mineral textures

Hydrothermal Geochemistry

Mineral textures

Mineral textures

Primary growth texturesMagmatic texturesOpen-space textures

Replacement texturesCooling-related textures

ExsolutionInversionThermal stress

Deformation-related texturesTwinningCurvature of crystals

Metamorphic-related recrystallization

Primary growth textures - Magmatic

Indicative of cooling from a meltHigh-temperature minerals show no obstruction of facesIf rapidly cooled, dendritic textures may be present

Poikilitic crystals Cocrystallization = mutual boundaries of different angles (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, some fractures are along a poorly defined cleavage (bottom center) 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 of South Africa

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



Mineral textures

14yb

Exsolution lamellae of ilmenite in magnetiteMagnetite, ilmenite, TiO2 minerals 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) of the 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 and pyrrhotite: 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) shows internal 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 which are '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, top left) and rounded iron-nickel alloy (white, very high reflectance, bottom left) are present in the plagioclase also. The high magnification and large variation in reflectance between the opaque phases makes accurate color photography difficult.

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


Mineral textures

Immiscible sulfide droplet in basalt, mid-Atlantic ridge

0.15 mm

Primary growth textures Open space

Main characteristic is unimpeded growth of crystal faces, particularly for crystals that rarely exhibit crystal forms

Comb structures, rhythmic banding, mineral zoningDissolution features and skeletal crystalsColloform and banded ores

56yb

0.25 mm

Vug filling of earlier euhedral sphalerite followed by galena

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sl

Galena and sphalerite. Shullsburg, Wisconsin, USA

Sphalerite (light grey) occurs as radiating aggregates of different grain sizes. Very fine-grained sphalerite is poorly 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 than the enclosing zoned marcasite. Bravoite shows higher reflectance cores and lower reflectance outer zones and has a pentagonal dodecahedral habit (center left). The enclosing marcasite is coarse-grained and also shows faint nickel-rich zoning (center). Nickel-poormarcasite is unzoned, has a slightly higher reflectance, and occurs on the margin of thenickeliferous marcasite (bottom right). Different crystals show reflection pleochroism (green-blue to yellow, bottom center).

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


Mineral 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 within zoned sphalerite. Zoning in the sphalerite is just visible as blue-grey (centre) and brown-grey (bottom right) 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, North

Pennines, Britain

This is the same field of view as 58d but in transmitted light. The fine scale of the growth banding insphalerite is very clear. In thin section or polished thin section, much of the fine detail would be lost. The intensity of the colours are due to variations in the trace element content of the growth bands, most importantly 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 their extreme anisotropy, variation in grain size, and twinning along (101) as coarse single twins (top left) and as polysynthetic twinning (center left).



Mineral textures

36yc

0.25 mm

Open-space growth of early native Ag followed byniccolite and thin maucherite rims Niccolite, native silver, acanthite and maucherite. Great Bear Lake, Canada

Native silver (white, scratched, center left) forms the cores to botryoidal niccolite (pink-brown) showing faint 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) has replaced native silver in the core of a niccolitedendrite. Dark grey areas are calcite showing faint bireflectance (top center). Black areas are polishing pits.


Cooling textures Exsolution

Separation of structurally-incompatible phases as T decreases, often in a characteristic pattern controlled by 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 laths that 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 which is crystallographically oriented along (100) of thebornite. Subhedral altered pentlandite (light yellow, center) is fractured. Dark grey areas are silicate gangue. Black areas are polishing pits. This type ofchalcopyrite-bornite texture can be the result ofexsolution or replacement processes.



Mineral textures

64yf

0.25 mm

Twinning of hematite: bireflectance of hematite makes this 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 above but with partially crossed polars. Hematite crystals show polysynthetic twinning along (1011). The silicate gangue shows light-colored internal reflections.

Hematite (white, right) is coarse-grained and shows very faint bireflectance along (1011) twin planes, which are oriented north-south but difficult to see in plane polarized light. Magnetite (pink-brown, bottom left) 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) carries abundant ilmenite lamellae (darker brown) oriented along (111) and the result of oxidation-exsolution. An incomplete magnetite rim around the euhedral crystal also carries exsolved ilmenite (center right). Very small grains of tarnished bornite (red-brown, center right) have replaced original chalcopyrite. Euhedral tosubhedral pyroxene (light grey, left) and plagioclase (dark grey, light internal reflections, bottom right) are the main silicate phases. Minor amounts of relict carbon-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 (light yellow, higher reflectance, top right) growing into a void (black). A very thin veinlet that is only just discernible (running north-south, centre) is marked by elongated polishing pits (black) and by stannite (lightgrey, higher reflectance than sphalerite, center) cutting across the central sphalerite star. Black areas are polishing pits.



Mineral 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 are associated with fractures in the pyrrhotite and are oriented along its (0001) plane. Black areas are silicates and polishing pits.


