TIIE AMERICAN MINERAI,OGIST, VOL 56, JWY-AUGUST, 1971

RATE OF AI_Si ORDERING IN SANIDINES FROMAN IGNIMBRITE COOLING UNIT

Ronnnr B. Scorr,l Dept. of Geology, Florida State Llnia., Tallahassee, Fla.32306;SnenoNW. BlcnrNsrr, Dept. of Geology Unia. of New Brunswich.

Fredericton, -AI. ,B., Canada; Roennr W. Nnssrrr, Dept. oJ Geology,Unia. of Adelaide, Adela'ide, Australio.. aNl Menrne R. Scorr,:1

DepL oJ Geology, Florida State [Jnia., Tallahassee, Fta. 32306.

AesrnacrX-ray difiraction studies of monoclinic alkali feldspars [016 (AbfAn)27] from a 300-

meter-thick ignimbrite indicate that those feldspars in the central portion of the unit aresignificantly more ordered than those near the margins. The variations in degree of orderare indicative of structural states that cover most of the more disordered half oi the rangebetween high sanidine and orthoclase. Further Al-Si ordering is believed to have becomeineffective at temperatures belou.'approximately 500"c, based upon the lack of significantK-Na unmixing The length of time required for feldspars to order to their observed statecan be estimated by calculation of the time required for the ignimbrite to cool from 800oc,the approximate emplacement temperature, to 500'c; this period was found to be a fewhundred to 650 years, depending upon the cooling effects of rain water from above andground water from below.

INrnooucrror.r

The limited information on the rates of ordering of aluminum andsil icon between tetrahedral sites of alkali feldspars (K, Na)AlSLOsrestricts a description of this ordering behavior to the rather impreciseterm "sluggish." Yet knowledge of these rates and their dependence uponthe environment of feldspar formation and evolution would be mostvaluable to the understanding of the thermal histories of many alkali-richigneous and metamorphic rocks. This paper examines the structural statevariations of alkali feldspars in a thick ignimbrite cooling unit where boththe initial structural state of the feldspar and the cooling history of theignimbrite may be estimated within reasonable l imits; such l imits allowestimation of the rate of ordering of these feldspars. The assumptioncan be made that the initial structural state of ignimbritic alkali feld-spars at the time of emplacement was close to that of the least orderedsanidines found in the ignimbrite sheet; the structural states of alkalifeldspars at various l iquidus temperatures are not known.

Previous studies of structural state variations of natural alkali feld-spars (e.g., Steiger and Hart, 1967; Wright, 1967; Til l ing, 1968) haveconcentrated on attempting to establish from field evidence the tempera-

1 Present address: Departnent of Geology, Texas A and M university, college station,Texas 77843.

1208

Al-Si ORDITRING IN SANIDINES l2O9

tures and thermodynamic orders of the phase transformations, ratherthan the rate at which ordering took place. Experimental work (e.9.,

Goldsmith and Laves, 1954; Tomisaka, 1962) has largely been directedtoward the same goal as field studies. Recently, Martin (1968, 1969a, b)has succeeded in hydrothermally synthesizing both low-albite and ortho-clase; he concludes that the presence of peralkaline hydrothermal fluidshas an imporLant catalytic effect on the rate of ordering. Investigationsinto the cause of the variability of structural states of potassic alkalifeldspars in Connecticut pegmatites (Godchaux and Orvil le, 1970)suggest that a complex of chemical and thermal factors control the order-ing process.

For the present study, sanidines from the Oligocene Windous ButteFormation (about 325 meters thick), a compound ignimbrite coolingunit from the Grant Range, east central Nevada, were selected becausethis calc-alkalic rhyodacitic unit is sufficiently thick to have undergonea lengthy cooling period, and the chemical and mineralogic properties ofthis ignimbrite have been studied in detail (Scott, 1966,l97la,b). Thisparticular cooling unit has the characteristic zones described by Smith(1960) which include a chilled, slightly welded basal zone grading upwardinto a dense black vitrophyre which is overlain by a dense devitrified zoneforming most of the interior; the less welded glassy zone found cappinguneroded young ignimbrites has been stripped from the Windous ButteFormation. Intratelluric crystals form about f of the rock and consist ofabout 30 percent qrartz,35 percent zoned plagioclase of andesine andoligoclase composition, 25 percent sanidine, 5 percent biotite, and lesseramounts of opaque minerals and hornblende. A small increase in crystalsize and abundance occurs within the devitrified portion; this suggeststhat some pre-eruptive magmatic variations existed but detailed studies(Scott, t966,197 la,b) indicate that these are not of the magnitude of thedistinct mineralogic and chemical zoning of ignimbrite zoning of ignim-

brite units farther south in Nevada (Lipman, et aI., 1966).

