl!i~~l(v in Fi~r~ .5 ...

·-The values of. va,n't Hoff ei al (190'3) alone .. aie higher' tha=Q._ tb;~e of l1ie

188 ~LAWRENCE:A. HARDiE)

pr~enf ·study. The''appniach of van'f Hoff'.: and':his; co::.·workets' to the problem. WaS brillian uY:· conceiv~d: .. Thi!y. fi.r~t pahiall y ·cqhy¢J;t~d: gyps:iip1 : to: anhydrite· m S.aty,rated:·s'dd.ium: chloride soiution ill a, d:Uatometer at, 70-"G.'·: The rate of con,v_er~ton of gypsu:m. to ari.hydrite•. an.d: tlice• 11er.S~' a1 ·<fi.ff¢renf:.tempe;ratu:res ·W<l$ the1;1. measuredby'the _rnte of ~h'a.ng~ o(vol:. :Um.e o$ the contents of-the:dilatomei:er~ Af-_the equilibrium tt:;p.iper~tute. llie~e should he no volume. chq.nge. Th.e}r gbsj$!V~cl :i voi~une'9~~~ea§~ ::tt' 2soc and a:rt: ii)-~re;;i.se at ,3~'?¢; ·so; die tran;;;_itiori.te_mperature was tikexi as 30°G.· The :pard?i. pr~ssure ·of H~O of the sdlu.tion co..iexisting: with.

0.7.5.0

0: Mo<lgin ond Suiafe.S .. (195Gt

:A.:.z~ri-ll965l.\CQico.itoted"f

e-. Hill.cirtd Wills (1938)

Jl Toper'<mi ond !;ch~lgfno (19451

0 van:T Hoff ef al (19u3~

TE:rJ PEB4TURE "C,

~G• .S;·T~mperah.rre.:a:e:ti~ty ofH:.O i:eliitloM.of gypsum_and ililhydrite at.atmospheriq. · J.)XesliUr~~ acomparioof.. of previo~s worii; Solid ctiiv~: thiS'study; da5hw q.l~_t~~! ma;pin.U'!Jl· and. nrl'ry,imum limits predtcred.·b)- thermodynamic .::ak_ulations.

g)'pS'li)n, anhydiite at~;dh.~_iit~·at ,10°C.was.tlwn·meas.ured,, giving;a.:v.alue qf 24' min.. The meth9d waS:.' repeated u:sing sat1lrated 'sodiitn:J:· bro:rii'afe solution and a transition teb1pe.i:ature of 'S0°C\vas· obfamed; th~:{~<~,te:t; · vapot pr~ssil,te pf. the .equilibrium solutibn was lUeas.ur~lf .. ~s .$~3~.$: i?i.nr.:.·JJ'.s'-":. ii:l.g_ these· two. ·.~oin~. in th~ tel.:ttion~h.ip · · · · . · · . .

lbg'p--~'·lo~f'-0 +· 4 ~- B(T: . they ,were; ,'able;.,' t-o extrapolate to: solutiilns :irt:- the $ystetri. Ca$P~...-R~O, Iillpljdt fu,·this approat:h,l; the'_p:r~cipie .. t.liat .the. gsrp.s'U;n:-ari:liy(lrite b:·a,nsition is. ~d~m·~n4eut·.-of the t~p,sf&t~e,nts: in s()I~Jio11 ll:~d.,. th,~t .. the eq~b~{~,.f:~tnp6r~.t~r~fs ~ fu.nctJqn:oftheratio~pfp'\ 'l'hi$ is,;of:course~ ·tJ.i~ P:~ill:t:i~l~ o:q.: w4.i~;:h . .Jh~~ pre~el),t_s t.udf'. is b.p.se~.! · ·

,p{gw.e: 5 also. shows, th!:lt althol;lgh llie resUlts of different work.¢.i:~ ~re in: .ver.)i:'J?oor igreemetrt;-.they: allAall' within; the Jin;J.i ts p~~dfctec{ ~y· .the -±h~rm:od:fgi:J.:rq.it' calculatr<i1is· (Appendix) l · ··

GYPSUJ.1-A)VRYDRITE EQl)riJBRll/M

GEOLOGICAL IMPLICATIONS . . ~ . . . .

'If hal)' -l?e~n,:sugge5t¢d by· ~:Otri~ '1/btkers thai most,= -if no.t all~ ·caldum· sulf~te: of" naturaL eyaporites was origin_a_ity 4~poshe(as gypsum J?6sn-­j~k~-::1940;:Ilundy;_1956;.Conle,y. and Eun.dy, 'i95(~Iurray, 19M;; Z~n,_ 1965). The agtument is· based_ ori: {lj petrographic observati<?~s that much_ a-i1h.ydrite is pseu&:mioi-pr(olis after twiniled gypsum, (2) tlie"sc_ar~ cify of anhydrlt~ in-mod_ern ~--~aporit"es <l.eposits, and (3).-"exgerimental evidence; that an"tiyru:ite_ canno-t pe s;rnthes1~ed undet pressl:l:re-=t~:tllpera:.. ture: (:tni.ditions=-tonsistent with patural evaporite environ:m.ents.-

_lt is dear ~hat the _present -.relationship be~ween-gypsUm, a~d: anh.y:­~;l.'d.te fu the· pr,e~.R~c(;!):lt m:a.rine= e.vap_orit~s of the world, tq _.a gJ;ea t e.iten tt is secondary,- due--to the effects of_:po;:;t-Q.epo~hi()nal buriaL :Gypsum_ at sudate may be- trated downward i~to·anliydrite ·sQ tbafat depth!)_-h~li:>W about· ti}0(}---3009. f~et gypsum- is pr~·ctically absent (MacDo~aid; __ i95~ i Stew;u-t, 1963). EVidence of replacemept is :a.bi.tridarit. Anhydrite psu_e­~omorphqus, a:fter -"twjrined. gyp&.Wn. ha~ b¢en =;;~ported· by S~haller- ~n.d H.enderson (19.32) -in the Sai~do for;matio~- of Te:Ji:as and New Mexico, byStewatt·(i953) in _the Permiail evaporites-of Yorkshire, Epghrnd,_ ~4 by BorChert ~nd Baier (1953) in the German Zechstt~i.ll:- At shallo~er depths gypsum has.-b_een sh(Jwn tci hav~- tepla~ed anhydrite ·(stewart, 19.'53; Go1dman·, 1952-; Oguiben, l9~S; S:und~- 1959); $U.t~ replhcement has b:een recorded rec.ently at a de~th as great as 3500 feet (1{urray:,·-19Q4); Theief()fe, both gy-psum and auhydr.ite in .sedin;ien.tary- deposit:> way. bt: mit~wprphtc- huU:hii? ~yid~nce does-riot prov.e:that the r_epl\lced gypsm1t

1 t?r_ anhydrite~ w~s prfu~!'l-ry in m;igii:-i, a_ p9int.e;p1'phasized by Zen

(1965-~ p. 't47)._ : - - .. ~1'ore- signintaht evidence is provided 'by the_ d1stribution of-_ gypstirQ. _

a:nO:'anliydiit~in Recent :rii:,trine and nonmarine evaporites where effects __ due to pur!al al;e.,nQ_t ~v.Qlve~. ·iu these d~posits gyps-um}~ ubiquitou~. and

