CPG Mineralogy & Petrology 2

Question 1

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

Accepted Answer

Phyllosilicate

Answer

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

Question 2

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

Accepted Answer

Isotropic

Answer

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

Question 3

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

Accepted Answer

Hornblende

Answer

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

Question 4

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

Accepted Answer

Olivine

Answer

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

Question 5

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

Accepted Answer

Topaz

Answer

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

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