07 textures

8/3/2019 07 Textures

1/17

drothermal Geochemistry

eral textures

Mineral textures

Primary growth textures

Magmatic textures

Open-space textures

Replacement textures

Cooling-related texturesExsolution

Inversion

Thermal stress

Deformation-related textures

Twinning

Curvature of crystals

Metamorphic-related recrystallization

Primary growth textures - Magmatic

Indicative of cooling from a melt

High-temperature minerals show no obstruction of faces

If rapidly cooled, dendritic textures may be present

Poikilitic crystals

Cocrystallization = mutual boundaries of differentangles (contrast to metamorphic)

Low-temperature minerals fill interstices

07yb

0.125 mm

Chromite: The Great Dyke, Zimbabwe

Chromite: The great dyke, Zimbabwe

Euhedral chromite crystals (grey) are fractured, somefractures are along a poorly defined cleavage (bottomcenter) and are accompanied by incipient alteration(higher reflectance areas, center right). Silicate (darkgrey) forms the matrix to the chromite and replaces it(bottom right).

Polished block, plane polarized light, x 160, air.

7ya

0.5 mm

Primary chromite, chalcopyrite and rutile: Bushveld

rut

cpy

Chromite, chalcopyrite and rutile. Bushveld, Republic ofSouth Africa

Euhedral, fractured cubic crystals of chromite (mediumgrey) are intergrown with silicate (dark grey). A singlelath of rutile (light grey, center) is partially enclosed inchromite and partly in silicate. Minor amounts ofinterstitial chalcopyrite (yellow, left center) arepresent. Black areas are polishing pits.



2/17


eral textures

14yb

Exsolution lamellae of ilmenite in magnetiteMagnetite, ilmenite, TiO2minerals and hematite.

Guernsey, Channel Islands, Britain

A diorite contains magnetite (light brown-grey, centreright) that has oxidation-exsolution lamellae ofilmenite (light pink-brown, center) parallel to (111) ofthe magnetite. The lamellae have altered to a fine-

grained intergrowth of TiO2 minerals and hematite(blue-white to light grey, center left). Amphibole(bottom left) shows cleavage and biotite (top right)

has light brown internal reflections.

Polished thin section, plane polarized light, x 80, air.

11yc

Immiscible pentlandite, chalcopyrite, pyrite andpyrrhotite: Merensky Reef, Bushveld

0.25 mm

Pentlandite, chalcopyrite, pyrite and pyrrhotite.Merensky Reef. Bushveld, Republic of South Africa

Pentlandite (light brown, center) is intergrown withchalcopyrite (yellow, right), pyrite (pale yellow-white,centre bottom) and minor amounts of pyrrhotite (lilac-grey, center right). Silicate gangue (grey) showsinternal reflections. Black areas are polishing pits. Thesulphides are interstitial to the silicates.


04yd

0.05 mm

Ilmenite laths in silicate matrix: unrestrained growthIlmenite, spinel and iron-nickel alloy. Apollo 17, Lunar

Sample 7018

A basalt fragment from the lunar regolith. Abundantsubparallel ilmenite laths (pale brown), many of whichare 'feather-like', lie within plagioclase (areas of light-colored internal reflection, bottom right) which isintergrown with euhedral rhombic pyroxene (lightgrey, few internal reflections, left center). Smalleuhedral equant chromite-ulvspinel (pale brown, topleft) and rounded iron-nickel alloy (white, very highreflectance, bottom left) are present in the plagioclasealso. The high magnification and large variation inreflectance between the opaque phases makesaccurate color photography difficult.

Grain mount, plane polarized light, x 400, oil.


3/17


eral textures

Immiscible sulfide droplet in basalt, mid-Atlantic ridge

0.15 mm

Primary growth textures Open space

Main characteristic is unimpeded growth of crystalfaces, particularly for crystals that rarely exhibitcrystal forms

Comb structures, rhythmic banding, mineral zoning

Dissolution features and skeletal crystals

Colloform and banded ores

56yb

0.25 mm

Vug filling of earlier euhedral sphalerite followed by galena

gn

sl

Galena and sphalerite. Shullsburg, Wisconsin, USA

Sphalerite (light grey) occurs as radiating aggregates ofdifferent grain sizes. Very fine-grained sphalerite ispoorly polished and shows reddish-brown or light-colored internal reflections (top and bottom right).The central bands of sphalerite have grown into a vugand hence have euhedral crystal terminations. Galena(white) has infilled most of this central vug. Black

areas are polishing pits.