7-_in ·a.u cedaib.ty~-prilnary- .(Bxan~amp ;"aild fow-ers, ~ 95_5; ~for!J~ and,

DiCkey-; 1957 ;··tv:r-asson, 1955; Phlege~ a.uQ.:E_yv_ipg; i~~i; W~IT$-, l9.-~2.'~ and otli~r_s}'-;vher~a5- anliydtite has been.reported_f:i:"om"only OJ;r~·ip~;:;llity; i~ Sl:lpratjdalfi~t. sedbPents on _t~e Trucial Coas~;:P~rsiau Guff'(Cuttis et al, :1963-; -~msnian; ".1964)_i This :sirigle- oi:c~rren~e of Recent sed1menfat:y. ·

1 Qfuer,:oc.eurrences of: Recent anhydrite have_, been: t¢pbrfe4 recently by ;B;UJ;~-t·:ef- ~~· :{1966, P• 59}.~- a_ surf~q! layer: in. Death Valley,.- cilifomi.a, aM by Mqioi~- arid G.'ov~~ (:(965) ~qm-a ~~~~-~J:I:ID!)-O~ ·q~yton.P~aya, Nev~a. In-bothtase5 ilie_anhydi:ite i:i~_ formed f~:qm gypsUp:t;' via bassap_it;e, ill the- absel;l.Ce of' a; liquid phase. This de~ydratlon process

·caso~~2R:O~.ca.so.-!B:~Q+- H:a~O" {g) - - - 1 _, __ · -· -- -: - : . :- -- "'

_-CaSO~-·%H~O~CaS04+ !H10(g/

-involve;; a set offuermoc:iieml.caf conditici~s y~'differeridrom those eiit-ciri:ilteted "inf'wet" evaporite deposits. -

tAWRENCE A .. HARDIE

aubydd.te is~ of g:reat import because,jf liot eyiden):;e ot primary pretipi" tatiori:of anhydrite, it i~; atl~asf;.pi:bof}hat :o.ietamorph£;m of gy!isum ~';m

·bm:ial kn<;>t e~se1it{a.t to· 8.~hyd1:ite: f~rmation. Even accepting that aDc­J:iy~ite can:btf l?~ri_ID<J.fY I1_1I1llC\)ns1stency~bet~e~n the field e~ideuce horn . p..1qd~r:n eyap_orit~s,and 13-V;;tilahle experlinenial_:evidente~xist'!,. T~tnpe);'a;-, t\!r.e:7saunHy conditions ne~essary· fpr a,n}iy~the s.t~bility 1 a:s _pr_~_d:t~:t~cl

:fi:o·ni s-olubility. exJ;'iejh):tent!? ~Il<:f·, ~b~O:Q.od)~nJ'l.OOP c.~i4ul~tlons~.: are. i:::o.m­.Jilonly {qljgd._in:.: __ mod~rit evapo~ite. environments; yet gy{Ysuiri fs the ·:co~~on phase··Qf such deposits .. :This obseivaHoD:,··Cbupled With the ih­·a~hity·of e.1l:perilnentets to s)rnthesize anhydiit~.~t:low tenipetatuJ,:~s, J:ps led w6i·kerssud{ai Mitrray ((9,64). fo' GQI?:Ciude·t1iatgypslim ~~:ll,iways the ·1Jt'irrrar:Y. pt.eqp~t~te .. Jti.lrtlrer;: un.d~ c,pp~it;io.ns the<ireti.cally f'~~:odng. -~P.Jiyddte;>his:· gypsum wiil persi~brieta;stably ( excep~ where t-e~pera­. ture.s, at surface are. yery hi'gh} untiltiuriar causes- dehyili;a tiQil to'-~i)hy'-~~ .

The'presetit ~:xpeuiiJ.eJ;lt_ai r~lJ..lts Jlaye'~ tw9-foid beari~Jg on· thfj pro o­lein:. 'ft}:!;t, it is :demonstra:ted that. aiiliydrite can be synthesi:zed· ... at nne ~Unospher~. pre&sw~- uU:d¢r g~ologibi.lly re&so:Iiable ·conditions of . terri­

. peratui:e and ·activity of H.20 ill a.' geolcigically· re.aso:p.able · t~~,, redwn~d iii:. -~onths, Piiinary- precip:ltitio~ of anhyru)te,.J~bwe~;er~. cqtl]d. not b·e: achieved; ill.deed, .h~s,_i;lo_t b~en_ a~bieved by pn;v1ous \Vorkers:. This wou):d s'\igg'est~ hut; ofcourse) not prove, tiiat''gypsuui'js __ al~"{a:ys.the first fO.rmed C.aS04' .phase on evap.oratlbn· of nai\ual waters.J; B~ that _as it may,. th~

,exP.eri:i:nents do :sho:~ _¢hat_ gypsU:tn,_ ;nai~t~inec(;jn. the·. stabiiity f(~ld 9f: .an~Y._drl\e;_ ~oUfd be 'd:ehydtat~d't'f> aniiydr~fe: S:QO!f ~t-~er <t<?p_O;;ltiQP; s:ec"' o1ld, it:i~ g~~tistr·a:t&d th~t h!gh~~ te~p~r~tur~l'Vth~n p;eYiuu~ly.'we.r~· ~.nt~r~~m~.d, ?.-r~,i:~_quhed f~r-anhydrite: formation, This~ qualitatively; iS' more1n keepillgwith the observation tha.f::~ypsum is ·the\omi:llo,~·r,-&a·s'e: in.:.Recen:f .evaporites'.

Qua:rititativ~· a:pP._li~t~(ni ·of: t~~- e;p~r~~~tal.:re~ul~ to: n:'J.tural \-'l~­p~sl:ts {S.; ya;ij.{(; 8:n~:f P.9s~fbie. hq( i~ hiAd!!r_e<f by ~~-paucity· ,of p;reci$e: -illJorml:l;tibp. o.1J; the..t~~p~:ra-ttl":f~ and· aazo ofna.tb:alsol.utions- c;o--existing .-with msu:m:_ ami..~rihyd~i te., F~;:funit~ly~ the ::Per~ia,p_. Gulf i~P.9.~itjs __ a]1 hnpo:rtant e~ception, Qliantihtlv~:d~t:a hay~Q~¢t.r~_oll~Gted.: !w'D-} J~ lprtsm~~- (Ih~rsofi.~f c:pmmlin1ca.tionJ 19()4)_, Bri:ne femperatuies range· ftoiq.2_4t t<'f'·$9f'C -~nd ~nhydriti;:- is foun:d. m-t:arbonate· muds of -tile· ;Sabkha1. '9x:-__ guptafidal saltdl~f~ where groun(t ·w.ater'.chioF1nitie3- exceed .4oout :i3oaf~~;t~bO'ut 24· pet.ce'~t. s-aiiJ}itit · PtJ,rt: .. ~t;t]ie alJ,hyf3i.it,e' 9-~P.:o-siy

-1· ~tis P<?.~si,bl~,: {)fpo~~~ tn~lt.naturiifWa.ters dl,ffer from e:xjJerini.¢htafly, tested ~ol\ifidn.S fu that fltey. f;:.ont~::additional.'toJ:rijJonehts -wlik~ would ind:uce. dii:ec(precipitatiiJn .o~. •anliyd.d.le, as, (or.C.u:niple; trace eiements· t;finli:iimties'j): J:i~ve:iieen 'found t.o· i'iiliuence 'the nudifation of'a:ragocite and ea1ci.'te(Wray a:D.d,:Damels,l9S7); . . . .

