Mineragrafía texturestarea

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Geoquímica hidrotermal

Texturas minerales

Texturas de crecimiento primario - magmáticos

Texturas de crecimiento primario

Indicativo de enfriamiento de un derretimiento Minerales de alta temperatura no mostrar ninguna obstrucción de caras si texturas rápidamente enfriadas, dendríticas pueden ser presentes cristales de Poikilitic

Texturas magmáticas Texturas de espacio abierto

Texturas de reemplazo Refrigeración-relacionados con texturas

Cocrystallization = límites mutuos de diferentes ángulos (contraste a metamórficas) baja temperatura minerales llenar intersticios

Exsolution Inversión Estrés térmico

Texturas de deformación relacionados con Hermanamiento Curvatura de cristales

Recristalización metamórfica

Cromita: El gran dique de Zimbabwe

Cromita: El gran dique, Zimbabwe Se fracturaron cristales de cromita definido (gris), algunos fracturas a lo largo de una hendidura mal definida (centro inferior) y se acompañan de alteración incipiente (zonas altas de la reflectancia, centro derecha). Silicato (gris oscuro) forma la matriz de la cromita y sustituye (abajo a la derecha).

Bloque pulido, plano polarizado luz, x 160, aire.

0,125 mm 07yb

Calcopirita, cromita primaria y rutilo: sabana

rodera

Cromita, Calcopirita y rutilo. Sabana, República de Sudáfrica Definido, fracturaron cristales cúbicos de cromita (medio gris) están pegados con silicato (gris oscuro). Un solo listón de Rutilo (luz gris, centro) se incluye parcialmente en cromita y parte en silicato. Cantidades menores de Calcopirita intersticial (centro amarillo, izquierda) están presentes. Las áreas negras son pulido hoyos.

Bloque pulido, plano polarizado luz, x 40, aire.

CPY 0,5 mm 7YA Multilizer PDF Translator Free version - translation is limited to ~ 3 pages per translation.

Texturas minerales

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Geoquímica hidrotermal

Exsolution laminillas de ilmenita en magnetita

Magnetita, ilmenita, hematites y TiO minerales. 2 Guernsey, Channel Islands, Reino Unido Una Diorita contiene magnetita (luz marrón-gris, centro derecha) que tiene oxidación exsolution laminillas paralelo de ilmenita (luz color de rosa-marrón, centro) (111) de la magnetita. Las laminillas han alterado a un fino grano 2 (azul-blanco a gris claro, centro izquierda).

Anfíbol intercrecimiento de TiO minerales y hematites (abajo izquierda) muestra escote y biotita (arriba a la derecha) tiene reflejos internos de luz marrón.

Sección delgada pulida, plano polarizado luz, x 80, aire.

14yb

Inmiscibles en pentlandita, Calcopirita, Pirita y Pirrotita: Merensky Reef, Sabana

Pentlandita, Calcopirita, Pirita y Pirrotita.

Filón de Merensky. Sabana, República de Sudáfrica Pentlandita (Centro marrón, luz) está intercrecido con Calcopirita (amarilla, derecha), pirita (pálido amarillo-blanco, centro de la parte inferior) y cantidades menores de Pirrotita (lila-gris, centro derecha). Ganga de silicato (gris) muestra reflexiones internas. Las áreas negras son pulido hoyos. Los sulfuros son intersticiales a los silicatos.

Bloque pulido, plano polarizado luz, x 80, aire.

0.25 m m 11yc

Listones de ilmenita en matriz de silicato: crecimiento desenfrenado

0.05 m m

Ilmenita, espinela y aleación de hierro y níquel. Apolo 17, la muestra Lunar 7018 Un fragmento de basalto del regolito lunar. Abundante listones con ilmenita (marrón claro), muchas de las cuales son ' pluma ', se encuentran dentro de plagioclasa (áreas de reflexión interna de color claro, parte inferior derecha) que está intercrecido con piroxeno rombal definido (gris claro, algunas reflexiones internas, a la izquierda centro ). Pequeño definido equant cromita-ulvöspinel (pálido marrón, parte superior izquierda) y redondeada aleación hierro-níquel (reflectancia muy alta, blanca, abajo a la izquierda) están presentes en la plagioclasa también. El gran aumento y gran variación en reflectividad entre Monte del grano, polarizado luz, xexacta 400, aceite. las fases opacasplano dificulta la fotografía del color.