57yf

0.05 mm

Euhedral, zoned bravoite enclosed by euhedral zoned marcasiteNickeliferous marcasite and bravoite. Oxclose Mine,

South Pennines, Britain

Euhedral, zoned bravoites have lower reflectance thanthe enclosing zoned marcasite. Bravoite shows higherreflectance cores and lower reflectance outer zonesand has a pentagonal dodecahedral habit (center left).The enclosing marcasite is coarse-grained and alsoshows faint nickel-rich zoning (center). Nickel-poormarcasite is unzoned, has a slightly higherreflectance, and occurs on the margin of thenickeliferous marcasite (bottom right). Differentcrystals show reflection pleochroism (green-blue toyellow, bottom center).

Polished block, plane polarized light, x 400, oil.


4/17


eral textures

58yd

0.5 mm

Zoning in sphalerite evidenced by weak color variationsSphalerite and gersdorffite. Nenthead, North Pennines,

Britain

Small gersdorffite crystals (white, right) occur withinzoned sphalerite. Zoning in the sphalerite is justvisible as blue-grey (centre) and brown-grey (bottomright) areas. Black areas are polishing pits.

Doubly polished thin section, plane polarized light, x 40,air.

58ye

0.5 mm

Zoning in sphalerite evidenced by stronger

color variations in transmitted light Sphalerite (and gersdorffite). Nenthead, NorthPennines, Britain

This is the same field of view as 58d but in transmittedlight. The fine scale of the growth banding insphalerite is very clear. In thin section or polished thinsection, much of the fine detail would be lost. Theintensity of the colours are due to variations in thetrace element content of the growth bands, mostimportantly the iron content.

Doubly polished thin section, plane polarized light, x 40,

air.

57yd

0.25 mm

Simple and polysynthetic twinning in marcasite

simple

polysynthetic

Marcasite. Ashover, South Pennines, Britain

An intergrowth of marcasite crystals shows theirextreme anisotropy, variation in grain size, andtwinning along (101) as coarse single twins (top left)and as polysynthetic twinning (center left).



5/17


eral textures

36yc

0.25 mm

Open-space growth of early native Ag followed by

niccolite and thin maucherite rimsNiccolite, native silver, acanthite and maucherite. Great

Bear Lake, Canada

Native silver (white, scratched, center left) forms thecores to botryoidal niccolite (pink-brown) showingfaint reflection pleochroism (light to dark pink-brown,center right) that is difficult to see. Thin rims of

maucherite (grey-blue, center bottom) surroundniccolite. Acanthite (light grey, bottom right) hasreplaced native silver in the core of a niccolitedendrite. Dark grey areas are calcite showing faint

bireflectance (top center). Black areas are polishingpits.


Cooling textures Exsolution

Separation of structurally-incompatible phases as Tdecreases, often in a characteristic pattern controlledby crystallography

Different from replacement textures because of

depletion of exsolved phase at intersections (spindle-shaped lath textures)

gib100x2.gif

kamacite ~7% Ni

taenite 27-65% Ni

Widmanstatten structure in Fe-Ni meteorites:example of exsolution (similar to mt-ilm)

13ya

0.5 mm

Exsolution of chalcopyrite and bornite from ISS during cooling

Note the spindle-shaped lathsthat taper at intersections

Bornite, chalcopyrite and altered pentlandite. Palabora,Republic of South Africa

Bornite (brown) is intergrown with laths and irregular-shaped areas of chalcopyrite (yellow), much of whichis crystallographically oriented along (100) of thebornite. Subhedral altered pentlandite (light yellow,center) is fractured. Dark grey areas are silicategangue. Black areas are polishing pits. This type ofchalcopyrite-bornite texture can be the result ofexsolution or replacement processes.