Additional cooling-related textures

Inversion: difficult to recognize, sometimes by twinning or pseudomorphs

Thermal stress:Common in pentlandite because it has a different thermal

expansion coefficient than pyrite or pyrrhotite

12yd

0.25 mm

Cooling-related thermal stress in pentlandite caused 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 free of flame-like exsolution bodies of pentlandite.

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



Mineral textures

31ye

0.25 mm

cs

cpy

bn

cc

Cooling and contraction of HT cassiterite with subsequent infilling by Cu minerals

csChalcopyrite, bornite, chalcocite, hematite and

cassiterite. Wheal Jane, Cornwall, Britain

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


Equilibrium textures Symplectic intergrowths

Wide variety of terms are applied to various textural variants 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 relief against chalcocite and its irregular shape suggests that it has been partially replaced by chalcocite.



Mineral 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

ProvenanceInclusion-free galena (white, center right) is intergrown

with bornite (brown, top) and stromeyerite, showing purple-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 the section 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 (light yellow-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 altered to 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 and featureless). Pyroxene (grey, top right) is present.


62yc

0.5 mm

Chalcopyrite disease in sphalerite:

Not an exsolution texture, but replacement or

epitaxial growth

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

Sphalerite (light grey, right) has chalcopyrite inclusions aligned along crystallographic directions and about grain boundaries. Hence, it shows chalcopyritedisease. It is rimmed by chalcopyrite (yellow, center) and pyrrhotite (brown, top left), together with minor galena (white, center bottom). Dark grey area is silicate, black areas are polishing pits.



Mineral textures

Replacement textures

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

Replacement of other minerals is dependent on:Presence of crystal surfaces for depositionCrystal structure of host mineralChemistry of fluid and host mineral

Replacement is often visible as a different mineral along crystal surfaces, cracks, cleavages, etc. that allow fluid entry

Compositionally-zoned minerals may exhibit selective replacement

Replacement of wood by pyrite, preserving the cellular structure

60yb

0.125 mm

TiO2 replacing ilmenite lamellae in titanomagnetite;magnetite is completely altered to limonite (brown) TiO2 minerals and sphene. Central Wales, Britain

A metadolerite in which a trellis-like intergrowth of a TiO2 mineral (light grey), often called 'leucoxene', has replaced ilmenite lamellae within a titanomagnetitewhich has been completely removed and is represented by iron-stained non-opaque minerals showing brown internal reflections. The originaltitanomagnetite aggregate can be seen to have comprised two crystals. Sphene (light grey, center right) 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, Northern

Territories, Australia

A pisolitic bauxite in which an angular fragment of an earlier pisolith has been extensively hematitized (blue-white, center). Hematite is the only mineral that can be identified by reflected light microscopy in this section. The matrix, which is light red-brown due to finely disseminated iron minerals, comprises gibbsite and boehmite which were identified by X-ray diffraction.



Mineral 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 but these 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, right bottom) 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 been preserved. 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 developed cleavage which extends into chalcopyrite (yellow, top center and left) and cubanite (blue-grey, center right) areas, suggesting that chalcopyrite and cubanite are replacing 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 of pyrrhotite. 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. English Lake District, Britain

Digenite (blue, top left) shows minor replacement by covellite (deep blue) along cleavage and small fractures. More extensive replacement is shown bybornite (brown-pink, center) which contains relict digenite. Minor amounts of chalcopyrite (yellow, right center) occur on the edge of bornite but are difficult to see. Two distinct generations of hematite are present. Hematite laths (light blue, hard, center bottom) occur within digenite and bornite, whereas most hematite (green-grey) is very fine-grained and replaces bornite along grain edges (bottom center). Quartz is dark grey.



Mineral textures

51ye

0.25 mm

Digenite completely replaced by covellite and bornite-chalcopyriteOnly cleavage remains to suggest original digenite

Covellite, bornite, hematite, wittichenite andarsenopyrite. English Lake District, Britain

Digenite has been totally replaced by fine-grained covellite which shows reflection pleochroism from dark to light blue (center). Only the cleavage of digeniteshows its former presence. Bornite (orange-brown) and minor wittichenite (cream, top center) surround covellite 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

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py

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Galena, sphalerite and pyrite. Shullsburg, Wisconsin, USA

Pyrite (yellow-white, center) as lath-shaped crystals has been 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 TiO2 minerals. St Bees Sandstone,

Cumbria, Britain

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



Mineral textures

34ya

0.25 mm

Covellite replacing arsenopyriteNote the characteristic shape of

the arsenopyrite

apy

cvArsenopyrite and covellite. Cligga Head, Cornwall, Britain

Characteristic rhombic crystals of arsenopyrite (white, high reflectance, center) occur within quartz (low reflectance, bottom center) and the main gangue phase, tourmaline, which shows bireflectance (greys, center). Banded covellite (deep blue, top left) has extensively replaced a large arsenopyrite crystal. Black areas 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 plucking along (100) to give characteristic triangular pits (black). It is altered and replaced by rhythmical aggregates of cerussite (light greys) showing faint bireflectance (bottom left) and anglesite (lower reflectance, poorly polished bands, center right). This is a fine example of a caries texture. Smaller crystals of galena are totally pseudomorphed by cerrussite and anglesite (bottom). Dark grey areas are fluorite, black areas 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 grain of ilmenite (brown, top left). The central magnetite crystals have oxidized to hematite. Blue-white hematite forms a rim around unaltered magnetite (center right) but also forms martite (white, center left) with relict magnetite (brown). Crystallographic control of the hematite oxidation along (111) planes of the original magnetite is clearly seen.