GYPSUM~-'ANFJ.YVRI'{E.EQUIL!BRIU.M

'l'ABL:E; s,.D.ATA ON Sor.urio~s' Co~ EXISTING 'WI:tli:Gwsu:M: OR A:NlrroRl~. rn NATVRA.t :Ev A"PoRitt -DEPostrs

Coast; Fersian, GUlf.

Salina£~-· ·• . . . . ' 1,o;[i~h, . . (Siimia.n)

a~ydnte; "halit~

.. l- :-··----··. :~ ::

· satd. 32-{8-"' I Nati .

I ... _ .. , .. :

i91

1. Anhydrite in the zpne of.a~.pill:ary clra,w •. Chlori:nities as given ardor the U:ndcrlymg gro)li!.dwaters.' i\et!.J.al.soli:i.P.9ns ill. ~hlcil .. anhyclrite fo~ed presuma,l;lly w.e.re more: con-. ~ent:t;aj~d~. · · · · · · ·

.2 :PreciSe locatio:tJ. m. 'Bocca.I)a wb.~re gypsum is pt¢Cipita}lng· is difa'q:Llt tb .reru(froxn liorri~ and DiCkey's descri:puon5, Value glv~n liere. iS ta:ken f.r.on:i their data for l~~ation C, w.h1ch J;Day be incorrect1y i~terpi:eted by the present author as the gY.Psum'site. . ·

·5_I)elicately pr<:J?erved "hpppa:" crysta;ls. w~ch have. de.a.rly· :not: s1]ifered alteration s~ce the~:( formati~rr:. ':(4e. saiiie argu~l~nt '.lll~t apply. to til;~ i!f1iiu!l-te~y- asSociated )m­}l.ydri.te. Tempera.hu:-e:s· of formation bt. tbe .hopper.h:~te..w~ d~terroiried by fl-qiq inf1usi\m ·~hu;lie5. . . . . .

occur5~m the zone ofcapillar.y draw above· the· p:tesen:t·water table b~t~. i:uore importa:p:t;-anhyddte is found iii dit~it contact With· brine ('}.'able. ·.5\· Where, grou~d ~ater~. ar.e: less conceritta,.ted. (up. to a'JiQll:~-_::?ltf.~-o .. chlorlnity) gypsum is preCipitated w~thin th-e ca,rbouate. m.U:d.s. These .dat?-, ·.t~gether w-ith the limit~4. inf:oim?-tio~}ro:m, ::1:· Jew other deP,o~~ts, ·are summarized· in Tabie 5. To· tompa:r..e these data, _chlorillity Vfl:lu~;s

,:.i

have. beeutecai.t.~11ttecl:_to~ actiVit~e~ .6£ u~o us1lig th~ yapo;r:·pressur~ a~ta for __ seawatet of· different·' thl9tirii:ti.e:S' gi'~ien by Ax.ons. <n1d, .. ki~nt~i~r· tt954);; The ilaJ:urai br1ue, (tat?-. aiJ.d. tbt; ~q1J,ilibxiill:n t].l.ty~ (iete:r:mined i~ the, p~e~ep..~; study· are plotted in>:Fig1Jre·6, Included iii this' figure is a gypsum-aubydrite:transition eunre c~m.puted Irom: the solubility mea-. S1\tem.enh3 oi Bock (1961) "lli· _the h11pl&~eyaporite system .CaSO~-N aCl-' H20:. -Hi~''_tE;s·uJ.cl'a:r~: consid~l-¢d n3presenfatfve of the. stability range for . gyp~m-·arid. anhydrite p_r~dicted by most existing solubility studies (see­Fig .. · 5):. If the n~tirral deposits are fo:'i;e interpreted: Jn terms :of this transitimt turYe, then it is.·cleat from Figur~ 6 that meta~tabie.· per~ili~ . tett£e "Of gypstitii in: ,naw:r.JJ· brines·!~ the p.1.k: On·· ~h~· oJJ:ter: Ji?n.~; t~e ·.·.· . ---- ..... .. . . . .

equ,iJiP.riitin.: ~ur.:ve.:·pf .tl;le ·p:re~~;:Q.t stJidY: is· r~ma:rkably compa_tibk ·qu4il.-:-t~.theiy7wit}J,· th~ d~t.a:. from· the natural deposits, patticularly that of the Persian; Gulf' (Fig:.6)~ This.woUid-sfi:ongly suggesitha.f chemiCal equilih­rilim.:·prevails· ill· ~ach Of· these. deposits:: Taken :one step: fmthfir~ this could nieali. that· the, ~car city i;l anl;tydrite .in--mo<).~r:Q. ev,:a.porite~epos~ts · ~s. simpiy a. re~e.ctjoJ1 th,at· .the. couditions. for .. .its fm:ma,tron are ~eidom ;~:ea.9).ecl·,. or1 1'!-t least;: main"t~i:i:red . .fot any length. of time" .M.etastable pet.:. sistence·of gypsum wouid. n·ot be· a necessity.

'While this is most. piau~ible i.t. ~- si.uely ~n.oy~.f~)Jiiplifiq,tion b~c.a:li~.~ gyp~~;¢ is fo1.¢d \md.er ·npnequ.llibri~m~ con,9.itibns iri some wo4~r.n ~v~p_ciritic. envirorn;I'lents, ·Foi example; in. Lagu:.na Ojo de Liebre~ .. Baja c:.·alifornia, gyp~Rtrii co?exists with halite at: tetriperatures ·u:p to· 27.°C (Phleg~r and ·Ewing, 1.962}-:Wb~re.llS tlie pte$ept '?xp~ri!lle11ta.l. data. pre­dict~ that l.ri .. a: s~awat~t hripJ! sat-dr~_thl ~ith.J~Mite; ((l:E:26;{o~7 s)·_g)ipsu;i:U should ·4ehydrate t9 aP,hydrit.e a~ t~~p~ra,i~es abo~e-·ab~ut is<:;e,: (see Fi"g:, 2):. .

The questions,:this. discussion. ta:ises~_are:intrlgul:rig .. bhes ih€{_P~rsian Gult.,. wher.e.~.ar-iliy4tite ·is fornling:, co;m.~ine: a freaJO.ih ~et' !Jf di¢¢j:cal

. ai?-d/ot:Qhls~~~Ccircu_msta.nc~s·ngt_ fo1.p1d.. iil ?t?e:~; w.oQ.er~ evappp:te ·en.:. W~W;rn~ht~J Or;a.r.e_fh.~ ~QgQ.~ti'onq._u*Q.~r)vhi.d! _gypf>UPi:~s found .. m'.BM~ C~J.iforpfa· (~nQ. .. p.~:~.·J~:;!.p~: .other areas} not m.aril:tained for !Qng enQugh periods of ti:ine: each .year- to produce a:b.]iydrite?. H. is dear: that the ptO.b,. 'le~'.J.s·one of. kil:icfics· .. whidi :n:i.u.st;. tl!h·efor¢, ·petor6e a,· most D:P.po~;tant .£~rtsidefatlon ·m.: mierfhetlng gyps~~,p.nhy4rit~· '(tepo~its, ._m.o.ci"erli}. __ , 0~ apcit:mt.

