A Key for Identification of Rock-Forming Minerals in Thin by Andrew J. Barker

By Andrew J. Barker

Structured within the kind of a dichotomous key, resembling these standard in botany, the mineral key offers an effi cient and systematic method of picking rock-forming minerals in thin-section. This new angle covers a hundred and fifty+ of the main mostly encountered rock-forming minerals, plus a couple of rarer yet noteworthy ones. Illustrated in complete color, with 330+ top of the range mineral photomicrographs from a world number of igneous, metamorphic, and sedimentary rocks, it additionally presents a complete atlas of rock-forming minerals in thin-section.

Commencing with a quick advent to mineral structures, and the homes of minerals in plane-polarised and cross-polarised mild, the mineral key additionally comprises line drawings, tables of mineral homes and an interference color chart, to additional relief mineral id. To minimise the opportunity of misidentification, and let much less skilled petrologists to exploit the main with self belief, the foremost has been prepared to prioritise these houses which are most simply recognised.

Designed for simplicity and simplicity of use, it's basically aimed toward undergraduate and postgraduate scholars of mineralogy and petrology, yet must also supply a helpful resource of reference for all training geologists facing rock thinsections and their interpretation.

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Isa region, Queensland, Australia. The staurolite (Fig. 24d), showed 1st order grey/white interference colours prior to ­insertion of the gypsum plate. With the plate inserted, and with length oriented NW-SE, it now shows its length-slow character, with lower colours (1st order yellow-orange) compared to the gypsum red, exactly the opposite of chloritoid and andalusite above. 6 Interference figures All previously described thin-section properties of minerals are observed in what is termed orthoscopic illumination.

21 shows examples of inclined extinction for actinolite (Fig. 21a) and ­plagioclase (Fig. 21b). When measuring extinction of multiple lamellar twinned plagioclase, look for crystals with equal illumination when in N-S position relative to cross-wires (Fig. 22b), then rotate left and right in turn (Fig. 22a-c) to measure extinction angle of each twin set. If the crystal is to be regarded as reliable for extinction angle determination, the value obtained turning left should be the same or very close to the value obtained when turning right.

Having selected the crystal of interest, the same procedure as that described above, for uniaxial crystals, is undertaken. Assuming the crystal has a favourable orientation, when the Bertrand lens is inserted, if a well-centred interference figure is produced, the pattern observed will either be of two boomerang-like curved isogyres arching towards the centre of the field of view (Fig. 26c), or else something that looks very like a uniaxial cross (Fig. 26a). The latter occurs in cases where the 2V angle (optic axial angle) is very low (0–10°), whereas the latter forms with higher 2V values.

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