Mrruops oF SrUDY

A suite of six samples were collected in vertical sequences from theWindous Butte Formation (Lat. 38o39'41"N., Long. 115'21'17"W.) ; the

sample positions from the base of the unit upward are'. Bl3 at t2 m., Bl4a t 3 0 m . , B l 5 a t 4 5 m . , B l 7 a t 8 0 m . , B I S a t 1 8 0 m . , a n d B t 9 a t 2 4 0 m .The sanidines in these samples were separated from the rest of the crushedrock by magnetic, heavy liquid, and handpicking methods. Small por-tions of each separate were homogenized at 1050"C for 24 hours and theweight percent of potassium f eldspar was determined to be 73 * 2 percent

by the method outl ined by Tuttle and Bowen (1968, p. 13). Partial

l2lo scoTT, BACTT(NSKI, NESBITT, AND SCOT',T

chemical analyses by atomic absorption spectrophotometry confirmthe X-ray determinations of Or content and also indicate that theanorthite content of these specimens is one to two percent CaAlzSizOa.The X-ray patterns of untreated feldspars from the interior of the coolingunit show a slightly broadened 201 peak; sample Bl7,located near thecenter of the unit, has the broadest 201 peak with only incipient splitt ing(Fig. 1). On the basis of this it can be said that no appreciable exsolutionof an albite-rich phase has occurred.

From X-ray diffraction traces of untreated pure mineral separatesof sanidine, the 20 positions of 28 to 33 peaks per sample which metwith the visual and statistical quality outl ined by Orvil le (1967, p. 61)were measured and recorded (scanning speed: f," f rnin., Ni-filteredCuKar radiation, Norelco X-ray diffractometer at Florida State Uni-versity). These data were used to compute the refined unit cell param-eters by the least square computer program of Prof. Charles W. Burn-ham, Harvard University; between 14 and 20 peaks were used in the finalrefinements. Direct and reciprocal lattice constants, c*fb* ratio, 20x6s,20211a, 202n, (20roo-20oor) and 2V, optical data for the samples are listedin Table 1.

Srnucrunal Srarns or ALKALT Fnr,ospars

According to both X-ray and optical data these feldspars are all classedas sanidines, but the relative amount of variation in their structural statesdepends upon the particular criterion used as the measure of degree ofAl-Si order.

The reciprocal lattice constant ratio c*fb* is inversely related tothe degree of order (Jones, 1966). The variation in c*fb* found in thisstudy (c/. Table 1) is small, about 20 percent of the range between highsanidine and the sanidine-orthoclase boundary arbitrarily defined by

Jones (1966). On the right side of Figure 2 the c*f b* ratios of our sanidinesare plotted as a function of stratigraphic position within the cooling unit.(The left side of the figure wil l be discussed below). As would be expected,sanidines displaying the greatest degree of order occur in the interior ofthe ignimbrite sheet; sample 815, the most ordered according to thismethod of evaluation, occurs at the 45 m. level.

A similar trend is obtained when the direct lattice constants, a and bof the sanidines are plotted (Fig. 3) in the diagram of Wright and Stewart(1968): feldspars from the marginal zones of the ignimbrite fall closer tothe high sanidine line while those from the interior of the unit are closerto the orthoclase line. If we regard the relative positions of the pointsrepresenting the projection of feldspars onto a line parallelling the o-axiscoordinate of Figure 3 as a measure of the degree of order, then according

2 0 . 5 2 t . 5D E G R E E S 2 0

Frc. 1. Shapes of 201 peaks in relative stratigraphic position. Note that ttre 201 peakfrom sanidines of the most centrally located sample, Bl7, is the broadest and that thesharpest peaks are those from samples 319 and 813, the uppermost and lowermost samples,respectively.

