Actinolite
Ca2(Mg, Fe+2)5Si8O22(OH)2
 |  | |
| Photomicrograph of actinolite in a groundmass of Mg-rich chlorite in PPL. | Photomicrograph of actinolite in a groundmass of Mg-rich chlorite in x-nicols. The photo shows the upper first-order to mid second-order interference colours of actinolite. | |
Actinolite is an amphibole with moderately high relief, symmetrical extinction, and typical amphibole cleavage.
| Properties | |||
| Light Behavior | anisotropic-biaxial | Extinction | basal sections-symmetrical, sections parallel to (100)-parallel |
| Colour/Pleochroism | colourless, sometimes pale to dark green/coloured varieties display pleochroism: colourless to shades of green | Interference Colours (birefringence) | upper first-order to mid second-order |
| Relief | moderate to high positive | Optic Sign | negative |
| Cleavage | two intersections at 56° and 124° (on {110}) | 2V | 75°-88° |
| Crystal Form | columnar, bladed, or acicular crystals, elongate parallel to the c-axis, basal sections (cleavage visible) are diamond shaped.0 | Alteration Products | chlorite, talc, carbonates |
| Twinning | simple and lamellar, {100} composition plane | | |
Andalusite
Al2SiO5
 |  | |
| Photomicrograph of andalusite in plane polarized light. Andalusite is colourless in ppl. | Photomicrograph of andalusite in crossed polars. The distinct chiastolite cross that is characteristic of andalusite is easily seen in this section. | |
Andalusite is an orthosilicate with high relief, and parallel extinction, with two good cleavages.
| Properties | |||
| Light Behavior | anisotropic-biaxial | Extinction | parallel |
| Colour/Pleochroism | colourless/none (sometimes weak pleochroism visible) | Interference Colours (birefringence) | first-order white or gray |
| Relief | moderately high positive | Optic Sign | negative |
| Cleavage | two at 90° | 2V | 71°-88° |
| Crystal Form | crystals are elongate prisms, with square x-sections (chiastolite-dark cross-shaped inclusions of granite along diagonals) | Alteration Products | sericite, chlorite |
| Twinning | rare | | |
Apatite
Ca5(PO4)3(F, Cl, OH)
 | ||
| Photomicrograph in PPL showing large apatite end section (indicated by arrows). Note: hexagonal shape. Green phenocryst is hornblende. Width of view is 0.85mm. | | |
Apatite is almost always present in most igenous and metamorphic rocks, but in small amounts. Look closely at feldspar crystals on medium to high power to find apatite. The only common mineral that closely resembles it is dahllite.
| Properties | |||
| Light Behavior | - | Extinction | Parallel |
| Colour/Pleochroism | Colourless | Interference Colours (birefringence) | Basal sections are commonly too small to give good interference figures. |
| Relief | Moderate, n > basal | Optic Sign | Negative |
| Cleavage | Imperfect basal {0001} shown as cross fractures. Larger crystals may show imperfect cleavage parallel to the length. | 2V | none |
| Crystal Form | Commonly found as minute hexagonal prismatic crystals common and widely distributed, but commonly occurs in small amounts. | Alteration Products | none |
| Twinning | none | | |
Augite
(Ca,Mg,)2Si2O6
 |  | |
| Photomicrograph of augite in ppl. This example is a basal x-section on which two cleavage directions are visible. The width of the field of view is 1.35mm. | Photomicrograph of augite in x-nicols showing low to mid second-order interference colours, and simple twinning. The width of the field of view is 1.35mm. | |
Augite is a clinopyroxene that displays simple twinning, high relief, birefringence higher than that of enstatite, and two cleavage directions intersecting at approx. 90°.
| Properties | |||
| Light Behavior | anisotropic-biaxial | Extinction | basal sections-symmetrical, longitudinal sections-parallel |
| Colour/Pleochroism | colourless, gray, pale green, or pale brown. If Fe-rich weak pale brown/green to pale yellow. If Ti-rich, pale purple | Interference Colours (birefringence) | low to mid second-order or below |
| Relief | high positive | Optic Sign | neg. or pos. |
| Cleavage | two at 87° and 93°. | 2V | 25°-70° |
| Crystal Form | crystals are stubby prisms, elongate along the c-axis, basal x-sections are 4 or 8-sided (2 cleavages visible) | Alteration Products | uralite, serpentine, chlorite, biotite, carbonates |
| Twinning | simple and lamellar | | |
Biotite
K(Mg,Fe)3AlSi3O10(OH,O,F)2
 |  | |
| Photomicrograph of biotite in ppl showing the characteristic red-brown colour, and one cleavage direction. Also note that the grains that are oriented differently have a different colour, giving some indication of pleochroism. The width of the field of view is 5.5mm. | Photomicrograph of biotite in x-nicols. Some of the crystals show interference colours (although they are often masked by the absorption colours of the mineral). The width of the field of view is 5.5mm. | |
Biotite is a phyllosilicate that displays pleochroism, parallel extinction, and one perfect cleavage direction.