S~RY

}.- :The·present. re~J~:.sho.~-t"bc~J::~P.hydtitt?..Cian ·9~ ~ylidi~si.i-e4 ~xpefi; m~:iJ.tilly' f.ro:Qi ·g)rp~uip.. iiP.,t;l;er :fi~. t ·?J~d a8'2~ <;on:i.lttf~u.s·. u~ason~bJ~ fp_J; · nq; Writ ~va.podt~ envu.on.men.ts ... ·· ...

~'- The gjrpsum-'an4ydrite,equilibrii::til1 temperatures. determined ill.. the . ···.· -.~ . -

0 N ::r: ..

. li.. 0.8.!; 0

>-f-

.:;>' h cj <!

.o.ao

0.70

GYPSU}yf,_ANRYDRI_TE EQUILIBRIUM

0.65~~--~~~~-L--~~--~~~~~~~ 0 _?O· 40 _qO· J30

. .

'TEMPERATURE <>C. FIG, 6. Tempe:rll fur~acq-.n.ty _Qf H:zO _reiatii).D.s of p_rines c~-~-<Qsting wjth gyps_ur(l ap.~-an_..:

hydrite in na-hiral evapf)rite deposits. ExpeclmeJ;Jtally ·deternrli:ted eq(lllib~um. rela1]o1;1s are shown, as curves: solid curve:. this study; itas7~ed, CunJe:. solubility d3:ta:. of Bock (1961)'. The. open.-syinbols rep_r~t solutiC)nS fu contact with.gyps~m and._the solid symbols are those in ·

_ ,~o11ta~;tw~th anhydrite .

. o_pen ~iic;F:e: SqiHI;res: ··Triangles~ Solfrl· circleS:· .Bqu;-.,

·j!-qriis" &< !ii~ey.: (l?;i?) . ")Jra.IDkani.p · ~ Po~ers {1955) .ffi.tdie· (1.965) · · · · Delh~g-(1~5$):

-· _E;insma.!l·· (pers: cm~n1l·-d9_~4j

. .

present st~dy. ar-e considerably--higher-than those based. on solublfity. measurements mid·· orr. thermodvnamic. calculations-. The new. data. are ~o~~fd~~ed mor~. reli~bi~ than. th~ eXisti~g dat~ beca.~~e· (~);·the· pres~nt ·results are ba.Sed on reversible -i:ea.ctl.ons whereas in availabie solub1litv &t'4d~~ ~l;ie sattmi.timi_~W:v-~ for in,P~~:~;d a,nbydrit~-w~re approa~h~d only fm+Uthe ·side: of und~rsatilr~-don: an(:(ther~fore', do:'rloJ·':l:t~t~s~aril.y rep~-esent ·equilibrium' curves·;= (b)' signifi.tanf lolcertaint!~s exist'in·the. a-.iailable' thennochentkal valuds' for· g).rpsunl:·and anh)r,drite.

3. Tb.e·new'equ1libriun1 values of'··1$1s:s1udy,·~re·ml)r~ ·cg:n;rpatlble ~th the· field obseYv:atioris that· gypsUln·is.the;coitunon·ph~se';·and anhydrhe rare~- in Uri.JP.etaniorphos~d· evapo'rite· d~posits:' FurtheJ.~j,these values are' q~ a.n ti~a:tively cop~i$ ten'£ '-wltb.. 'th¢ liH~::-1:: ~qnditi\ms . l,inder w hkh :gyp~· S~Ji:l.- and' arihydrite are f~.und· in tl;le :R~~ent ~yaporite .. d~posit of ili.e. Trudai .C0ast; Persian GUJ( ·

AGKN'OV.lLEDC!ffiNTS

Professor :fiaJ;tS· p; Eugsfer:)'uggested the project •. gui&!i th~ teseard1.. and criticallyreyiewed t!l~ nianus~ript:, }'o hj~; I a~'geepfyin.d,ebfed: The ~a.nus~:rlpf was also re~~w~d:by Owen P~ ;Bd~ker; }a.1-n.e~ L.'~lunoz a.~ d. George B.: Skipp en_ a:nd their c·o~eDis led. to a· considerable im:.. prpve:JP.eP.t in the fu<l.ri~s-(::fipt. 'Nir ... A~:thur Everh;:t,rt desig~ed and built the.rela.ys·vse.d.to··c.olltP)l th!:!. wfl.te.:r~b~th uniw. A ~p~ci.aJ tri'Qut¢)nust be_ paid to· the:late,.ThQmas Jlanks (The Jolins- H:opkins"uni-v~rsity and Ha:tvaid Unive'rsity)_ for· h-elpful dfuc:i.J.ssi-bns and sfimulatior(duririg the ~atter:'part. of the sh~dy. . .

. The·r~earch wassuppoited, l.n.-·part, by:Gra11t 680.-A.f.i.'om:t.he·P~ho:: leum Research Fund· t~ The Johns Hopkins. U:niv~rsity. · · · ·

APPxNI>ci:.. Tm::riroD-iNAIDc CoN.Sm:ER..,.noNs

· C.tJkt.ilfl.ff.01~ of. tlttJ: gyps'!£.1ii~at?Jiyd~i#e ttr,i'n#ti<»~ (t11tje,raturc·iii · t!ze 'systefft:Ta$04-H 20 at vne almasplzer'e' pie.ssu~e: .R~ey-; Southard JJ,.nd A,ndm~n: (l94l} m~11red tl;te· thennochemi¢al ptop~ties of ilie,solid J.lb~es o(ilie system ca.so~-JI20 {J),bl~ '(it .

For tli.e reactibn.. ·

caso~~~~d =:. ti.l.Sb~ +2H::2o m

Kelley· ~rq!.: (1C)~J.. .. p. #}:ob~me9. 4-G~ = _,_i495- ·6s:11 fliJg-t + o.oi15':t2i+·1~.89;P: '(i).

:}'his·equ~tip~ gives 3U°K·(40"C) a5 tb.e_temp.ti.ature ·a.rwhl~ll gypsuili, anbydJite and li.Crr~d _w.ate.r_a:i;e·:Q:l equililir!~ at;.:op.e !! t'!llo_sph~!;toi:al pr~e; in. !li1IP.ri5.ingly good 3.-g~ee~ .m~1: 'o/.!~ tl;le.:va.lue 9f ~"G degv.ed fro!ll sQlU.'{}il~tY data' (.131~; l?~7_i:P(;¥1jak., i938): ~--

Zen (i9t:i2i'.196S) has pQmtell out _tl_iat !'!qlJ.ail9~-(1) was·o1?~ined.ftow.. ip.co,n#ent 4abl,: Tlu:'pt~ent.'(:alcuiations co~i;n zen:·:s criticism .. Dlffe~ep.tiati9n: of equation (i) with re-·

spect to temp~ratu:te, ·-cf.6.G6/dT=tJ.S'\ }~eids · '· .

