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Chlorite
A group of sheet silicates, like micas but with no K or Na. The general formula is A4-6Z4O10(OH,O)8. A=Al, Fe2+, Fe3+, Li, Mg, Mn, Ni, Zn. Z=Al, Fe3+, Si.
By far the commonest are Fe-Mg chlorites clinochlore and chamosite (don't worry about the species names). They are the stable Fe-Mg silicates at low temperatures, and hence form during low-grade metamorphism or alteration of other Fe-Mg silicates like biotite, pyroxenes, amphiboles, garnets etc.
Associated minerals are quartz, calcite, pyrite, muscovite, and sometimes biotite or garnet. Chlorites are normally pretty easy to identify:
A) Hand Specimens
Coarsely crystalline chlorites look like dark green micas. Like micas, they have a perfect cleavage, are soft, and can be easily bent. Flakes tend to be less springy and elastic than mica. When fine-grained this may not be obvious, but the green colour, together with the low hardness (< 2.5) are distinctive. Many low-grade metamorphic rocks get their green colour from chlorite - use a hand lens to spot the little green flakes.
Chlorites form at lowish temperatures, sometimes during weathering, but mainly during low-grade metamorphism(the greenschist facies is typified by the presence of chlorite) or by hydrothermal alteration of ferromagnesian minerals.
Chlorite (the species chamosite) commonly forms green ooliths in sedimentary ironstones - the ooliths are aggregates of extremely tiny chamosite grains. Ultra-small crystals of chlorite are an important component of clays and mud-grade sedimentary material, but you can't identifiy them in thin section.
B) In thin section
PPL: Chlorites have a rather low relief, and are usually apale green colour. Chlorite is often intergrown with biotite, from which it forms by low-temperature alteration. It commonly replaces or forms rims around garnets during retrograde metamorphism. Sedimentary chamosite is usually ultra fine-grained, but often forms dark greenish or greenish-brown ooliths.
CP: Chlorites are very distinctive between crossed polars. They have extremely low birefringence (maximum colour: grey) , but this is usually masked by "anomalous" dull purple or brown colours quite unlike most other minerals. Once seen this is never forgotten (well, maybe..)
Epidote
Epidote is the commonest of a group ofcalcium-rich aluminosilicates containing Si2O7 groups formed by two SiO4 tetrahedra sharing an oxygen. The general formula is A2B3Si2O7(SiO4)OH, where A is normally Ca, (or rarely REE, in igneous rocks), and B is Fe3+, Al, Fe2+ or Mn3+. Epidote itself is Ca2Al2Fe3+Si2O7(SiO4)OH.
It is typical of low-medium grade metamorphosed basaltic rocks - the greenschist (ep+chlorite+actinolite) or epidote amphibolite (ep+hornblende) facies especially.It also occurs in hydrothermal veins, and as an accessory mineral in igneous rocks such as granites.
Associated minerals: quartz, calcite, plagioclase, chlorite, actinolite.
A) Hand specimen
Colour is a distinctive green with a definite yellow-green tint - this is very distinctive, and distinguishes epidote from most other green silicates which usually have a bluish or greyish tone.Occasionally classic "pistachio green" colour, like the nuts. Crystals are usually columnar and striated along their length, and may form radiating or fibrous aggreagtes. Cleavage along length of crystals. Hard - around 6.5.
B) In Thin Section
PPL: high relief, colourless to distinct yellowish green tint. Strongly coloured varieties may be pleochroic in shades of yellowish green.
CP: Typically very bright 3rd order colours, but may be lower (sometimes anomalous blue and/or yellow colours) in Al-rich epidotes. Extinction normally parallel to elongation of crystals.
The combination of high relief, colour and high birefringence are distinctive. You might mistake epidote for actinolite or tremolite (no yellowish tints, amphibole cleavages, max extinction angle 10-20o), or staurolite (low bireferingence).
Titanite
CaTiSiO5 (may also contain traces of elements such as U , and REE)Formerly known as sphene. One of the commonest titanium minerals in igneous and metamorphic (particularly meta-igneous) rocks.