I2I2 SCOTT, BACHINSKI, NESBITT, AND SCOTT

TABLE l . LATTTCE CONSTANTS, CRtTtCAL 2€ PEAK pos tT toNS, AND OPT|CAL DAIA

to the "Wright-Stewart criterion" the most ordered sanidine is sample817 (from the 80 m. level) I it occurs at a position in the figure represent-ing about 40 percent of the distance between high sanidine and ortho-clase. The relative degree of order of our sanidines expressed by anarbitrary scale is shown on the right side of Figure 4 as a function ofstratigraphic position, as in Figure 2.

B t 4B t 5

a ( A ) q.5058 8 . 5 1 4 6 8,4948 8,491'1 8 ,4956 9./'Rt4/b ( A ) It:88?8 It:88tr3It:889X '1:8812It:8891 It:8d?6

c ( A ) 7 . t 172+ .00 | 4

7 . 1 8 2 5+.0045

1 . t757+ , 0 0 2 8

7 . 1 7 4 7+ . 0 0 | 4

7 , 1 1 1 4+.0020

1 . t730+ . O 0 t 2

. x ( d e g . ) 90 90 90 90 90 90

p roes . I | | 6 .0596+ . 0 | l 8

l r 6 . i l 6 l+.0422

I t 6 . 0 3 0 1 | 6 .0366+ , O 1 2 4

I 1 6 . 0 6 4 4+ . 0 1 9 1

I t6 .o29 l+ . o t 2 l

Y ( o e s . t 90 90 90 90 90 90

V ( A - ) 7 t3.21. 3 7

7 t 4 , 2 8. 9 4

7 t 2 , 4 0 t 2 , 3 8 7 t 2 . 5 3. 4 4

7 1 t . 3 7

a r . | 5090+,00004

. | 3080

.00007. | ] t 0 |.00005

t3091.00002

. | ] t 0 l,00003

. t 5 i l 8

.00003

b r ,01681+.0000 |

.07688,00005

.07689,00002

.07690+.0000 I

,o768'l+ .00002

.07686+.0000 |

c. r 5 5 t 0

+ .00002,15506

+. 00005. t5509.00004

. t 5 5 t 2

.00002. t 5 5 t 0

+ .00002. 1 5 5 t 5

+,00002

1 t 90 90 90 90 90 90

f * 63.9404+ . 0 | 5 8

63.8839+.0422

63.9699+.0275

63.9634+ . o 1 2 4

63.9356+ , 0 t 9 1

63.9709+ , 0 t 2 1

I * 90 90 90 90 90 90

c * / 6 x 2.0t161! ,ooo32

2 . O 1 1 7 3+.00075

2 . O t 7 t 4+.00069 2 , 0 1 7 2 4

+.000292 .0 t759+.00050 2 , 0 1 8 5 1

A^-^ dea. 4t .620 4 t . 6 t 4 4 t . 6 2 1 4t .638 4 | .606 4 t . 6 t 7

Eor- "-"' 50.847 50.846 50.839 50.851 50.826 50.821

2e deo,201

2 l t 8 7 2 1 . t 6 6 2 t . 2 3 7 2 t . 2 0 3 2 t . t 9 1 2 t . 2052fo+o - 2%oz a.s . 2 4 7 ,289 . 292 . 2 1 72 V | ( 0 t 0 ) t 0 24, 38 42, 40 2 i )

0 l ( w e l g h + 72 75 13 72 13 73

AI-Si ORDERINC IN SANIDINES t2 t3

ASH

kr o.oo3

O F F E L D S P A R

H E A T O F

CRYSTALLIZATIOI I I

A D D E D

A S H , k : O . O 0 3

2 0f9 2.018 Z.Ot? 2.016)f

+ c,/, */ b

Frc. 2. Comparison of the variation of degree of order as indicated by the c*/b* ratio

on the right side with the time position curve of the 500'C isotherm of the Windous Butte

Formation taking into account variable thermal diffusivities and the heat of devitrifica-

tion on the left. Development of this cooling curve is discussed below in the text. The dashed

line of the time-position curve does not inciude the heat of devitrification factor. The range

of error for e*fba values is indicated by the horizontal bars through the values shown as

solid circles.

