(a) Describe common minerals of igneous and metamorphic rocks, and the diagnostic features of these minerals. (15 marks) (b)…

(a) Common minerals of igneous and metamorphic rocks and their diagnostic features (15 marks)

Igneous rocks.

• Quartz (SiO₂): colourless, no cleavage, low relief, first-order grey to white, undulose extinction, uniaxial positive.
• Orthoclase and microcline (KAlSi₃O₈): low relief, cloudy from alteration to clay, two cleavages at about 90°; Carlsbad twin in orthoclase and tartan twinning in microcline; biaxial negative.
• Plagioclase (NaAlSi₃O₈–CaAl₂Si₂O₈): polysynthetic albite twinning, zoning, inclined extinction that varies with anorthite content.
• Biotite: brown, strongly pleochroic, one perfect cleavage, "bird's-eye" extinction.
• Muscovite: colourless, one cleavage, bright second- to third-order colours.
• Hornblende: green to brown, pleochroic, two cleavages at 56° and 124°, extinction 15–25°.
• Augite: pale green or colourless, two cleavages at about 87° and 93°, extinction up to 45°.
• Olivine (Mg,Fe)₂SiO₄: colourless, high relief, no cleavage, irregular cracks, altering to serpentine.

Metamorphic rocks.

• Garnet: pale pink, very high relief, isotropic, no cleavage, euhedral or poikiloblastic.
• Staurolite: yellow, pleochroic, twinned as a cross.
• Kyanite: colourless, high relief, two cleavages, elongated blades.
• Sillimanite: prisms and fibrolite needles.
• Andalusite: pinkish, pleochroic, square section, chiastolite cross.
• Chlorite: green, anomalous blue or brown interference colours.
• Epidote, tremolite–actinolite, talc, wollastonite, diopside and calcite are diagnostic of low-grade, contact and calcareous rocks.

(b) Optical properties of rock-forming minerals and their use in identification (15 marks)

Optical properties.

• Plane-polarised light: colour, pleochroism, habit, cleavage, relief (refractive index against the mounting balsam, 1.54) and the Becke line, alteration and inclusions.
• Crossed polars: isotropic or anisotropic behaviour; birefringence, shown by interference colour on the Michel-Lévy chart (a 0.03 mm section of quartz shows first-order grey to white); extinction, whether parallel, inclined or symmetrical, and extinction angle; twinning and zoning; sign of elongation with the gypsum plate.
• Convergent light (conoscopic): the interference figure shows whether the mineral is uniaxial or biaxial, its optic sign and 2V.

Method of identification.

• Observe first in plane-polarised light, noting colour, pleochroism, relief, cleavage and habit.
• Insert the analyser and note birefringence, extinction, twins and zoning.
• Use the conoscopic figure on suitable grains to find uniaxial or biaxial character and sign.
• Compare the whole set with tables of properties, for example quartz (uniaxial positive, low relief) against plagioclase (twinned, biaxial).

(c) Twinning and twin types in feldspars (20 marks)

Twinning. Twinning is the symmetric intergrowth of two or more crystal parts related by a symmetry operation, reflection across a twin plane or rotation about a twin axis, that is not a symmetry of the single crystal. Twin elements are the twin plane, the twin axis and the composition plane. Twins may be contact or penetration, simple or repeated (polysynthetic or cyclic), and may arise by growth, by deformation or by transformation, for example when a monoclinic form inverts to a triclinic one.

Feldspar twin laws.

• Carlsbad: penetration, simple, twin axis parallel to c; common in orthoclase, sanidine and plagioclase; it divides a crystal into two halves.
• Baveno: contact twin on the plane {021}.
• Manebach: contact twin on the plane {001}.
• Albite: polysynthetic twin on {010}, the most common twin in plagioclase, with fine parallel lamellae.
• Pericline: twin axis parallel to b, composition plane the rhombic section; lamellae are also polysynthetic.
• Combined albite and pericline twinning gives the cross-hatched, "tartan" pattern that is diagnostic of microcline.

Significance. Albite twinning identifies plagioclase and, with the extinction angle, gives the anorthite content (Michel-Lévy method). Carlsbad and albite twins are often found together in the same crystal. The lack of twinning helps to recognise untwinned potassium feldspar and quartz. Tartan twinning marks microcline formed by inversion.