6/17


eral textures

64yf

0.25 mm

Twinning of hematite: bireflectance of hematite makesthis look like two different minerals that are exsolved!

mt

hm

Haematite and magnetite. Skye, Scotland

This is the same field of view as the third plate abovebut with partially crossed polars. Hematite crystalsshow polysynthetic twinning along (1011). The silicategangue shows light-colored internal reflections.

Hematite (white, right) is coarse-grained and showsvery faint bireflectance along (1011) twin planes,which are oriented north-south but difficult to see in

plane polarized light. Magnetite (pink-brown, bottomleft) is well polished and does not show twins. Darkgrey areas are silicates, black areas are polishing pits.

Polished block, plane polarized light, x 80, air

05yd

0.125 mm

Ilmenite exsolution from magnetite,

resulting from oxidation during cooling

mt

ilm

Magnetite and ilmenite. Derbyshire, Britain

Euhedral magnetite (light brown, center) carriesabundant ilmenite lamellae (darker brown) orientedalong (111) and the result of oxidation-exsolution. Anincomplete magnetite rim around the euhedral crystalalso carries exsolved ilmenite (center right). Verysmall grains of tarnished bornite (red-brown, centerright) have replaced original chalcopyrite. Euhedral tosubhedral pyroxene (light grey, left) and plagioclase(dark grey, light internal reflections, bottom right) arethe main silicate phases. Minor amounts of relictcarbon-coating are blue-grey (bottom center).


18yb

0.22 mm

Exsolution stars of sphalerite in chalcopyrite from coolingChalcopyrite, sphalerite and pyrite. Jersey, Channel

Islands, Britain

Chalcopyrite (yellow) contains exsolved sphalerite stars(light grey, center) and two crystals of pyrite (lightyellow, higher reflectance, top right) growing into avoid (black). A very thin veinlet that is only justdiscernible (running north-south, centre) is marked byelongated polishing pits (black) and by stannite (lightgrey, higher reflectance than sphalerite, center)cutting across the central sphalerite star. Black areasare polishing pits.



7/17


eral textures

10yc

0.25 mm

Exsolution pentlandite in pyrrhotite [along (0001)]

Pyrrhotite and pentlandite. Kambalda, Western Australia

Pyrrhotite (brown, center) carries flame-like exsolutionbodies of pentlandite (light brown, higher reflectance,center right). Many of these exsolution bodies areassociated with fractures in the pyrrhotite and are

oriented along its (0001) plane. Black areas aresilicates and polishing pits.


Additional cooling-related textures

Inversion: difficult to recognize, sometimes by twinningor pseudomorphs

Thermal stress:

Common in pentlandite because it has a different thermalexpansion coefficient than pyrite or pyrrhotite

12yd

0.25 mm

Cooling-related thermal stress in pentlanditecaused cracking:note the difference between

pyrrhotite and pentlandite polish

po

pn

mt

Pyrrhotite, magnetite, pentlandite and chalcopyrite.Strathcona Mine, Sudbury, Ontario, Canada

Pyrrhotite crystals (light brown) have granularpentlandite crystals (light brown, higher reflectance,center left) along their grain boundaries but are freeof flame-like exsolution bodies of pentlandite.

Magnetite (grey, bottom left) encloses a crystal ofchalcopyrite (yellow, bottom left). Black areas arepolishing pits.



8/17


eral textures

31ye

0.25 mm

cs

cpy

bn

cc

Cooling and contraction of HT cassiteritewith subsequent infilling by Cu minerals

cs

Chalcopyrite, bornite, chalcocite, hematite and

cassiterite. Wheal Jane, Cornwall, Britain

Equant and prismatic cassiterite (dark grey-brown, wellpolished, center) is intergrown with fine-grainedhematite (light blue-grey, pitted, center bottom).Chalcopyrite (yellow) is veined by bornite (brown, topright) and chalcocite (light blue, top left). Chalcocitealso forms a rim around the oxide minerals. Blackareas are polishing pits. A single crystal of cassiterite(top right) is present within the copper-iron sulfides.The cross-cutting relationships show that thealteration sequence is chalcopyrite to bornite tochalcocite.