Optical data also confirm the general trend; 2V, values of 0-10ocharacterize sanidines near the edges of the ignimbrite while those fromthe inner portions have2V, values in the 20-40o range (see Tuttle, 1952).However, the generalization of increased order in feldspars from the in-terior of the cooling unit is not well supported by a plot of 20oaors.20zoq(after Wright, 1968, Fig.3), and a comparison of thefeldspars based onthe (200m-2000:) method suggested by Borg and Smith (1969) shows notrend at all. Because the latter two methods are each functions of thepositions of only two X-ray peaks, it is perhaps not surprising to find

O R D E R I N G

\ ' ' oa\

o 200 400 600

Y E A R S

xooe

oo ;U -L :

aF

Uo

1214 SCOTT, BACHINSRI, NESBITT, AND SCOTT

7.22

7.20

7. t8

7 . r 6

t *n*,*o*

$\cRocv\$e

\ \ q-ko-F--

-'\ x - - - t \

\ - - - - - - '- { . o \ - \

- -

- \ e o . . \ - \-< \_ -Y

. L I T E R A T U R E

STUDY

r3.ooo t3.o4ob i

Frc. 3. Degree of order indicated by the plot of direct lattice constants a and b (Wrightand Stewart, 1968). Open circles represent sanidines from this study, solid circles representfeldspars from the literature. The average range of error for a and b constants for this studyis plotted in the lower right corner.

ORTHOCLASE

Frc. 4. Comparison of the projection of the sanidine positions on the oaxis of Figure 3 with the cooling curve of Figure 2.

12.960

HIGHSANIDINE

AI.Si ORDERING IN SANIDINES Lzr5

r / r . - t 6 G , f ) - 0 (2 -E , r l J

T= kto2E.+

o. tz lh icknessk' fhermol d i f fus iv i ty

o.r ^( | ro

Frc. .5. PIot of T/To ratio versus r with g contours for an extrusive sheet' where I:temperature of the sheet, ?o:initial temperature of the sheet, z:dimensionless time

number defined in equations in this figure, and g: 6i-"tr.ionless position ratio also defined

in this figure. d (€, r) and 6 (.2-E, r) are error functions defined b1' Jaeger (1968). See de-

scription in text for further details.

these methods less sensitive to minor changes in structural state than

those of Wright and Stewart (1963) and Jones (1966) wherein structural

state criteria are based on refined and reciprocal lattice parameters'

respectively.

TnBnlrar Hrsronv oF IcNTMBRTTE CooLrNG Unrrs

An estimation of the rate of cooling of a volcanic sheet and, therefore,an estimate of the time required for the feldspars to order to their ob-

served state can be readily calculated using an infinite sheet with a dis-

continuous upper boundary and a continuous lower boundary as a model.

Ingersoll, et atr., (1954) and Jaeger (1968) have developed equations and

presented data from which we constructed a graphical expression of

cooling rates. From Jaeger's cooling-temperature/initial temperature

ratio, T/Ts, written as a function of the dimensionless position ratio fand the dimensionless time number z, a plot oI T/Tg us. r with t contours

was drawn for the asymmetric cooling conditions of an extrusive sheet

and is shown here as Figure 5. The time necessary for any position, r,

I2L6 SCOTT, BACHINSKI, NESB]TT, AND SCOTT

in the unit to cool to a temperature 7'from an init ial temperature Is iscalculated as follows: (1) calculate T/Ts; (2) determine the positionratio by the relationship (:x/a, where a:one half the thickness of thesheet, f* values are above the position o, and -f values are bei<-rw o1(3) find the intercept between the line representing the T/To value andthe appropriate { contour; (4) the proiection of the intercept on the r axisgives the dimensionless time ratio, r, for the case in question; (5) thecooling time, l, can be calculated from l: razf K af.ter a suitable thermaldifiusivity, K, is chosen for the cooling unit; (use cgs. units).

The init ial emplacement temperature of the ignimbrite is estimatedto have been about 800"C; the choice of this value is based upon twoIimiting l ines of reasoning: (1) A minimum lower l imit is based upon thearguments of Boyd (1961) that take into account the heat loss duringemplacement and the minimum welding temperature requirement. (2) Amaximum upper Iimit is based upon the presence of hydrous ferromag-nesians (biotite and hornblende) and upon the fact that compositionsof coexisting plagioclase and alkali feldspar are more typical of feldsparsthat crystall ized from hydrous melts than dry melts (Scott, 1965);bothof these characteristics require a depression of liquidus temperatures be-cause of the presence of water. 800"C is an intermediate value betweenthese limits.

