Mineralogy and Petrology Flashcards

Which crystal system is characterized by three unequal axes, all intersecting at oblique angles?

Answer: Triclinic

The triclinic system has three unequal axes (a≠b≠c) with all three axial angles differing from 90°.

A rock containing plagioclase, hornblende, and biotite with a porphyritic texture most likely formed as a:

Answer: Hypabyssal intrusive igneous rock

Porphyritic texture with those minerals indicates a hypabyssal (shallow intrusive) or volcanic rock where phenocrysts grew slowly before rapid cooling.

The mineral sphalerite is the primary ore of which metal?

Answer: Zinc

Sphalerite (ZnS) is the principal ore mineral of zinc and commonly occurs with galena in hydrothermal deposits.

Which metamorphic index mineral first appears at the lowest grade in Barrow's zones of regional metamorphism?

Answer: Chlorite

Chlorite defines the lowest-grade Barrovian zone, appearing before biotite, garnet, staurolite, kyanite, and sillimanite with increasing P-T conditions.

Orbicular texture in igneous rocks refers to:

Answer: Spherical masses of concentric mineralogic shells

Orbicular texture consists of spheroidal structures with concentric layers of alternating mineral compositions, found in some granites and diorites.

The mineral aragonite and calcite are best described as:

Answer: Polymorphs

Aragonite and calcite are polymorphs — both are CaCO₃ but with different crystal structures (orthorhombic vs. trigonal).

An ultramafic rock composed almost entirely of olivine is called:

Answer: Dunite

Dunite contains >90% olivine and represents the most olivine-rich end-member of the peridotite family.