Two pegmatite minerals from Bhandara: crystal system by morphology or by optics?
A college in Nagpur receives two euhedral crystals from a zoned pegmatite near Bhandara. Specimen A is brown, adamantine, hardness 7½, specific gravity 4.6, white streak, no cleavage, conchoidal fracture. A contact goniometer gives 90° between adjacent prism faces, and a prism-to-pyramid angle of about 123°. In thin section it is uniaxial positive, with very high relief, strong birefringence and straight extinction. Specimen B is white to cream, vitreous, hardness 6, specific gravity 2.6; it has two cleavages meeting at 90°, is biaxial negative with 2V near 70°, shows extinction inclined by about 15°, and displays repeated fine parallel lamellae under crossed polars. In the practical record a first-year student has written 'A is cubic because all the angles are 90°' and 'B is orthoclase because the cleavages meet at 90°'.
1. Critically examine the student's identification of specimen A. Assign a crystal system and support it with the evidence.
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The student has used only one line of evidence and drawn the wrong conclusion from it. Interfacial angles of 90° are compatible with three systems - isometric, tetragonal and orthorhombic - so the goniometer alone cannot decide between them. The reasoning 'all angles are 90°, therefore cubic' is invalid.
The optical data settle the matter. An isometric mineral is optically isotropic: it would be dark between crossed polars in every orientation and would show no birefringence and no extinction position. Specimen A is strongly birefringent with straight extinction, so it cannot be isometric. A single optic axis (uniaxial) narrows the choice to tetragonal or hexagonal/trigonal, and the presence of prism and pyramid faces at 90° to each other, plus a 123° prism–pyramid angle, is consistent with a tetragonal prism and dipyramid.
The properties together - H 7½, SG 4.6, adamantine lustre, uniaxial positive, no cleavage - point to zircon, a common accessory in pegmatites. Correct answer: tetragonal, 4/m 2/m 2/m, identified from optics plus morphology, not from angles alone.
2. Identify specimen B and explain which features separate it from orthoclase. Is the 90° cleavage a diagnostic criterion here?
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Specimen B is a plagioclase feldspar, most probably albite–oligoclase from a pegmatite.
The 90° cleavage is not diagnostic. Both orthoclase and plagioclase have two good cleavages at approximately 90°, hardness near 6 and specific gravity near 2.55–2.6; a student relying on cleavage and hardness will confuse the two every time. Both are biaxial negative as well, so the optic sign alone does not separate them either.
The discriminators are three. (1) Twinning style: plagioclase shows repeated parallel lamellae - albite and pericline laws - while orthoclase typically shows a single Carlsbad interpenetration or the cross-hatched microcline grid, not fine repeated lamellae. (2) Extinction: plagioclase sections cut near (010) extinguish at an appreciable angle, here about 15°, and the extinction angle increases with anorthite content; orthoclase is nearly parallel, within a few degrees. (3) Alteration and association: plagioclase alters to fine sericite and zoisite specks, orthoclase to kaolin and sericite in patches, and in a pegmatite, association with muscovite and quartz is expected for either.
A practical confirmation is the Michel-Lévy determination of extinction angle against the albite-law lamellae to get the composition, which also tells you whether this is sodic plagioclase consistent with a pegmatite.
3. Cassiterite is reported from the same pegmatite and looks superficially like specimen A. Which single determination would you make first, and why?
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Measure specific gravity, ideally on a clean, weighed fragment using a pycnometer or Jolly balance rather than by eye. Reason: the two minerals converge on almost everything except density. Both are tetragonal, both are uniaxial positive, both can be brown with adamantine lustre, both have hardness around 6–7½, and both may lack cleavage. Morphology and optics therefore offer no quick separation, and a student who stops at 'tetragonal, uniaxial positive' will record the wrong mineral.
The numbers diverge sharply: zircon is about 4.6–4.7 and cassiterite about 6.8–7.0. A heavy-liquid check is the field equivalent; cassiterite sinks even in methylene iodide (SG 3.33), while most gangue floats, though care is needed because zircon also sinks in that liquid. The decisive thresholds are therefore a measured SG above about 6.5, supported by X-ray diffraction or a microchemical tin test on a polished grain.
Report both the SG and its method in the practical record; an unrecorded or eyeballed density is not evidence.