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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.
Polished block, plane polarized light, x 40, air.
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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.
Polished block, plane polarized light, x 80, air.
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.
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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.
Polished block, plane polarized light, x 80, air.
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.
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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).
Polished block, plane polarized light, x 80, air.
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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.
Polished block, plane polarized light, x 80, air.
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.
Polished block, plane polarized light, x 40, air.
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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).
Polished block, plane polarized light, x 160, oil.
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.
Polished block, plane polarized light, x 90, air.
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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.
Polished block, plane polarized light, x 80, air.
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.
Polished block, plane polarized light, x 80, air.
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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.
Polished block, plane polarized light, x 80, air.
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.
Polished block, plane polarized light, x 80, air.
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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.
Polished block, plane polarized light, x 80, air.
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.
Polished block, plane polarized light, x 160, oil.
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.
Polished block, plane polarized light, x 40, air.
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drothermal Geochemistry
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.
Polished block, plane polarized light, x 160, oil.
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.
Polished block, plane polarized light, x 40, air.
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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.
Polished block, plane polarized light, x 80, air.
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.
Polished block, plane polarized light, x 160, oil.
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.
Polished block, plane polarized light, x 40, air.
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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.
Polished block, plane polarized light, x 80, air.
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.
Polished block, plane polarized light, x 80, air.
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.
Grain mount, plane polarized light, x 180, oil.
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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.
Polished block, plane polarized light, x 80, air.
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.
Polished block, plane polarized light, x 80, air.
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.
Grain mount, plane polarized light, x 180, oil.
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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.
Polished block, plane polarized light, x 40, air.
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.
Polished block, plane polarized light, x 80, air.
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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.
Polished block, plane polarized light, x 180, air.
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.
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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.
Polished block, plane polarized light, x 80, air.
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.
Polished block, plane polarized light, x 80, air.
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
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8/3/2019 07 Textures
17/17
drothermal Geochemistry
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).
Polished block, plane polarized light, x 80, air.
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.
Polished block, plane polarized light, x 40, air.
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.
Polished block, plane polarized light, x 160, air.