Equilibrium textures Symplectic intergrowths

Wide variety of terms are applied to various texturalvariants of these equilibrium textures:

Lamellar, emulsoid, myrmekitic, etc.

32yb

0.25 mm

Chalcocite-bornite symplectic intergrowth

cc

bnpy

Chalcocite, bornite and pyrite. Levant Mine, Cornwall,Britain

Chalcocite (blue) has a symplectite-like intergrowth withbornite (brown, center right). Euhedral to subhedralpyrite (light yellow-white, center bottom) shows reliefagainst chalcocite and its irregular shape suggeststhat it has been partially replaced by chalcocite.



9/17


eral textures

35yc

0.25 mm

Chalcocite-bornite symplectic intergrowth

gn

bn

cc

st

tt

tt

py

st

Stromeyerite, bornite, galena, chalcocite andtetrahedrite group mineral and pyrite. Unknown

Provenance

Inclusion-free galena (white, center right) is intergrownwith bornite (brown, top) and stromeyerite, showingpurple-grey (left center) to blue-grey (bottom center)reflection pleochroism. Stromeyerite occurs in asymplectite-like intergrowth with chalcocite (light blue,center, bottom right) which is accentuated in thesection by relief differences. Subhedral tetrahedrite(green-grey, moderate reflectance, center left,extreme bottom right) is pitted and is associated witheuhedral quartz (dark grey, center left). Pyrite (lightyellow-white, high reflectance, center) is subhedral toeuhedral.


05yc

0.125 mm

Intergrown magnetite-silicate mixtureMagnetite, ilmenite and haematite. Clee Hills,

Shropshire, Britain

A large equant crystal of magnetite (pink-brown, left) isintergrown with, and encloses, plagioclase (dark grey,featureless). Oxidation-exsolution lamellae of ilmenite(pink-brown, lighter colored than magnetite, topcentre) are present. Magnetite has extensively alteredto hematite (white-blue) and minor TiO2 phases (lightgrey) along fractures and crystal boundaries (center

bottom). Lobate ilmenite (right) is unaltered and isintergrown with plagioclase (dark grey andfeatureless). Pyroxene (grey, top right) is present.


62yc

0.5 mm

Chalcopyrite disease insphalerite:

Not an exsolution texture,but replacement or

epitaxial growth

Sphalerite, chalcopyrite, pyrrhotite and galena. GreatGossan Lead, Virginia, USA

Sphalerite (light grey, right) has chalcopyrite inclusionsaligned along crystallographic directions and aboutgrain boundaries. Hence, it shows chalcopyritedisease. It is rimmed by chalcopyrite (yellow, center)and pyrrhotite (brown, top left), together with minorgalena (white, center bottom). Dark grey area issilicate, black areas are polishing pits.



10/17


eral textures

Replacement textures

Problems with complete replacement, recognition ofreplaced material: fossils & organic structures

Replacement of other minerals is dependent on:

Presence of crystal surfaces for deposition

Crystal structure of host mineral

Chemistry of fluid and host mineral

Replacement is often visible as a different mineral alongcrystal surfaces, cracks, cleavages, etc. that allowfluid entry

Compositionally-zoned minerals may exhibit selectivereplacement

Replacement of wood by pyrite, preservingthe cellular structure

60yb

0.125 mm

TiO2 replacing ilmenite lamellae in titanomagnetite;

magnetite is completely altered to limonite (brown) TiO2minerals and sphene. Central Wales, Britain

A metadolerite in which a trellis-like intergrowth of aTiO2 mineral (light grey), often called 'leucoxene', hasreplaced ilmenite lamellae within a titanomagnetitewhich has been completely removed and isrepresented by iron-stained non-opaque mineralsshowing brown internal reflections. The originaltitanomagnetite aggregate can be seen to have

comprised two crystals. Sphene (light grey, centerright) is present. The matrix is silicate.