| Properties | |||
| Light Behavior | anisotropic-biaxial | Extinction | parallel |
| Colour/Pleochroism | brown, brownish-green, reddish-brown/light tan or pink to shades of brown and green; exhibits | Interference Colours (birefringence) | third or fourth-order (may be masked by its colour) |
| Relief | moderately high positive, low if Mg-rich | Optic Sign | negative |
| Cleavage | one perfect cleavage | 2V | 0°-25° |
| Crystal Form | euhedral, tabular crystals parallel to {001}, with a roughly hexagonal x-section, sometimes micaceous grains with irregular form. | Alteration Products | chlorite, sometimes clay minerals, magnetite |
| Twinning | rarely visible in thin section | | |
Calcite
CaCO3
 |  | |
| Photomicrograph of calcite in ppl showing its lack of colour in plane polarized light. Scanned thin section of marble. | Photomicrograph of calcite in x-nicols showing upper-order interference colours, and lamellar twinning. Scanned thin section of marble. | |
Calcite is a carbonate which displays high birefringence, a change in relief with rotation, polysynthetic twin lamellae (common), and rhombohedral cleavage.
| Properties | |||
| Light Behavior | anisotropic-uniaxial | Extinction | inclined or symmetrical |
| Colour/Pleochroism | white or colourless/none | Interference Colours (birefringence) | creamy high-order colours. |
| Relief | moderate to high negative-changes with rotation | Optic Sign | negative |
| Cleavage | three rhombohedral | 2V | 15° |
| Crystal Form | crystals are rhombohedral, although they are usually anhedral | Alteration Products | dolomite ? |
| Twinning | lamellar-parallel or oblique to long diagonal or parallel to the edges of the cleavage rhombs | | |
Chlorite
(Mg,Al,Fe)3(SiAl)4O10(OH)2(Mg,Al,Fe)3(OH)6
| | | |
| Photomicrograph of actinolite in a groundmass of Mg-rich chlorite in PPL. | Photomicrograph of actinolite in a groundmass of Mg-rich chlorite in x-nicols. The photo shows the upper first-order to mid second-order interference colours of actinolite. | |
Chlorite is a phyllosilicate which is colourless to green, may display pleochroism, with a low birefringence.
| Properties | |||
| Light Behavior | anisotropic-biaxial | Extinction | parallel (maximum of 9°) |
| Colour/Pleochroism | light to medium green/colourless to shades of green | Interference Colours (birefringence) | first-order white or yellow; anomalous purple or brown colours possible |
| Relief | low to moderate | Optic Sign | negative or positive |
| Cleavage | one perfect on {001} | 2V | 0°-40°/0°-60° |
| Crystal Form | crystals are commonly platy (as in micas), they are rarely tabular parallel to (001) | Alteration Products | fairly resistant, some clay-minerals, iron-oxides |
| Twinning | composition planes on {001} are difficult to recognize | | |
Cordierite
Mg2Al3(AlSi5)O18
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| Photomicrograph of cordierite in PPL showing its lack of colour. Also note the clouding pinnate which indicates that it has been altered. | Photomicrograph of cordierite in x-nicols showing its low interference colours, and cyclic twinning. | |
Cordierite is a tectosilicate that displays low relief, low birefringence, and parallel extinction. The cyclic twinning is distinctive. It is found in Al-rich metamorphic rocks.
| Properties | |||
| Light Behavior | anisotropic-biaxial | Extinction | parallel |
| Colour/Pleochroism | colourless/not common | Interference Colours (birefringence) | first-order white or gray |
| Relief | low negative or low positive | Optic Sign | negative or positive |
| Cleavage | 1 fair on {010}, 2 poor on {100} and {001}, (not usually obvious in thin section) | 2V | 40°-90° |
| Crystal Form | commonly as anhedral grains or irregular porphyroblastic grains, less commonly as euhedral hexagonal prismatic crystals | Alteration Products | pinite (an aggregate of chlorite, muscovite, other silicates) |
| Twinning | lamellar and cyclic | | |
Dolomite
CaMg(CO3)2
 | |
| Zoned feorran dolomite in thin section, PPL, showing typical dolomite rhombs. ? Hard ground surface. | |
Dolomite is a carbonate that displays high birefringence, a change in relief with rotation, twin lamellae, rhombohedral cleavage, and is usually euhedral. It is common in sedimentary rocks.