(2)

and a vali.ie of 12·.85- cal;&g:fo'r.29&0~ •. Thlsu fucoQ.Sist~n~\.:.i~ tlfe su,ro qf t!ie indiv:i<t.t~al: entropies at.298cK as ~l:ven b:r Kelley e.t cl, (s¢e T:a'bie (j)~_j:ha:_t ~. ··

~~S.:"'"-25.5 +(~X 16:8) -: 46.4'~ 12j ~al/d~g·_ .The disc~epancyansei bmn: the l).s~ 0:{ an i;D:t~'~aj:iQ:Q. g5):).stan:t qt· ..:-_ 33 .1,._$'· :WmrJl;~ tli~'i?.cit.n : ,?f.~cir. owri cori¥stent_va:t:ue.of:.~33.03 :~url.:l.h~ ~<!:iUej)f:;:33..3~.o~t'a~e~ ~rQ.Pi',th~ in<W~f.~ vaMr.preSsui:es ofTo~iumi arid Hai:ii (1934): The iesulting;sm.all ep:ot m entr9_PY: has··fl. con:

GYPSUM.:-ANH.YDRITE.EQT)ILlBRIUJ.f

Reference! -~~ Cop~ cal/deg mole 5° (298"K) j·

, : (29&0--450~) qiljdeg .i:J:!o.l~ t

..i--gy~;p-,~~ZH"""'. :",o~ .. ~c___,;:l2L&if0 0'16T .· •. 46.4 ± 0.41 I (id6 & p ;~~ .

Ca_S_Q,~-. anh .·drite . y ..

:1 l4;,10+Q,_0~_3:( ·! 16. 78+040236T '

25-~. ::!;0,:4 1 (p. 36 & p;>1_9) .nr (p,;46}

_ H"a (liquid) -wa.tei'

=t

1 i=KeJley et irl. (t941) II ~Kelley' (1950)

ill:=KeUey (196o)-

18.02

18..04

16_._8 i6.75±0_03

t:4S+0.002t 45: f~ . i :f-5.13±0~03:

7 .3o+0,00246T ~ ••. '·• •' ~ "'·· • ,r~ ~ •

r (p. 36} II (p. 105)

I:,:I (p. SO)

sjd~ble effect qn ilill :free ene.rgy.val:Ues ~nee it is fu-corpo:rated mto the aT t!=IDl of equa-uon (1). . . . . . . . . ' ' .

R.ocateuiation ~.f ilie: entropy: fur the teattion ti:smg ~e a:cceptect v~tire oi s"';gs = .i6.i5 cal/deg-mole for liquid V<--ater (Giauque and. Stout, 1936; Kelley, 1960) yie]ds .1S02gs,;;,255 .+(2-X.16,75)-46.f=:-12,6.<;al/deg; · . .

I;l~ce, 'eq'?-ation (2) becom~'

Mf :::=..-~ 135.84 f 65.17log·T- 0.04.-3 'l'

Usip.g (Kelley it Gl,;194ViJ·-#);

4JJ!; ..;; -- u?s + 4a,3Q :r- o.o.-41_;? p . the' free .energy ea..;pres'sion l;>ecomes_ .

.6.(ii. =; ,...::. 2495 + 164.14 T :__· 65.17 T lo«·T-+'O.OilS t'i. . . . . ~ ... ·.

(3}

(4) .

(?)

'this equatiot'l. gives ~19o:k {46°C) as th~ gypsUII).-anhydrite equilibri1rm temp~a:f4!!;· an iJ?.crease of 6°C o:.er tile value obtained from· the e~ression.ofKdiey·et al (194i) (Equa-

. til)n_ (1)). , , · . ·-The tree energy expre~on <:a,n,·oo fu~ili.er modified b~ employing the reviS.O~ hea-t= . C?.-pacity .QI anhydrite. given \)y KeUey ·(19~0, p~ 46), T,he follo~ng. relationship~: for 't;he reaciiilli are then obt3.i.i:ted: ..

(6)

196 . .

LAWRE.iVCE A. HAJiDIE

Sub~titutfug tl:ie mean vaiue,.liH'\I;s =.4o36 <;aii (Kt<Hey: ~- ~{ . (94 i )-, ofthe hea\: 9f solutlbll, mea,S;W(!IJ!ents iii ~-q~<t-tio:\'1 (1): , . . . .. . . . . . . . . .

sa that

. o· ·: .. · .· . ·. .. . ··.-· LlS~pa-'':--':15.5 +_(~X 16;75) ---' 46.4::==-:1·2.6 raljdeg. Lb.

• 0 .• .o· .. - .. . .

~S'£. = ~S~+ 3L02lnT -~,%i4 T

.Js; := - 148.55 f 3i.02ln T- o.o524'1''

I<:rpin" eqri;t_tion.S"_\7)- aJ14 @}.it .foll(>"'s t~af

':(7).

A~-;= - 2890 + 17957:: r +. o:oz6z n - 71.44 :t JOg T (9)-

thl.s. iquitiO"n· giyes _ T( ~qull) = 46"C, 9-ei:nonstrating- that the. eq¢-li!)rimri ~e/fipe~ture _is' insens1tive to small- '?ar.ia.tions in the beat C!l-p;l:dty of ;mhydrite. ·

If the uncertainties of measurement assigm;d to ea.ch..o~e of t)J.e thermodynamic values (TaP.l.e 6)-used in the _de:Jjvation of equation (9) are a.ssembled, then the· confidence to be placed G_ _i:his .. !OQ.mi.tioh c~ be ~sessed. From the rria.-.mtini and mirumun;_posSible Vaiuis" . of each proper.ty, ex:tep~the h~t capacity; we obtain tyto limiting free energy eqnlJ.~Ol_lSi

.acP = .... 29io· + ·11s.1i t+ ·o.oi62 rz'-'- 1·vu t:Ibg r: O,t)

' Eq~ation (10)", ~~-siiu~ ~o ii\re.11: ~~1i~~ iemp~~e, wM. d~ri~~ uskcr Mf0

19a= {Q;?Q +20==4050 cal_afrqM"2~=·25.1+(2XliY.72)·-4o.8,;; 11.74 caljae~ Th,e eqillvaleilt vatiies -~or eqlfation (11), wJrich. giveS a .mininitun. temperatUie, _are .6l:Z0.29a·=4J)30-20=-@10 cat -a.n-dl\S02sa=25.9+(2xi6:78)-,-46.d:=i3,46cal/deg,. · · · · ·· · - ·

The:~uilil;!riurn· t~nree#tures given by equations- UO) ~d (11} !l-n!,68~C. an!h _25°C respectlveiy, It is clea,r.~t t:p:e avai~b)~ Ut.et!llochernkal"dafl! ca.:n lPc. the gypsun:ka,nJi-y--'dri,te·transiti'on p~if1bl( ~o bette"t ·thfl.ll. ~±22"C:! · ·. · ·

(;{i.Jculaiirm iJf tlweJiect)f rlissoiliea.sal:iS' an--tlie gy_pstmi~dtdtydrite tta-1tsiii0ft. 'temi_erM1I-re:a.t ~t-mo!ip"heric · PreS'Sflrei Fori the·. coli version:· of. gypsum to a:iil:iydrite' in the presen:ee· of: any ::iqueo-lis-soluti<m ·co~t~~_disSQl~:~lts, th_e.reac.!i?n. ma-y: J:ie\'i~tten · . - ·- ,:· - -

G?-.so~ ·ZJ:1:20- == ca~p4· +2Er20cli'q, •• ~»,). ~ d~p~ed)>quilib~hmrconst:ant for this r_eacti~n- can-be a.pp¥ed u: til:e comp(jSi~~n$ Q:dh~ 'sQUd pha.ses:-te:i:riam ·unchanged'- (pu.re liqUid :H:~ and tlie p~e solids at .t a.tni~spiter~ 'being. t:lk_e-ri-as the~tiind~rdstates): · · - ·. ·- ··. -: -

( "·)" • . . 2.