49ye

0.5 mm

Hematite replacement of bauxite pisolith:

note the two generations of bauxite formation Gibbsite, boehmite and haematite. Gove, NorthernTerritories, Australia

A pisolitic bauxite in which an angular fragment of anearlier pisolith has been extensively hematitized (blue-white, center). Hematite is the only mineral that canbe identified by reflected light microscopy in thissection. The matrix, which is light red-brown due tofinely disseminated iron minerals, comprises gibbsiteand boehmite which were identified by X-raydiffraction.



11/17


eral textures

50ya

0.25 mm

po

Supergene replacement; pyrrhotite altering to marcasite

along (0001); pentlandite altering to violarite

pn vi

Crystal structure often controls replacement

Pyrrhotite, violarite and altered pyrrhotite. Kambalda,

Australia

The primary ore was pyrrhotite and pentlandite butthese minerals have suffered extensive supergenealteration. Relict pyrrhotite (brown, well polished, left,center right) has an alteration rim of zwischen-produkt (light brown-white, highest reflectance, rightbottom) and clearly shows that the alteration ofpyrrhotite is crystallographically controlled along(0001). Pentlandite has been totally pseudomorphedby violarite (brown-white, finely pitted surface, center)but its cleavage and crystal boundaries have beenpreserved. Silicates are black.


12yc

Pyrrhotite replaced by chalcopyrite and cubanite: note the (0001)cleavage of pyrrhotite extends into the replacing minerals

pocpy

cb

0.125 mm

Chalcopyrite, pyrrhotite, cubanite and pentlandite.Stillwater. Montana, USA

Pyrrhotite (dark brown, top right) has a well developedcleavage which extends into chalcopyrite (yellow, topcenter and left) and cubanite (blue-grey, center right)areas, suggesting that chalcopyrite and cubanite arereplacing pyrrhotite. Dark brown areas withinchalcopyrite are relict pyrrhotite (bottom left).Pentlandite (pale brown-white, bottom) forms flames

which are parallel with the basal (0001) cleavage ofpyrrhotite. Silicates are black.


51ya

0.5 mm

Digenite replaced by covellite along fractures and more

extensively replaced by bornite and chalcopyrite

bn

di

Digenite, bornite, haematite and chalcopyrite. EnglishLake District, Britain

Digenite (blue, top left) shows minor replacement bycovellite (deep blue) along cleavage and smallfractures. More extensive replacement is shown bybornite (brown-pink, center) which contains relictdigenite. Minor amounts of chalcopyrite (yellow, rightcenter) occur on the edge of bornite but are difficultto see. Two distinct generations of hematite arepresent. Hematite laths (light blue, hard, centerbottom) occur within digenite and bornite, whereasmost hematite (green-grey) is very fine-grained andreplaces bornite along grain edges (bottom center).Quartz is dark grey.



12/17


eral textures

51ye

0.25 mm

Digenite completely replaced by covellite and bornite-chalcopyrite

Only cleavage remains to suggest original digenite

Covellite, bornite, hematite, wittichenite and

arsenopyrite. English Lake District, Britain

Digenite has been totally replaced by fine-grainedcovellite which shows reflection pleochroism from darkto light blue (center). Only the cleavage of digeniteshows its former presence. Bornite (orange-brown)

and minor wittichenite (cream, top center) surroundcovellite and are rimmed by fine-grained hematite(green-grey, top left). A euhedral crystal ofarsenopyrite (white, high reflectance, center) occurs

within covellite. Dark grey areas are quartz crystals(top left). Black areas are polishing pits.


56ye

0.25 mm

Pyrite extensively replaced by galena and sphalerite:

note the original crystal shape is retained

gn

py

sl

Galena, sphalerite and pyrite. Shullsburg, Wisconsin, USA

Pyrite (yellow-white, center) as lath-shaped crystals hasbeen extensively replaced by galena (blue-white,center) and minor sphalerite (light grey, center right).This replacement is crystallographically controlled.Inclusion-free galena (center right, bottom center) isintergrown with replaced pyrite and with sphalerite.Sphalerite (light grey, bottom right and left) is

inclusion-free. Dark grey areas are carbonate (top)grains.