Associated minerals: plagioclase, amphibole, quartz.
A)Hand specimen
Titanite is usually brown, with a very bright resinous or adamantine lustre. Small crystals are common in intermediate and granitic rocks, and are usually somewhat flattened or tabular with a diamond-shaped cross section. You'll normally need a hand-lens to see them though… Hardness around 5.5-6.
B) In thin section
PPL: Titanite has extremely high relief - it stands out more than any other common mineral. It often has a faint brownish tint. Habit is usually as fairly large equant grains or euhedral crystals, often with the distinctive diamond-shaped cross-section.
CP: Very high briefringence - nearly as high as calcite and carbonates, so may show pinky-buff birefringence colours. These colours can be hard to see given the extremely high relief and faint brownish colour, so at first sight a titanite might not seem to change much in appearance when switching from PPL to CP. Some grains, esp those showing lower birefringence colours, may not extinguish properly and instead of going black, show dull orange or bluish colours.
The relief, crystal shape and birefringence of titanite are distinctive. Garnet has a lower relief and is isotropic. Staurolite has a lower relief and low birefringence. Carbonates have similar birefringence, but have low relief. Other wierdo accessory minerals such as zircon or monazite may resemble titanite, but tend to form much smaller crystals, and are usually very hard to see, even with a good microscope.
Apatite
Apatite is strictly a group name for phosphate minerals of formula Ca5(PO4)3(F, Cl, OH), which may also contain carbonate or sulphate. U and LREE may substitute for Ca at 0.0x to 0.x% levels. Apatite group minerals are the main host for phosphorus in crustal rocks and living organisms.
Associated minerals: Many! Apatite almost universal in basic igneous rocks (often in areas of late-crystallising "mesostasis"). Especially common in intermediate igneous rocks rich in opaque Fe-Ti oxide minerals. Also widespread in other igneous and metamorphic rocks.
A)Hand specimen
Most commonly seen as pale green to grass greenmassive material with lots of cracks. Vitreous to slightly greasy lustre. Softer than most silicates (H=5). Cleavage absent or poor. Green commonest, but can be almost any colour. Mn2+ -rich material may be quite intense blue-green, but the streak is always colourless. Sedimentary apatite rocks (phosphorites) are usually ultra-fine grained, and dull or earthy. They may contain pisolitic
Apatite (contd.)
structures, and/or bones, or teeth - also made of apatite! The small apatite crystals typical of normal rocks can't usually be seen in hand-specimen.
B) In thin section
PPL: Apatite has medium relief. Most commonly it forms very small, colourless elongate crystals which always show a round or hexagonal outline in cross-section. Cleavage is usually absent, or poor - normally cracks across the length of crystals. Biogenic or sedimentary apatite in thin-sectionis almost always ultra fine-grained, and often brownish in colour. It often shows concentric banding.
CP: apatite has very low birefringence - lower even than quartz and feldspars. The maximum colour is a mid-grey - shown by long crystal sections. Hexagonal or rounded end-sections are more or less isotropic, and will be close to black in crossed-polars. Long crystals always show straight extinction.
Apatite is normally pretty easy mineral to identify in thin-section, at least in normaligneous and metamorphic rocks. The combination of small, well-formed hexagonal crystals, moderate relief, and low birefringence with straight extinction is more or less unique.
Opaque minerals
Any mineral which is opaque will appear black in thin section. This does not mean that they would appear black in reflected light. For example, magnetite and ilmenite are true blackish minerals; however, in thin section they look exactly the same as pyrite , other sulphides or elements such as gold.
In igneous and metamorphic rocks, the commonest opaque minerals are oxides of iron and titanium such magnetite and ilmentite. However sulphides can also occur (eg pyrite in slate, pentlandite in gabbros etc), or elements such as graphite (in carbonaceous metmorphic rocks).
In sediments, opaque minerals may be detrital grains of opaque oxides, or commonly, diagnetic sulphides such as pyrite.
You may be able to tell the difference between pyrite and Fe-Ti oxides by examining a thin section in reflected light using a hand lens. Pyrite will often have a greenish-grey colour, while Fe-Ti oxides are black.