Because common occurrence of perthitic sanidines and orthoclasesin nature is clear evidence that K-Na unmixing takes place more readilythan Al-Si ordering, and because the bulk composition of the WindousButte sanidines (Or73) intersects the sanidine-high albite solvus at at-mospheric pressure (Thompson and Waldbaum, 1969) at a temperatureof approximately 525'C, 500"C is taken by us to be the temperature atwhich effective ordering ceased as evidenced by the lack of significantexsolution in these feldspars. The time required for this 500oC isothermto reach various stratigraphic levels in the Windous Butte formation wascalculated using Figure 5.

The presence of slightly- to non-welded ash along the top and bottommargins of ignimbrites and of densely welded vitrophyre and devitrifiedrock in the interior introduces a complication into the calculations be-cause the thermal diffusivity of ash is about 0.003 while that of vitrophyreor devitrif ied rock is about 0.01 (Ingersoll, et al., 1954, p. 289). Clearly,a realistic cooling model of an ignimbrite must take into account majorzonation present in the degree of welding. Such a position us. time curvefor the 500"C isotherm is denoted by the dashed Iine on the left side o{Figure 2. Devitrification to quartz and feldspars is estimated to have pro-duced about 60 calories per gram of glass; when this additional thermal

AT-S| ORDERING IN SANIDINES I2I7

effect taken into account as in Jaeger (1961), the solid curve of Figure 2is constructed. Thus, considering both the variation in thermal diffusiv-ity and heat of crystall ization, about 650 years are required for the in-terior of the unit to cool to 500'C.

Lrlrrrs oN ORDERTNc Rerns on Narunnr, SeNrornns

The calculated curve representing the cooling history of the ignimbriteunit is compared with the relationship between stratigraphic positionand degree of Al-Si order of the Windous Butte sanidines, based on theJones (1966) criterion, in Figure 2 and, based on the Wright-Stewart(1968) criterion, in Figure 4. Although the expected correlation be-tween longer cooling periods and increased degree of order is observable,sanidines possessing the greatest degree of order (indicated by bothmethods of measurement) occur Iower in the cooling unit than the the-oretically slowest cooling portion of the ignimbrite. Furthermore,sanidine -B19, collected from near the top of the unit, is considerably lessordered than feldspars that existed in an apparently similar thermalenvironment near the base of the unit. These discrepancies suggest wemust question the assumptions that: (1) the cooling curve of Figure 2 and4 is an accurate model of the cooling history of the ignimbrite, and (2)the degree of order of the feldspars is exclusively a function of coolingrate.

No attempt was made in the above calculation of the cooling curve toevaluate the effect of environmental factors such as rain water or groundwater on the cooling rate. Although the Great Basin is rather arid atpresent, Axelrod (1956) indicates that climatic conditions were" con-siderably wetter in Nevada during the Tertiary. Given a temperateclimate with a meter of rainfall per year, the entire Windous ButteFormation could have cooled to 100oC in less than 200 years providedthat perfect penetration and a homogenous distribution of rain waterprevailed. Ilowever, it is much more realistic to assume that only aportion of the rain water penetrated into the porous upper zones andthat the zone ol rain water penetration would be l imited by the vaporiza-tion isotherm underlain by a sharp thermal gradient. Such a downward-migrating cooling front would greatly decrease the cooling time for theupper portion of the unit and displace the slowest-cooling portion to aposition Iower in the unit. This may explain why the stratigraphicallyhighest sanidine is the least ordered and the most ordered sanidinesoccur lower in the unit than would be predicted from the calculated cool-ing curve of Figures 2 and 4. Penetration of rain water into the porousand permeable upper zones of an ignimbrite should be considerably more

1218 SCOTT, BACI]INSKI, NESBITT, AND SCO:TT

effective than into the relatively impermeable crust of Kilauea Iki whereAuIt, el al. (196I) found rain water cooling to affect only the uppermeter in a five month period.