42yd

0.11 mm

Hematite replacing phyllosilicates along cleavage (left)Hematite and TiO2minerals. St Bees Sandstone,

Cumbria, Britain

Very fine-grained hematite lies along the fabric ofphyllosilicate grains (left). Both the green-white colorand incipient red internal reflections are characteristicof this type of fine-grained hematite. TiO2 (pink-brown) forms euhedral crystals with faint light-coloredinternal reflections (bottom right), but is present as arounded detrital grain that forms the light brown core(centre) to euhedral lanceolate hematite crystals(white, center). The original iron-titanium oxide grainis now pseudomorphed by a fine-grained intergrowthof very minor hematite (white) and TiO2 (pink-white).Other white areas are hematite.



13/17


eral textures

34ya

0.25 mm

Covellite replacing arsenopyrite

Note the characteristic shape ofthe arsenopyrite

apy

cv

Arsenopyrite and covellite. Cligga Head, Cornwall, Britain

Characteristic rhombic crystals of arsenopyrite (white,high reflectance, center) occur within quartz (lowreflectance, bottom center) and the main ganguephase, tourmaline, which shows bireflectance (greys,

center). Banded covellite (deep blue, top left) hasextensively replaced a large arsenopyrite crystal. Blackareas are vugs and polishing pits.


50ye

0.25 mm

Galena replaced by anglesite and cerussite:

a classic example of replacement caries texture

Effect of chemical composition:Often just a change in oxidation

state of cation (e.g. pyhm)

Galena, cerussite and anglesite. South Pennines, Britain

Galena (white, top) shows well developed pluckingalong (100) to give characteristic triangular pits(black). It is altered and replaced by rhythmicalaggregates of cerussite (light greys) showing faintbireflectance (bottom left) and anglesite (lowerreflectance, poorly polished bands, center right). Thisis a fine example of a caries texture. Smaller crystals

of galena are totally pseudomorphed by cerrussite andanglesite (bottom). Dark grey areas are fluorite, blackareas are polishing pits.


44ya

0.11 mm

Placer magnetite grains with alteration rims of hematite

Magnetite, ilmenite and hematite. New Zealand

A river placer containing euhedral magnetite (brown)that has a slightly deeper color than an irregular grainof ilmenite (brown, top left). The central magnetitecrystals have oxidized to hematite. Blue-whitehematite forms a rim around unaltered magnetite(center right) but also forms martite (white, centerleft) with relict magnetite (brown). Crystallographiccontrol of the hematite oxidation along (111) planesof the original magnetite is clearly seen.



14/17


eral textures

Oxidation effects in the Fe-S-O system

A common replacement

sequence in samples isfrom pyrrhotite topyrite/magnetite tohematite

This is an expected resultof oxidation

Deformation-related textures

May be seen in some minerals not normally thought tobe metamorphosed

Twinning:

Growth twins: lamellar, irregular width, uneven

distributionInversion twins: spindle-shaped, intergrown networks

throughout grain

Deformation twins: uniformly thick lamellae, associatedwith bending, cataclasis; twins often cross grainboundaries

Curvature & offset of linear features

Infill and flow of softer sulfides around harder ones

Fracturing and brecciation

18yd

0.5 mm

Molybdenite showing basal cleavage anddeformation-related kink banding Molybdenite. Jersey, Channel Islands, Britain

Coarse blades and laths of molybdenite show strongbireflectance and reflection pleochroism. The strongbasal cleavage of molybdenite (left) parallel to (0001)is clearly seen, as are deformation effects similar tokink banding (center). The dark grey area (bottom) isquartz. Four trigonal carbonate crystals showingbireflectance are lighter grey (bottom left). Black

areas are polishing pits.


41yb

0.25 mm

Deformation twins in stibnite

Uncrossed nicols

Stibnite. Unknown Provenance

Stibnite showing deformation twins (center), 'pressurelamellae', and strong bireflectance and reflectionpleochroism. Black areas are polishing pits.