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Texturas minerales

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Geoquímica hidrotermal

Gotas de sulfuro inmiscibles en basalto, Mid-Atlantic ridge

Crecimiento primario texturas – espacio abierto

Principal característica es el crecimiento sin obstáculos de caras cristalinas, especial para cristales que raramente muestran formas de cristal Estructuras, bandas rítmicas, zonificación cristales esqueléticos y características de disolución del mineral delypeine Coloforme minerales congregados

0.15 m m

Relleno de cavidad de la matriz de Esfalerita definido anterior seguido por galena

Galena y Esfalerita. Shullsburg, Wisconsin, Estados Unidos Esfalerita (gris claro) se presenta como radiación de agregados de tamaños de grano diferentes. Muy de grano fino Esfalerita está mal pulida y muestra rojiza o de color claro reflexiones internas (superior e inferior derecha). Las bandas centrales de Esfalerita se han convertido en una cavidad y por lo tanto tienen terminaciones de cristal definido. Galena (blanco) tiene la mayoría rellenos de esta cavidad central. Las áreas negras son pulido hoyos.

Bloque pulido, plano polarizado luz, x 80, aire.

SL GN

0.25 m m 56yb

Definido, dividido en zonas Bravoita rodeado de Marcasita definido zonas

Niquelífero Marcasita y Bravoita. Oxclose mina, Peninos del sur, Gran Bretaña Definido, bravoites zonas tienen reflectancia más baja que la Marcasita zonada envolvente. Bravoita muestra mayor reflectancia corazones y reflectancia inferior zonas externas y tiene un hábito dodecaédricos pentagonal (centro izquierda). La Marcasita envolvente es de grano grueso y también muestra débil ricos en níquel zonificación (centro).

Pobre en Marcasita reflectividad y ocurre en el margen deníquel la Marcasita es unzoned,(abajo tiene auna ligeramente mayor cristales niquelífero la derecha).

Diferentes muestran Pleocroísmo de la reflexión (rojo-verde-azul a amarillo, centro inferior).

0.05 m m

Bloque pulido, plano polarizado luz, x 400, aceite.

57yf Multilizer PDF Translator Free version - translation is limited to ~ 3 pages per translation.

Texturas minerales

Hydrothermal Geochemistry

Zoning in sphalerite evidenced by weak color variations

Sphalerite 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.

0.5 mm 58yd

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 in sphalerite 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.

0.5 mm 58ye

Simple and polysynthetic twinning in marcasite

Marcasite. Ashover, South Pennines, Britain simple

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).

polysynthetic Polished block, plane polarized light, x 80, air.

0.25 mm 57yd

Mineral textures

Hydrothermal Geochemistry

Open-space growth of early native Ag followed by niccolite 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) surround niccolite. Acanthite (light grey, bottom right) has replaced native silver in the core of a niccolite dendrite. Dark grey areas are calcite showing faint bireflectance (top center). Black areas are polishing pits.

0.25 mm

Polished block, plane polarized light, x 80, air.

36yc

kamacite ~7% Ni

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 (spindleshaped lath textures)

taenite 27-65% Ni

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

Exsolution of chalcopyrite and bornite from ISS during cooling

gib100x2.gif

Bornite, chalcopyrite and altered pentlandite. Palabora, Republic of South Africa Bornite (brown) is intergrown with laths and irregularshaped areas of chalcopyrite (yellow), much of which is crystallographically oriented along (100) of the bornite. Subhedral altered pentlandite (light yellow, center) is fractured. Dark grey areas are silicate gangue. Black areas are polishing pits. This type of chalcopyrite-bornite texture can be the result of exsolution or replacement processes.

Polished block, plane polarized light, x 40, air.

0.5 mm

Note the spindle-shaped laths that taper at intersections 13ya

Mineral textures

Hydrothermal Geochemistry

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

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. Dark grey areas are silicates, black areas are polishing pits.

hm

mt 0.25 mm

Polished block, plane polarized light, x 80, air 64yf

Ilmenite exsolution from magnetite, resulting from oxidation during cooling

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 to subhedral 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).

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

mt

ilm

0.125 mm 05yd

Exsolution “stars� of sphalerite in chalcopyrite from cooling

Chalcopyrite, 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 (light grey, higher reflectance than sphalerite, center) cutting across the central sphalerite star. Black areas are polishing pits.

Polished block, plane polarized light, x 90, air.

0.22 mm 18yb

Mineral textures

Hydrothermal Geochemistry

Exsolution pentlandite in pyrrhotite [along (0001)]

Pyrrhotite and pentlandite. Kambalda, Western Australia Pyrrhotite (brown, center) carries flame-like exsolution bodies 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.

Polished block, plane polarized light, x 80, air.

0.25 mm 10yc

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

Cooling-related thermal stress in pentlandite caused cracking:note the difference between pyrrhotite and pentlandite polish

Pyrrhotite crystals (light brown) have granular pentlandite 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 of chalcopyrite (yellow, bottom left). Black areas are polishing pits.

pn

mt

po

Polished block, plane polarized light, x 80, air.

