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Identification Tables for Common Minerals in Thin Section

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Identification Tables for Common Minerals in Thin Section Identification Tables for Common Minerals in Thin Section These tables provide a concise summary of the properties of a range of common minerals. Within the tables, minerals are arranged by colour so as to help with identification. If a mineral commonly has a range of colours, it will appear once for each colour. To identify an unknown mineral, start by answering the following questions: (1) What colour is the mineral? (2) What is the relief of the mineral? (3) Do you think you are looking at an igneous, metamorphic or sedimentary rock? Go to the chart, and scan the properties. Within each colour group, minerals are arranged in order of increasing refractive index (which more or less corresponds to relief). This should at once limit you to only a few minerals. By looking at the chart, see which properties might help you distinguish between the possibilities. Then, look at the mineral again, and check these further details. Notes: (i) Name: names listed here may be strict mineral names (e.g., andalusite), or group names (e.g., chlorite), or distinctive variety names (e.g., titanian augite). These tables contain a personal selection of some of the more common minerals. Remember that there are nearly 4000 minerals, although 95% of these are rare or very rare. The minerals in here probably make up 95% of medium and coarse-grained rocks in the crust. (ii) IMS: this gives a simple assessment of whether the mineral is common in igneous (I), metamorphic (M) or sedimentary (S) rocks. These are not infallible guides - in particular many igneous and metamorphic minerals can occur occasionally in sediments. Bear this in mind, even if minerals are not marked as being common in sediments. (iii) Colour in TS etc: the range of colours for each mineral is given, together with a description of any pleochroism. Note that these are colours seen in thin-section, not hand- specimen. The latter will always be much darker and more intense than thin section colours. (iv) RI: the total range of refractive index shown by the mineral with this coulour is shown: This covers any range due to compositional variation by solid solution, as well as the two or three refractive indices of anisotropic minerals. (v) Relief : is described verbally, followed by a sign indicating whether the relief is positive or negative (ie greater or less than the mounting medium of the thin-section - 1.54). Minerals with refractive indices close to 1.54 have low relief, those with much higher or lower refractive indexes will have high relief. (vi) Extinction: angles are only given where minerals usually show a linear feature such as a cleavage and/or long crystal faces. For plagioclase feldspars (stippled) the extinction angles given are those determined by the Michel-Levy method (see a textbook for details). (vi) Int. Figure: this gives details of the interference figure. Any numbers given refer to the value of 2V (normally a range is given), followed by the optic sign. For uniaxial minerals the word "Uni" is given, followed by the sign. Your course may or may not have covered interference figures. If not, ignore this section! (vii) Birefr: Birefringence is described verbally, In some cases the maximum is given as a colour, in other cases you will need to cross-refer to an interference colour chart. (viii) Twinning etc.: a few notes about twinning, or other internal features of crystals may be given. If no twinning is mentioned, then the phenomenon is not common in thin section, but this does not mean that it NEVER occurs. (ix) Notes: general tips on appearance, occurrence and distinguishing features. May include indication of whether the mineral is length fast or slow - again a feature not covered in all courses - but a useful and easily-determined property. John Faithfull 1998 Mineral IMS Colour in TS etc. RI Relief Extinction Int. Figure Birefr. Twinning etc. Notes COLOURLESS fluorite IS colourless, blueish 1.43 v. high - ISOTROPIC Late mineral in granites - often purple due to radiation damage. Also a or purple cement in sandstones. High -ve relief, colour and isotropy distinctive. analcime I colourless 1.49 mod - - vv weak Usually in groundmass of dolerites and basalts filling spaces between plagioclase crystals. Colourless and nearly isotropic distinctive. "orthoclase" IMS colourless 1.52-1.53 v.low - 70, -ve low: up to Carlsbad (2 halves) Often turbid. May show patchy or thready perthitic texture. grey microcline IMS colourless 1.52-1.53 v.low - 80, -ve low: up to cross-hatch Twinning very distinctive. grey albite IM colourless 1.52-1.54 v.low +- -20 to -12 70-80, +ve low : up to Albite twins along Common in low-grade meta. rocks with chlorite-muscovite-actinolite etc), and (plagioclase) (Michel-Levy) cream length, also often clear and untwinned - can be very hard to identify. In igneous rocks Carlsbad + others usually twinned, as rims on more calcic plagioclase, or on perthite, or as a late replacement of other feldspars. oligoclase IM colourless 1.53-1.55 v.low +- -12 to 12 70-90, -ve low: up to Albite twins along In igneous rocks often with biotite and K-feldspar. Also in low-med grade (plagioclase) (Michel-Levy) or +ve white length, also meta rocks. Crystals in igneous rocks often rectangular laths. Often zoned: Carlsbad + others usually An-rich cores and Ab-rich rims. An-rich cores may alter more easily to sericite - a fine-grained turbid mica. Twinning distinctive - otherwise process of elimination. nepheline I colourless 1.53-1.55 v.low +- 0 Uni -ve low: up to dk Euhedral xls blocky: hexagons or rectangles, but only seen in lavas. grey Elsewhere anhedral. Often cloudy. May have highly briefr. alteration. Mostly in igenous rocks - v. scarce elsewhere. cordierite M colourless. 1.53-1.57 v.low - low +- 40-80, low : up to sometimes radiating Mostly found in low-pressure metapelites (eg hornfels). One of the hardest mostly -ve white hexagonal, striped, minerals to identify - often looks like quartz or plagioclase. Twinning may or untwinned. look like plag twinning in coarse grained rocks. Often has "blebby" inclusions. May have yellow pleochroic haloes around zircons etc. Tends to alter to darker coloured crud than plagioclase. andesine IM colourless 1.54 -1.56 v.low + 12-28 70-90, -ve low : up to Albite twins along In igneous rocks, often with hornblende, also in med-high grade meta rocks. (plagioclase) (Michel-Levy) or +ve pale grey length, also Crystals in igneous rocks often rectangular laths. Often zoned: usually An- Carlsbad + others rich cores and Ab-rich rims. An-rich cores may alter more easily to sericite - a fine-grained turbid mica. Twinning distinctive - otherwise process of elimination. quartz IMS colourless 1.54-.155 v.low + Uni +ve low : up to rare Usually irregular grains. Clear and unaltered. May show strained extinction. cream labradorite IM colourless 1.55-1.57 low + 28-39 70-90, -ve low : up to Albite twins along In igneous rocks often with clinopyroxene and or olivine. Also in high-grade (plagioclase) (Michel-Levy) or +ve white length, also meta rocks. Crystals in igneous rocks often rectangular laths. Often zoned: Carlsbad + others usually An-rich cores and Ab-rich rims. An-rich cores may alter more easily to sericite - a fine-grained turbid mica. Twinning distinctive - otherwise process of elimination. calcite IMS colourless 1.55-1.65 low to mod + Uni -ve extreme. rhombic lamellae Relief usually changes as stage rotated. Hard to tell apart from other Pinky-buff carbonates in thin section unless stained. bytowonite IM colourless 1.56-1.58 low + 40-52 80-90 -ve low : up to Albite twins along In igneous rocks often with clinopyroxene and or olivine. Also in high grade (plagioclase) (Michel-Levy) cream length, also meta rocks. Crystals in igneous rocks often rectangular laths. Often zoned: Carlsbad + others usually An-rich cores and Ab-rich rims. An-rich cores may alter more easily to sericite - a fine-grained turbid mica. Twinning distinctive - otherwise process of elimination. muscovite IMS colourless. 1.56-1.61 low + 0 30-40, -ve high: up to flakes with 1 perfect cleavage. Habit and birefringence distinctive. In up. II order aluminous granites + many metamorphic rocks. anorthite M colourless 1.57-1.59 low + 52+ 80-90 -ve low : up to Albite twins along Anorthite commoner in very high grade metamorphic rocks than in igneous (plagioclase) (Michel-Levy) cream/yello length, also rocks. Often zoned. Twinning distinctive. w Carlsbad + others tremolite M colourless 1.60-1.64 med + ca.20 80-88, -ve mod to high: Crystals usually elongate. Only found in carbonate-rich metamorphic rocks (amphibole) up to mid II (eg marbles). End sections show 2 cleavages at 120 degrees, Long sections order. show 1 cleavage along length, Length slow. anthophyllite M colourless 1.60-1.67 med + 0 70-90, +ve mod: up to In Mg-rich meta rocks, esp. low-grade metabasites - often with talc. Like (orthoamphibol low II order actinolite, but parallel extinction. Length slow. Amphibole cleavages on end e) sections: 120 deg. John Faithfull 1998 Mineral IMS Colour in TS etc RI Relief Extinction Int. Figure Birefr. Twinning etc. Notes COLOURLESS (continued) actinolite M colourless to pale 1.60-1.70 med. + 10-20 75-80, -ve mod-high: Crystals usually elongate. Lo-grade meta. rocks - esp. basic igneous rocks, (amphibole) green up to mid II or sediments containing basaltic detritus. End sections show 2 cleavages at order 120 degrees, Long sections show 1 cleavage along length. Length slow. apatite IM colourless. 1.63-1.65 mod + 0 Uni -ve low - grey Usually as tiny elongate crystals with round or hexagonal outline.
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