Optical Mineralogy Questions and Answers

Answer: C

The correct matching is P-ii; Q-i; R-iv; S-iii.

• Zircon has refractive indices near 1.92 to 1.98, far above the mounting medium, so it shows very high positive relief (ii).
• Orthopyroxene (hypersthene) shows characteristic pink to green pleochroism (i).
• Muscovite has a birefringence of about 0.036 to 0.049, giving bright upper second to third order interference colours (iv).
• Cordierite forms cyclic (sector) twins that divide a grain into three or six segments (iii).

Options giving muscovite the cyclic twinning, or zircon the pleochroism, swap properties that belong to cordierite and orthopyroxene.

Answer: D

The intensity of light does not control the interference colour.

Interference colour depends on the retardation between the two rays travelling through the crystal:

\[ \Delta = t\,(n_{1} - n_{2}) \]

where t is the thickness of the section and \(n_{1} - n_{2}\) the birefringence of the section, which depends on its orientation with respect to the crystallographic axes. The orientation of the vibration directions relative to the polariser and analyser also matters, since the mineral goes dark at extinction and shows its colour most brightly at 45°.

Changing the brightness of the lamp makes the colours brighter or dimmer but does not change which colour is seen.

Answer: A

The mineral that shows internal reflections is orpiment.

Internal reflections are seen in ore microscopy when light enters a mineral that is partly transparent, is reflected from cracks and grain boundaries inside it, and comes back out coloured. They are typical of translucent ore minerals: orpiment shows bright yellow internal reflections, and sphalerite, cinnabar and realgar show them too.

Magnetite, pyrite and molybdenite are opaque. Light does not penetrate them, so they show only surface reflection and no internal reflections. Pyrite is known for its high reflectance and pale yellow colour, molybdenite for its strong bireflectance.