15/17


eral textures

41yc

0.25 mm

Deformation twins in stibnite

Crossed nicols

Stibnite. Unknown Provenance

This is the same field of view as the previous sectionbut with crossed polars. Stibnite showing stronganisotropy along complex deformation twins and'pressure lamellae'.

Polished block, crossed polars, x 80, air.

32yd

0.11 mm

Polysynthetic twinning along (0111)twin planes of curved hematite laths Hematite. Devon, Britain

Curved laths of hematite show anisotropy andpolysynthetic twinning along (0111) twin planes.


26yb

0.25 mm

Replacement and fracture fill of bornite after pyrite

bn

py

Bornite, pyrite and chalcopyrite. Aarja, Oman

Radiating pyrite aggregates (light yellow-white, centertop) have been fractured and cemented by quartz(top center). They have been extensively replaced bybornite (brown, center), which locally is intergrownwith minor amounts of chalcopyrite (yellow, centerleft). In the cores of the original pyrite aggregates,where bornite replacement is complete, there is analmost total absence of relict pyrite. Bands of bornite(top left) between pyrite are probably fracture infillingrather than replacement. Quartz (grey) is the gangue.

Polished thin section, plane polarized light, x 80, air.


16/17


eral textures

08ya

0.25 mm

twin planes

polishing scratches

Twinning of ilmenite with exsolutionhematite Hemoilmenite. Allard Lake, Quebec, Canada

Large crystals of an ilmenite host (brown) containirregular exsolution discs of hematite (white) plus veryfine-grained hematite exsolution bodies (bottom left).Multiple twinning is present (north-south orientation,

lower reflectance, right), some of which can beconfused with parallel scratches (northwest-southeastorientation, left) in plane polarized light. Black areasare polishing pits and fractures, many of the polishing

pits are concentrated along twinning of the ilmenite.


61yc

Mobilization of sulfides into interstitialzones of silicates during metamorphism

sl

po

gn

silicate

0.25 mmSphalerite, pyrrhotite and galena. Unknown Provenance

Pyrrhotite (brown, bottom) is intergrown with sphalerite(light grey, left) and galena (white, center). A centralsilicate crystal has curved cleavage planes alongwhich galena, pyrrhotite and sphalerite havepenetrated. Dark grey areas are silicates.


Metamorphic-related textures

Most common is annealing, which results in equant

crystals with 120 interfacial angles

Metamorphism results in increased grain size,development of idioblastic or porphyroblastic textureswith zoned inclusions

Sketch of a largepyrite from Ducktown,TN showing rotatedinclusions in aporphyroblasticcrystal


17/17


61ya

0.25 mm

Annealing texture inpyrrhotite + sphalerite

po

sl

Pyrrhotite, sphalerite, chalcopyrite and galena.

Unknown Provenance

Pyrrhotite crystals (brown) show bireflectance andreflection pleochroism (light brown to darker brown,centre). They are equidimensional and have triplejunctions which suggest they have recrystallized.

Inclusion-free sphalerite (light grey, bottom) isintergrown and encloses a grain of galena (white,bottom right) and chalcopyrite (top center).


49yd

0.5 mm

Metamorphosed BIF: noteinterfacial angles Hematite. Little Broken Hill, Australia

A highly metamorphosed iron formation. Coarselycrystalline hematite (white) has totally replacedmagnetite and so is martite. The poorly polished cores(center top) show less complete replacement than thewell polished rims. The matrix comprises a mosaic ofequigranular garnet (light grey, center bottom) andquartz (dark grey) with characteristic 120 angles

between adjacent crystals. Black areas are polishingpits.


13ye

0.125 mm

Bornite, chalcopyrite and valleriite. Palabora, Republicof South Africa

Bornite (pink-brown, top) is replaced by chalcopyrite(yellow, center right) along (100). Valleriite (goldenyellow, center top and blue-green anisotropy colors,top center) forms an incomplete rim around thecopper-iron sulfides. The gangue is trigonal carbonateand shows curved deformation twins (center top) andvery faint internal reflections. Both valleriite andtrigonal carbonates are strongly anisotropic, althoughthe anisotropy of carbonates is often masked by theirstrong internal reflections.


07 textures

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