J_G J"'"l·.:t:=:' ~ii1(1'

l" Kelle:r ~~ al (1?41? -p: iS) ~~~ :f 20_qtls. as·fue ~cert;tint:Y.' in this value.

};(;~~·=;:.tiC!. 1-R:Ci.ll Ka' Thei#oie, fOJ.'.tllt dehydration ·af_gyp~um tQ ·anliydr~te

uGT ;;,. A~ + 2kT ln a}l.;o: (12} . . ~~~

. It follow:s . .U:~m ~s equation that ·a, lowering of the activi1;y o"f n~o of. the s~i~tion (e.g:, 'by jp.q~g .the salli.;tity) wou~d decrease_ the frf:e ~nergy bf reacQon: _'I:ht; •effect would be tO" }ower. the.dehjdra~wi tem.p~ture. To evaluate tli.is quantitatively it iS·rtecessary t9 ~uow aG.0i- (:teactionY~rni thean:,o 9{the solutions ill which the r~ctiOn occurs,. ·

An-exp:ressi~n for .tlG;,'I'-(:reaction) as a furictio~ of temperature has been derived (equa~ tioJi ~ a~o~e): ·· · · · · '

'l11ereiore, the change in eqU,iiioriiim temper~ture. wi$ c~gejil .ih:~o' (lf ~e co"eristlng sotU.tic;m can be determined from . . . . . . ..

NJ-r ""'· .....,_ 2.1~.91) + 1.79,57 r- + o:.o.~92 P ~, 11:-~ T ~og·z+ zm_: In.a.:p,q' (13)

An: .fiXPreS:iJoii siriiilar. to (13) biit: based orl_ fue !tee,eliergy' equation of·_ Kelley ei.d {1941}. (equation- 1};;: v.-tts derived by: Macbonaid (i953, -p •. 889) usilig· a sllghtly different tll.~rmody~a.pUc trea.tme.nt: >

!:..GJ. =- 2495 +t63~89T+'o:021S y2·_ 65J7TlbgT+.iRT.2.303i.Ogpjpo (i4-) . . . .. . . ·. . . - . ·.. . . . . ~ . ,. . ~ . .. .- .

. From, i:hisr MaCDoba1d deteJ;min~d the tratiSi.tidn.t!!I!lper!J.ture a~ a'functio!:J. of Conce~tra~ tion of sodium: chloride. · .. ~ell~y e£¢, (19~1, Fig. 8, p-. 41) a.lSo)aii. !=Onsklered the cltect ohctiVity of H!!O, on. tlie. gyp~-anh~dri.te. trjl,I),5.i~qn t~rrrp~i:i, ~urei T)lei prei;e!lted.: the. r~ulis in diagtaiiimatic form. 6nly aii.d did not give th.e~~tion used i.p. the cqlj:qla!;i9n. ·Thl.s; ~oweveriis mos~ cer-·

, tain:to he the equation given b:y-Ma:c])onald, who used the~"( data. ;in~ pro~~: ex~ctly ~uivalei:lt results... .

Eq~l\fJon (13) !il:V~ 20°Q andequa.tipn.(t;4) )5°(;. for the 'i:ral)SitiOli tempe~~e in .. the pr~eD.i:e·pf h~t~ ii:dhe.system C~Oi-:-NaCl-.H..O (a~Q=0-.15),. The uncertainties in . . these f~P~!ltures·willl:etn.ain in the order:: of ± i2e_C, i:he uncerta,inty range f~r-the solu-tion of equation. {9). · ·

REFE:R;E~CES'

.~_-<WDER,. G. :S.'r. W ~- M; !IEswN" :~'W H .. J.4 Jnx.x~ (195,4} TA6 s9._lnbility .of'3:morphous .silie3cin 'w;tter.i, Pliys: Ckem.: $8, 453::..455. • . .

XNs, J.D' (1933rDitiLliffl.n:gsgleitlige~hte.det $ysteme iier·.SaJze ·a.~eani$cher SalzabfjJgeh~t~ · 'g~n_. Kaliforschungsanstii.lt,_ Berlin. ·

-.-.. -.-.-.. D_:_ 'gR:EDTSc~ERi· H .. EICK, .urn li,: 'E:·Fruimm (1955). Uritersnclt:ungen ifl:!er. die· _Ca1aUiiJ._sulfat~. K¢i Ste;insal~, no, 9, 17~38_ · · ·.

-·-.AND p,_HOFE~ (i9;>7) Ube~ das System CaSO~-~:Po~~~20. AtJgw, ,f:;h.f:lilt. so,:toi-104:.. . . . . . . . . . .

AR.qNs; lL B. AND _r:;. •. F •. KIE..>n:ZLER (1<};14). Vapor pres~~p;e of sea-salt soitrtio~0'fran:\\ Am_cr .. {;eopf{ys. Unfqp,afi; i2z~i2B: · · · · _ .. · · ·. ·

~.A:cKS:ril9:~K; ·u. L J .. (1921} Uber clJI! Affiiltt~t der · Araggnit~Cl!-1cit-'--Uro~.adiUn.~.· Z: Pity~. Chet_n, 97, 179-228. · ..

BocK:~ ·E. (i961) On l:he sohibiiity ofallhydrous ca.ki1lin "s).ti.pbat~:a;nd Qfgypsum. in: conce~-:-.

i98.

hated sohitions .. of sodinm chl()Iide at 25~c;· 3o~c; 4ooci apq soc c. t~?i. I bur, Ckifm._ 39; 17~i75L . . . - - . -

BeyiHERT, R ~j:ro· E, B~R. a 953)-ZU:(l\feta:riio"tphose ozeariefGi})sablagei:iJ.Iigen, ~Veues. - lafprb, JW11~~rat-, ilbh:' s6, io3-t5.4. · - · Bil.~. R, A. AND R~ "\.V. .. Pd>i.'F:R!lf19S:S)" T\vo Pei:Sian Gulf-lagQons (ab~-tact). L

Sedi1n~1it. Pet;di: iS; 139-iMJ.. . -- --. . - , . -- . .. - -- . BtND_Y; w. M:;, (1~56) ,P.~:g:_ol9~ 'Q( gypsu¢:::8.n}iy~l'rite P!!!~OI;tts tr;( sa.utl}western fndlana.