0.25 mm 12yd

Mineral textures

Pyrrhotite, magnetite, pentlandite and chalcopyrite.

Strathcona Mine, Sudbury, Ontario, Canada

Hydrothermal Geochemistry

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

cs

Chalcopyrite, bornite, chalcocite, hematite and cassiterite. Wheal Jane, Cornwall, Britain

cpy

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). Chalcocite also 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 to chalcocite.

cc cs

bn

0.25 mm

Polished block, plane polarized light, x 80, air.

31ye

Equilibrium textures – Symplectic intergrowths Wide variety of terms are applied to various textural variants of these equilibrium textures:

Lamellar, emulsoid, myrmekitic, etc.

Chalcocite-bornite symplectic intergrowth

Chalcocite, bornite and pyrite. Levant Mine, Cornwall, Britain Chalcocite (blue) has a symplectite-like intergrowth with bornite (brown, center right). Euhedral to subhedral pyrite (light yellow-white, center bottom) shows relief against chalcocite and its irregular shape suggests that it has been partially replaced by chalcocite.

cc

Polished block, plane polarized light, x 80, air. py

bn

0.25 mm 32yb

Mineral textures

Hydrothermal Geochemistry

Chalcocite-bornite symplectic intergrowth bn st gn py

tt

cc st

0.25 mm

tt

Stromeyerite, bornite, galena, chalcocite and tetrahedrite group mineral and pyrite. Unknown Provenance Inclusion-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 a symplectite-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 with euhedral quartz (dark grey, center left). Pyrite (light yellow-white, high reflectance, center) is subhedral to euhedral.

Polished block, plane polarized light, x 80, air.

35yc

Intergrown magnetite-silicate mixture

Magnetite, ilmenite and haematite. Clee Hills, Shropshire, Britain A large equant crystal of magnetite (pink-brown, left) is intergrown with, and encloses, plagioclase (dark grey, featureless). Oxidation-exsolution lamellae of ilmenite (pink-brown, lighter colored than magnetite, top centre) are present. Magnetite has extensively altered to hematite (white-blue) and minor TiO 2phases (light grey) along fractures and crystal boundaries (center bottom). Lobate ilmenite (right) is unaltered and is intergrown with plagioclase (dark grey and featureless). Pyroxene (grey, top right) is present.

0.125 mm

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

05yc

“Chalcopyrite disease� in sphalerite: Not an exsolution texture, but replacement or epitaxial growth

Sphalerite, chalcopyrite, pyrrhotite and galena. Great Gossan Lead, Virginia, USA Sphalerite (light grey, right) has chalcopyrite inclusions aligned along crystallographic directions and about grain boundaries. Hence, it shows chalcopyrite disease.

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.

Polished block, plane polarized light, x 40, air.

0.5 mm 62yc

Mineral textures

Hydrothermal Geochemistry

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 deposition Crystal structure of host mineral Chemistry 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

TiO replacing ilmenite lamellae in titanomagnetite; 2 magnetite is completely altered to limonite (brown)

TiO minerals and sphene. Central Wales, Britain 2

A metadolerite in which a trellis-like intergrowth of a TiO mineral (light grey), often called 'leucoxene', has 2 replaced ilmenite lamellae within a titanomagnetite which has been completely removed and is represented by iron-stained non-opaque minerals showing brown internal reflections. The original titanomagnetite aggregate can be seen to have comprised two crystals. Sphene (light grey, center right) is present. The matrix is silicate.

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

0.125 mm 60yb

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 (bluewhite, 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.

Polished block, plane polarized light, x 40, air.

0.5 mm 49ye

Mineral textures

Hydrothermal Geochemistry

Supergene replacement; pyrrhotite altering to marcasite along (0001); pentlandite altering to violarite

Pyrrhotite, violarite and altered pyrrhotite. Kambalda, Australia The primary ore was pyrrhotite and pentlandite but these minerals have suffered extensive supergene alteration. Relict pyrrhotite (brown, well polished, left, center right) has an alteration rim of zwischenprodukt (light brown-white, highest reflectance, right bottom) and clearly shows that the alteration of pyrrhotite is crystallographically controlled along (0001). Pentlandite has been totally pseudomorphed by violarite (brown-white, finely pitted surface, center) but its cleavage and crystal boundaries have been preserved. Silicates are black.

pn vi

po 0.25 mm

Crystal structure often controls replacement

Polished block, plane polarized light, x 80, air.

50ya

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

cpy

po

cb

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 within chalcopyrite 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.

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

0.125 mm 12yc

Digenite replaced by covellite along fractures and more extensively replaced by bornite and chalcopyrite

Digenite (blue, top left) shows minor replacement by covellite (deep blue) along cleavage and small fractures. More extensive replacement is shown by bornite (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.