Mineral textures

Oxidation effects in the Fe-S-O system

A common replacement sequence in samples is from pyrrhotite to pyrite/magnetite to hematite

This is an expected result of oxidation

Deformation-related texturesMay be seen in some minerals not normally thought to

be metamorphosedTwinning:

Growth twins: lamellar, irregular width, uneven distribution

Inversion twins: spindle-shaped, intergrown networks throughout grain

Deformation twins: uniformly thick lamellae, associated with bending, cataclasis; twins often cross grain boundaries

Curvature & offset of linear featuresInfill and flow of softer sulfides around harder onesFracturing and brecciation

18yd

0.5 mm

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

Coarse blades and laths of molybdenite show strongbireflectance and reflection pleochroism. The strong basal cleavage of molybdenite (left) parallel to (0001) is clearly seen, as are deformation effects similar to kink banding (center). The dark grey area (bottom) is quartz. 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), 'pressure lamellae', and strong bireflectance and reflectionpleochroism. Black areas are polishing pits.



Mineral textures

41yc

0.25 mm

Deformation twins in stibnite

Crossed nicols

Stibnite. Unknown Provenance

This is the same field of view as the previous section but with crossed polars. Stibnite showing strong anisotropy 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 and polysynthetic 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, center top) 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, center left). In the cores of the original pyrite aggregates, where bornite replacement is complete, there is an almost total absence of relict pyrite. Bands of bornite(top left) between pyrite are probably fracture infilling rather than replacement. Quartz (grey) is the gangue.

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


Mineral textures

08ya

0.25 mm

twin planes

polishing scratches

Twinning of ilmenite with exsolution hematite Hemoilmenite. Allard Lake, Quebec, Canada

Large crystals of an ilmenite host (brown) contain irregular exsolution discs of hematite (white) plus very fine-grained hematite exsolution bodies (bottom left). Multiple twinning is present (north-south orientation, lower reflectance, right), some of which can be confused with parallel scratches (northwest-southeast orientation, left) in plane polarized light. Black areas are polishing pits and fractures, many of the polishing pits are concentrated along twinning of the ilmenite.


61yc

Mobilization of sulfides into interstitial zones of silicates during metamorphism

sl

po

gn

silicate

0.25 mm Sphalerite, pyrrhotite and galena. Unknown Provenance

Pyrrhotite (brown, bottom) is intergrown with sphalerite(light grey, left) and galena (white, center). A central silicate crystal has curved cleavage planes along which galena, pyrrhotite and sphalerite have penetrated. Dark grey areas are silicates.


Metamorphic-related texturesMost common is annealing, which results in equant

crystals with 120 interfacial anglesMetamorphism results in increased grain size,

development of idioblastic or porphyroblastic textures with zoned inclusions

Sketch of a large pyrite from Ducktown, TN showing rotated inclusions in aporphyroblasticcrystal


Mineral textures

61ya

0.25 mm

Annealing texture in pyrrhotite + sphalerite

po

sl

Pyrrhotite, sphalerite, chalcopyrite and galena. Unknown Provenance

Pyrrhotite crystals (brown) show bireflectance and reflection pleochroism (light brown to darker brown,centre). They are equidimensional and have triple junctions 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: note interfacial angles Hematite. Little Broken Hill, Australia

A highly metamorphosed iron formation. Coarsely crystalline hematite (white) has totally replaced magnetite and so is martite. The poorly polished cores (center top) show less complete replacement than the well polished rims. The matrix comprises a mosaic ofequigranular garnet (light grey, center bottom) and quartz (dark grey) with characteristic 120 angles between adjacent crystals. Black areas are polishing pits.


13ye

0.125 mm

Bornite, chalcopyrite and valleriite. Palabora, Republic of South Africa

Bornite (pink-brown, top) is replaced by chalcopyrite(yellow, center right) along (100). Valleriite (golden yellow, center top and blue-green anisotropy colors, top center) forms an incomplete rim around the copper-iron sulfides. The gangue is trigonal carbonate and shows curved deformation twins (center top) and very faint internal reflections. Both valleriite andtrigonal carbonates are strongly anisotropic, although the anisotropy of carbonates is often masked by their strong internal reflections.


mineral textures

Documents

left center

magnetite light browngrey

plane polarized light

center parallel

silicate dark grey

silicate gangue grey

polished block

silicate matrix