ASciUineni: Peirol. 26 ·Z40"'252. - · --. .. . ... -: . ~ .. . . . -::.. . ~ . ~ . . . . :: . . . ,. - . . -- -C{)X<1:-Jl:¥~ R .. F. __ P.Nn··W;. I~{._Bi;J:~--u¥: (1958) Mechapism of. gypsification, Gwcli-im. CoSTIUF

chi1,i_, Act-a r5;-~i---7i. · ' . - '·-;Cu~trs; R., G. EvANS;_ D.~ J.~ J;-~SW.N -A.--"''D_ :0.;.-J,- ~~ARl{,A_~- (V/.63)·ASs_ociati6n of dpio-· . mi te:<i,l{d :anhyQfife inRec(!n i sedimen 1:? Qf the:f!~tsiau GulL ~v c!-f.u-re, 197 ;:·679:-680:. DEER, W.A., R, A>B'owiE'MU) ,J .. iuss:r.rAN (i9~11 J,iixk~Fomii1ig i1inerals. v. 5, John

Wiiex mrg $ons,,New York.:,3'71 p. -- · -· ;D~i)!.?JrG;,L ~-- (1955) Ong~of the.S.alina salt of Michigan:J: Sru#Ji'iefi.t, ;Fe/;yol-,;,25, 83~110 .. ,·FYFE;·w-,·~-:;_r;_;;,'l;':o:~~R.-~J.Y.ia,E;oQ_~~--·(jljs,S).i:ie,t~qr_P.l]i_c;,~i{ns_;.i:nd.li1.~ta-

m6rphi~facic5,·G~ql; Soc. 4-~r. :J_fim_. 73,:259 P- · · _ _ .. · _ .... _ GIAuQuE, W _ ·F:_:-A~"'D J.. W~ STOUT (1936). ',The entrop}i ol.water and. the 'J:hlrd Law ot

.. 'i'he~9dyrui.mid. The' heat capadt)r .of ice'frmn,15,; to 273oK·: ~~~4~ner:. C!tem. Soc. 58, it44-11so. - . - -

'GO:i.nMi."'~--M-.:I. (1952).D~fm;n,1at~em.,o metii.motpW~m; a!!~ mi~eraliZa'ti'-cm:in gypsum~an-· . hydiite ~aP:-rock; Sulphur$ii.lttiome; Lo]lisiana~ Ueol; Soc .. A mer . .M em, Sol 169 p: .. _~DIE, L, A. . (1965) Phase· eqtiiiibna irivolvmg riiinerals of. tl1e system ~aSO,-Na2~0•-: .. :H2o: Ph.n.··:ni~is~ TheJ~hri.s1Iopkins University, Baltimore, Maryland. ·HARNED) .H.- $._.AND· B-. B:_ OWEI'f. {1958); Th,e pl:iysi!;al cheJ'rli~try o~ el~ti-olyti~ ~olQ.tiop.s,..­

· :?rd ed. Rdrilwfd Publishing Co,, New York. &03 p. 1;{1):.L, K E;~ (1Q34). T~~ary. systertlSj xrx; C:;i.Jciinl;i '&ulfati!, potassium' sulf'ate. and w•atel; .

.r. A mer. Ch'em~.Sot. 56;- iott, · · · -~- (i-93i} The ttansitioii..tein:pera turiOf gyp;mm·-to a:riiiydflte.-f.;A nJU~ Chem: Soc. 59;_:

2242-2244. . . . . .

-·-: -· · ___ A:t-.--p \Vrr.;r.s, J. 'H; (193.8) Ternary. 'systems -xxxv. caiclun:i 'siilla'te, Sodium ,Su~: fate-!!-~~iWi!..ter.J.~4n..zt/ Chem . .S¢c.:6o,'.647-l.655, · . - ·

~Q:PG.-~. c-~ _D. (1953) .Handbook; o/ Cli~1iistry· a.1iil. Physi.cs:,)5thoEdltloi). Chemidi _. :Rl!b~r-Publi;iiing- Cq\1· Clev-eland~ .3163. p,'

Ho:n', J. li vi.Nt.T,:E. F. ARMSTR9~G, Vf\ J!iJ!:TRI9f.JEN; ):<~, WE;J;<;;~_RT -~ G: Jbl?T· (11/D;3} Gipsu:qd Aryiydrit. Z.d'hy-?~C1iem.';!:5;-2,5'l~~06.

~]~:F;~, G.: A:.J+.'W!. L. :!;:._ .1\L~~ (-L~J)2i TJJ.e.~o!U:b)lity 9f gyp51irp_ .. .l- 4ili.er: Chem, S_oc: 24, . . :§61:'-619:. .· . . .

'11:--o:Nx, c: B., T:· w. :Ro:emso'N; w. A. :BowrJ.s .-urn· A~ L. WASRB~ CW66}Hydrologi.c: .. _ _basin Death ~alley <:::.alifarJ:lia. l! ~- G~~tStu·~-::frof;::f_ap_ .. W+.:B; l38 p, _ ''J?~ierrwiidttal'{::ritical_ T_a~les_ (1 \}.28) v:.-:to: aiJ.dN. ~.f!=P:ia,w-I!ill )39o1t .. Qo.,-N~w york:: ~L~{,iv; I,::;L AND K~ _:r: .. z,.\dw.osi:iK~ (19.35) bain:p{~parin1mg~ der- fhQeyphoxsii,utec_

)OSUflgen~ z~ A norg. ci~,. 224t 8i5-82o. . . . . KEL~, k·-K.''{1950) CqntnbuttOD$ to'fue:_d~ta on th-eP!=~ti4~·metallqrgy.- xi: Enb;ppi~

of inorgan.ic'sub-stauces·, Revfsto~ '(I~) o:f·4:a,ta inl~~~~ds.6i calat!ation:~ '[J~S, B'!fr:· M'i'n.e$ 'au11.- 477-. ·

--.. --~ -·· -_tf9.6Qft~ntri6utioris to'ih~ data o;Ii- t1i'eoietleai:in~tail¢gj,,XItLH:ign-Jem'[H;ra­ture_.hea,t-content; :heat._&padty, _and entropy. dl;l,ta,.for.·tli,;; elen:rents·.anc:l illo~gM.ic \:omrounds: us-: Bun_¥£~~~:) Bult: 584:- . ..

. . :· .· ....... · .

,.

GYPSUM -ANHY !)RITE .!JQT.JI4IlJR,f[J1fi.

-· -·-r c: So~RD.').r<ll ~,-T.-.:I\NDERS9N (1941)Thermpdynawicproperties ofgyp$'um.: ifndl\~~fehydratio;products. T/.S. B~~r·.1liine; Te.E!z. Pajr,62S.

X).~SMAN; D, i J. (1965) .Dolonritb..atimi and evap9.i:ite q~ye~opment,: inf?l~dingia!lhy-d,*e in Jagoorialsedllnents, Pe!-~Ian Gulf. (aj)Ji\:J:~ct)~ G.eill. Sop, i.1ner;' Spt:'c. Pap; BZ·, 108,-· 109. . . ..

KR.i\UsKOPl!', $.:,;.' . .iL (i95.6)· ~solutiqQ.. <J.l:!~ pr¢dpitatl~n · Qi siiica.· at. low·. tempenttures~ Ge~him. Cqs~n.oclitm: A cia, .lQ; 1----26:

M.i\cDONALD, G. J~ F. (1953) AUhyqri~e-gypsiu.n eq~prium ~Ja.ti{)_ns. At~- hSc.~~ 2511

884--898. :U.iADarn, W- M~ AND D. A- Sw,\LES (l956) So1ubilities iri the system·Ca.SO~';NaCf-I:f20 at

25" and 3_5°, 1: App~. Cfiem,, 6; 482-:-'!SL. .·. . . . . . MAm.G:'NA!:!, t. (1874) Sut Ia solubilite.du sulfate de chaux. An1~- C!im.: plzys:; 1. 27~282'; MAE.sAL;·D~R:rrn (1952) Der Efufliiss d~ Druckes auf das $y:>t~bi: C.iSO~-HiJ..:H eiilel6er.:. ·

gerBCiLAi it~eral. P~irol. 3~ .289'-?.96. · . . MA8scm, P. H. (19~5) ,AJ:.i, oceurie11ce of gypsu:inJn Southwest 'r.em:.J~ Sea.-tment. PetroL

25···7i-'-79: .. ·' . . . .. . . MoroLA, R. 1- AND E. D. Gwm.'(i96SJ Recent anhydrite from ChiytQn J?laya.1 Nevada.