| Properties | |||
| Light Behavior | anisotropic-uniaxial | Extinction | inclined or symmetrical |
| Colour/Pleochroism | colourless, may have brownish colour/none | Interference Colours (birefringence) | creamy high-order white or gray |
| Relief | low negative to high positive-changes with rotation | Optic Sign | negative |
| Cleavage | perfect-rhombohedral | 2V | none |
| Crystal Form | crystals are rhombohedral, and they are usually euhedral | Alteration Products | may be pseudomorphically replaced by other carbonates |
| Twinning | lamellar-parallel to long or short diagonal | | |
Enstatite
(Mg,Fe)2Si2O6
Enstatite is an orthopyroxene that has low birefringence, pale pink to green pleochroism, and two cleavage directions intersecting at approx. 90°.
| Properties | |||
| Light Behavior | anisotropic-biaxial | Extinction | basal sections-symmetrical, longitudinal sections-parallel |
| Colour/Pleochroism | pale coloured/subtle pinkish to greenish | Interference Colours (birefringence) | first-order yellow or below |
| Relief | moderately high to high positive | Optic Sign | positive |
| Cleavage | two at 87° and 93° | 2V | 50°-132° |
| Crystal Form | crystals are stubby prisms, basal x-sections are 4 or 8-sided (2 cleavages visible), longitudinal x-sections are rectangular. | Alteration Products | serpentine or talc |
| Twinning | none (may have a lamellar structure that resembles twins) | | |
Epidote
Ca2Fe3+Al2O(Si2O7)(SiO4)(OH)
Epidote is a sorosilicate which displays high relief, and weak pleochroism (pale yellow and pale green)
| Properties | |||
| Light Behavior | anisotropic-biaxial | Extinction | variable |
| Colour/Pleochroism | light yellow-green/colourless to pale yellow or pale green | Interference Colours (birefringence) | upper first-order to third-order; anomalous berlin-blue |
| Relief | high positive | Optic Sign | negative |
| Cleavage | one perfect (basal), 1 poor (rarely seen) | 2V | 64°-90° |
| Crystal Form | anhedral grains or granular aggregates, crystals are columnar, bladed, or sometimes fibrous, elongate parallel to b-axis | Alteration Products | none |
| Twinning | lamellar on {100} may be found | | |
Garnet
X3Y2(SiO4)3
 |  | |
| Photomicrograph showing chloritoid and garnet in plane polarized light. Both minerals display high positive relief. Chloritoid is light green and pleochroic in ppl whereas garnet is commonly colourless or a light version of the hand sample colour. | Photomicrograph of chloritoid and garnet in crossed polars. Garnet is isotropic and is therefore black. Chloritoid has a good cleavage and often displays simple and lamellar twins. Birefringence is first order yellow colours. | |
Garnet is an orthosilicate that is isotropic, has high positive relief, and is usually six or eight-sided in cross-section (in thin section). It is commonly found in metamorphic rocks, and it may contain lots of inclusions.
| Properties | |||
| Light Behavior | isotropic | Extinction | n/a |
| Colour/Pleochroism | colourless to pale reddish-brown/none | Interference Colours (birefringence) | none (black) |
| Relief | high positive | Optic Sign | n/a |
| Cleavage | none | 2V | none |
| Crystal Form | dodecahedral or trapezohedral crystals, which give six or eight-sided x-sections in thin section | Alteration Products | chlorite, occasionally hornblende, epidote Fe-oxides |
| Twinning | none | | |
Hornblende
(Na,K)0-1Ca2(Mg,Fe2+,Fe3+,Al)5(Si,Al)8O22(OH)2
 |  | |
| Photomicrograph of hornblende in ppl showing the characteristic amphibole cleavage at 56 and 124 degrees. Also note that the colour is different for differently oriented crystals, giving some indication of pleochroism. The black rims are "opacite", a fine grained mixture of magnetite and pyroxene formed from dehydration and reaction with the magma in volcanic rocks. The width of the field of view is 1.35mm. | Photomicrograph of hornblende in x-nicols showing the interference colours (up to second-order), and the characteristic crystal form (diamond-shaped x-section). The width of the field of view is 1.35mm. | |
Hornblende is the most common amphibole. It displays pleochroism, inclined extinction, and two cleavage directions intersecting at 56° and 124°.
| Properties | |||
| Light Behavior | anisotropic-biaxial | Extinction | inclined (between 14° and 25°) |
| Colour/Pleochroism | green, yellow-green, blue-green, or brown/shades of green, yellow, and brown | Interference Colours (birefringence) | upper first-order to lower second-order |
| Relief | moderate to high positive | Optic Sign | negative or positive |
| Cleavage | two at 56° and 124° | 2V | 23°-130° |
| Crystal Form | prismatic to bladed crystals, with a diamond-shaped x-section (showing cleavage) | Alteration Products | biotite or chlorite, magnetite, "opacite" rims. |
| Twinning | simple and lamellar | | |
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