Heat transfer by ground water migration can be a profound factor inthe cooling of an extrusive sheet (Christiansen and Lipman, 1966). In afew Grant Range localit ies, evidence of heat transfer by ground watermigration has been observed at the base of the ignimbrite sheets whereunderlying water-lain tuffs have been fused to a depth of one meter.Because fusion of glass shards in rhyolitic tuffs requires a temperature inexcess of 550'C (Aramaki and Akimoto, 1957; Bovd, 1961) and since thecontact temperature does not exceed one half the emplacement tempera-ture (Jaeger, 1968), heat transfer by convection or migration of super-heated vapors (in addition to simple conductive heat transfer) must haveoccurred; otherwise an impossibly high emplacement temperature inexcess of 1100"C would be required to produce the observed fusion.Fusion of sediments beneath the Windous Butte Formation has notoccurred, however, and the influence of ground water circulation uponthe cooling history is impossible to evaluate. VlcBirney (1968) suggeststhat fiamme in ignimbrites may be created by depression of the meltingpoint of the glass by introduction of vaporized ground water; althoughsuch an origin for the black vitrophyre of the Windous Butte Formationis possible, it is unlikely because of the uniform thickness of the vitro-phyre over variable underlying terrains. Also no quantitative estimateof the amount of water vapor involved or the cooling effect can be made.Nonetheless, such convective heat transfer may have played a significantrole.

With recognition of the imprecisely known magnitude of the coolingeffect of rain water and ground water, the best estimate of the time re-quired for the sanidines of the Windous Butte Formation to order to theirpresent state is between a maximum of 650 years and a minimum oIperhaps a few hundred years.

These time limits on ordering do not include the effects of any chemicaland physical aspects (beyond the purely thermal aspect) of the environ-ment of cooling ignimbrites, but such effects are believed to be negligible.A possible pressure effect (as noted by Martin, 1968, I969a) can be dis-counted because the pressure differential is too small, less than 20 to 30bars, even in thick ignimbrites. Studies of Martin (1968, 1969a) haveshown a substantial increase in the rate of ordering in the presence of avapor phase with a high alkali ion/hydrogen ion ratio but electronmicroprobe analyses of margins and interiors of sanidine crystals fromthick ignimbrites (Scott, unpublished results) show the same compositionwhich suggests that reaction between alkali feldspars and coexisting

AI-S| ORDERING IN SANIDINES t2t9

vapor has not taken place. However, we cannot eliminate the possibil i tythat the vapor phase catalyzed the ordering process without exchangingalkalis with the feldspar; until independent evidence for such an effectcan be found, we will assume that time and temperature are the onlymajor variables affecting the degree of order in this environment.

It is not possible to extrapolate our results and compare them to thoseof McConnell and McKie's (1960) and Parsons' (1968) studies of order-ing in albite-rich alkali feldspars, not only because of the uncertaineffects of feldspar composition on rate of ordering, but also-and moreimportantly-because of differences in research aim between our workand theirs. They were concerned with the attainment, or rate of attain-ment, of the equilibrium degree of Al-Si order at various temperatures,whereas we have dealt only with the time required to attain a givendegree of order with no consideration of whether or not the observedstate represents the equilibrium Al-Si distribution for 500oC (which, asnoted above, we believe to be the minimum temperature of effectiveordering).

Although the rate of ordering devised here may well be generallyapplicable to feldspars in ignimbrites, it would be unwise to assume thatit can be extrapolated to other petrogenetic environments. Extensionof this study to a wider ordering range and different environments shouldinclude investigation of the structural states of alkali feldspars in thermalaureoles of shallow plutons which have intruded sanidine-bearing air-falltuff.

AcxNowmoceunlts

'fhe National Science Foundation Grant GA-679 supported this research project in

part. We wish to thank Prof . Philip Orville, Yale University, for his advice, Prof. CharlesBurnham, Harvard University, for the use of the least square computer program for re-

fined unit cell parameters, and Dr. Robert L. Smith, U. S. Geological Survey, Washington,

D.C., for the encouragement he gave us. The constructive reviews of this manuscript by

Professors Philip Orville and Sydney Clark, Yale University, Dr. Thomas Wright, U.S

Geological Survey, Washington, D.C. and Dr. Alexander McBirney, Center for Vol-

canology, University of Oregon is sincerely appreciated.

RnrrnnNcrs

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1220 SCOTT, BTICIIINSKI , NDSBITT, AND SCOTT

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