Polished block, plane polarized light, x 40, air.

di bn

0.5 mm 51ya

Mineral textures

Digenite, bornite, haematite and chalcopyrite. English Lake District, Britain

Hydrothermal Geochemistry

0.25 mm

Covellite, bornite, hematite, wittichenite and arsenopyrite. 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 digenite shows 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 of arsenopyrite (white, high reflectance, center) occurs within covellite. Dark grey areas are quartz crystals (top left). Black areas are polishing pits.

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

Polished block, plane polarized light, x 80, air.

51ye

Pyrite extensively replaced by galena and sphalerite: note the original crystal shape is retained

Galena, sphalerite and pyrite. Shullsburg, Wisconsin, USA

0.25 mm py gn

sl

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) is intergrown 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.

56ye

Hematite replacing phyllosilicates along cleavage (left)

Hematite and TiO minerals. St Bees Sandstone, 2 Cumbria, Britain Very fine-grained hematite lies along the fabric of phyllosilicate grains (left). Both the green-white color and incipient red internal reflections are characteristic of this type of fine-grained hematite. TiO2(pinkbrown) 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 2 of verywhite minorareas hematite (white) and TiO (pink-white).

Other are hematite.

0.11 mm

Grain mount, plane polarized light, x 180, oil.

42yd

Mineral textures

Hydrothermal Geochemistry

cv

Arsenopyrite 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.

apy

Polished block, plane polarized light, x 80, air.

0.25 mm

Covellite replacing arsenopyrite Note the characteristic shape of the arsenopyrite 34ya

Galena replaced by anglesite and cerussite: a classic example of replacement “caries texture�

0.25 mm

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.

Effect of chemical composition: Often just a change in oxidation state of cation (e.g. py„hm)

Polished block, plane polarized light, x 80, air.

50ye

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.

Grain mount, plane polarized light, x 180, oil.

0.11 mm 44ya

Mineral textures

Hydrothermal Geochemistry

Deformation-related textures

Oxidation effects in the Fe-S-O system

May be seen in some minerals not normally thought to be metamorphosed Twinning: 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

A common replacement sequence in samples is from pyrrhotite to pyrite/magnetite to hematite This is an expected result of oxidation

Curvature & offset of linear features Infill and flow of softer sulfides around harder ones Fracturing and brecciation

Molybdenite showing basal cleavage and deformation-related kink banding

Molybdenite. Jersey, Channel Islands, Britain Coarse blades and laths of molybdenite show strong bireflectance 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 showing bireflectance are lighter grey (bottom left). Black areas are polishing pits.

Polished block, plane polarized light, x 40, air.

0.5 mm 18yd

Deformation twins in stibnite

Stibnite. Unknown Provenance Stibnite showing deformation twins (center), 'pressure lamellae', and strong bireflectance and reflection pleochroism. Black areas are polishing pits.

Polished block, plane polarized light, x 80, air.

0.25 mm

Uncrossed nicols 41yb

Mineral textures

Hydrothermal Geochemistry

Deformation twins in stibnite

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.

0.25 mm

Crossed nicols 41yc

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.

Polished block, plane polarized light, x 180, air.

0.11 mm 32yd

Replacement and fracture fill of bornite after pyrite

Bornite, pyrite and chalcopyrite. Aarja, Oman py

Radiating pyrite aggregates (light yellow-white, center top) have been fractured and cemented by quartz (top center). They have been extensively replaced by bornite (brown, center), which locally is intergrown with 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.

bn

0.25 mm

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

26yb

Mineral textures

Hydrothermal Geochemistry

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.

twin planes

polishing scratches

Polished block, plane polarized light, x 80, air.

0.25 mm 08ya

Mobilization of sulfides into interstitial zones of silicates during metamorphism

0.25 mm

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.

silicate

gn

Sphalerite, pyrrhotite and galena. Unknown Provenance

Polished block, plane polarized light, x 80, air. sl

po 61yc

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 textures with zoned inclusions

Mineral textures

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

Hydrothermal Geochemistry

Annealing texture in pyrrhotite + sphalerite

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) is intergrown and encloses a grain of galena (white, bottom right) and chalcopyrite (top center).

po

sl

Polished block, plane polarized light, x 80, air.

0.25 mm 61ya

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 of equigranular garnet (light grey, center bottom) and quartz (dark grey) with characteristic 120° angles between adjacent crystals. Black areas are polishing pits.

Polished block, plane polarized light, x 40, air.

0.5 mm 49yd

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 and trigonal carbonates are strongly anisotropic, although the anisotropy of carbonates is often masked by their strong internal reflections.

0.125 mm

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

13ye

Mineral textures