Aine;-. Mineral. 50i 2063-2069: · ' · ··. . · . .

Monis, R. C~ AND P. A. PrCKEY (1951) MDder:n evapoHte de~itiiin in Peru, Am.er . .iss. J;etrol, Geol.-:Bulr; 41,: 24q7-2414. .

Mu-RR.t).Y, R. C (1964) Origin and· dl~enesiS o£ gypsum and anhydrite. L $editlten/. . PB#:.ol, 34, 512-5~.

()GNmEN, L. (1955). In.verse.giaded bedqillg,in pririia:ry gypSUJil of chemical deposition. i. Seaime1~t. P~trvl, 25, i73-28L ·

Osno'FF,, A .. G. (19.64) <::<J:n.versioJ;I. of gym;uJJl ·to anhydrite in aqi;l,eOUs salt solutions. Geo~him. Cosmqclr:im. Acta. 28, 1363-1.372. · ·

J.'AR~~~. }';~ .?.- AND ,A.· H. Wmrn (1929) Tlie solubility of" calcium sulf~te}rom {)0 to 200°C, J. Afner. Che'm. Soc. 51, 36Q-~7Q... ·· · ·

PH:LEG'ER, J?· •. B. Al<i> G. C. Ew.m:a (1962)· S~ent6logy ai)4 ~Q.~ogra.j)hy. of ~a.Shil kgoons.ln :Baja C;illforpia, Meriro~ Geo~. Soc.A11t~• Buil. 731 145::-:i~j; · ·

lf'os~.AK;EtJG)lli (1938) l'he system CaSO~--HzO: Amer-1. Sci.35A, 241-.Tl'l. i"OSJ.'<"}AK, EUGJO<N. (1940}: Depo~ition of ca.i~iu~ sUlfate from sea·wat~r . . A~er.; 1. $'Cf. ~B,

559-568. ~tlPENS~UCH, G. c~ (i886) tiber die· :Bestlnm~ung der _Usli.~~c!( einiger sa'JZe i.it

Wasser b¢ verschiedenen Temperattii~n .. Sitzungsbcr. Aiida.. fV' iSs. W ie1~ 9Zir, 463--491. SCRALLER1 W: .. T. AND_ ;E. P. !f:&NDERSON {1~32Y:~.fiiuiralogy 6f drill <;oreS'fi;om the potash

field of.N:ew M~xicq. and Texas~:us:· Geol SitrD. Bull. 833~ p. 124.: -SEIDEL!,; fl.. (1958) Sotub,il1.ties. lnDrganir; .at~ ~Wal--orgctJZi& ComP.o.tmds. v. 1; 4th 'ed,- by;

W~F.Lhike, van Nostrand CO'., i:ribc~ton, i487p. . . ·. .. · S'IE'\vAR'i', F.·H. (1953)-Early gyp!iJ.lmin ·the :Pertnla.u ev.apor~tes ~>fNistth-eastern.Eng1a1uL

·.·Proc.·Ettgi: Gecl. Ass. 64; 33.~39. · ·· · -Smw.u-r, F .. H. (1963) Marinc.e'Vap'Orltt;?: Chap. Y-11~ M. FJeischerr (ed:):D.ata oj'Geo--

ckemistry_. oth e.iJ. u.s. Ge<il. Sun.Pr&j;Fap.4Aoc.Y, p. 52. . . . .. SuND; J. 0. (1959) Origin of the New Brunsmck. gypS1).111 deposits.: C(,n. 'Aaning kf et. B.uJI. ~w~u · ·. _

:'Sw.AN'soN'~ H: E.; R K. VUY.u AJ:ID G. ·u. UGRlNNic (19.55) Standi.fd..X--ray·dilfraction ·PO\'I'dex:pa~tern~. N at •. Bur::Stanit. (U.S,.) Ci1-c;.539, 4, 67'. .. _

>rcm:g;:MI;. T: ;um R. HARA' (1934) On the futisi.uon of CalciUm. sUlfate in water and in con;-. - centrat~ sea.water: J. Clmn. Soc. Japan 55, .iOSl-1059 (in Japanese),

-... -.-.-.. T~:RUWAIIAJL<\ AND J{.· B':~_{f9~~) Ori:tn~ ca1c_~gn:r-suliaf~ ili seaw-liter.ll; s6Iu~ . bilities_ of calcium. suJphMe.hexPiliydrate ill·i;ea:_ wa{er 9.t vaJ:iO.us; qmcenfx:ations at 6sqsoo; Tech1~ot."Rep.-icltokii'ititP:U11i.i!.)2;:s6iS-~7.t · · · .

WnGHER; ~:;T (H)62.). SfaruJq,rl_tit~JiiJIJ,s dJ. clz&-.1niCai ·anal~~#~ vbi., i; par:l:-'A, 6th. ~& V?:n: Nq~trand,:co.,-~x:in.::eton,.12!)2 p~· · · · · . · · · · ·

Wnx,s; A,,J; (196"2)1~e(;efi.l; doiomiteiu th~P~iSjan.Gt~!Lffal:Un~ 194, 2'14<:-2?5~. Wrnca:iri,_ A.: .N: (1933) El~Jiien_t~ aj Optuiil M·irterr;iogy;)>_wt)j_-:·D~s,c/iP.ijvns oflfine;ats,

-3rd ed. John Wiley-and Sons,. Ne~ York· . ·· . . · · . . ·· -- ·· Wo9~TERi ~~- ~~ (1936) On 1:he ciystai squ~tut~ _of.fn.i>sum, ·caSoi-i.H~"-i:jdistaJJogr . .

94i 3'ls:: ·· · · · ·· · · · .· ,.: , ··· · : . · · · W"RAY~-J. :1:-. -~F. _DAN,IELS .(19.57} Precipi!;!lqoil of~tof~ and·aragorlite~ X·A 1net'~ Cltem~

sOt;: ~9, zo3t:.:.fo34.. - · · · · . :zm•fr ~:AN (l962). :Pliise'equilibnum 'studl'eir9(tlie "syst~m· Ca$0i.-:t'faCl-H!O~-a.t lo~- ~~m~

pera.tuies and 1 atnioSl_)here pressure (abstr.). G~ot, Sdc.A~. Spea. i'~Pi 68~_30Q~ · -· --.. (1965)'sOlubiUty-m~~w:~ments·in thesy~tem CaSO~-NaCl~ii~dih35°, 500 a~d 700c

and_ on~ atniOEJ~heie:p~e5s~~.-J: Fetroi, ·6; i24-:-~64. .·

.M anitscrip{received~ ilarc!d9.i I9.66.i accePfd for:f?1~bli.Gq.Jfl!ft1 ~ug_u;i,1.9; ~9~([.