Mineralogy & Petrology Flashcards

In which silicate structural class does each SiO4 tetrahedron share three oxygens with neighboring tetrahedra to form continuous sheets?

Answer: Phyllosilicate

Phyllosilicates such as micas and clays have tetrahedra sharing three basal oxygens, producing sheet structures.

A mineral in thin section stays completely dark under crossed polars in every orientation as the stage is rotated. It is most likely:

Answer: Isotropic

Isotropic minerals such as garnet and other cubic minerals do not split light into two rays, so they stay dark (extinct) under crossed polars.

A dark prismatic mineral in a hand sample shows two cleavages that meet at about 56° and 124°. Which mineral is it most likely?

Answer: Hornblende

Amphiboles such as hornblende have two cleavages at about 56° and 124°, while pyroxenes have cleavages at roughly 87° and 93°.

On the discontinuous branch of Bowen's reaction series, which mineral crystallizes first from a cooling mafic magma?

Answer: Olivine

Olivine crystallizes at the highest temperature on the discontinuous branch, followed by pyroxene, amphibole, and biotite.

Which of the following minerals is hard enough to scratch quartz?

Answer: Topaz

Topaz has a Mohs hardness of 8, so it can scratch quartz, which has a hardness of 7.

In a thin section viewed under crossed polars, a feldspar shows fine, parallel polysynthetic (albite) twin lamellae. What is it most likely?

Answer: Plagioclase

Albite-law polysynthetic twinning is diagnostic of plagioclase, while microcline shows a cross-hatched (tartan) twinning pattern.

Which mineral fizzes vigorously in cold, dilute hydrochloric acid without first being powdered?

Answer: Calcite

Calcite reacts readily with cold dilute HCl, while dolomite fizzes only weakly unless it is powdered or the acid is heated.