CGC Crystal Structure & Mineralogy 3

Question 1

What is the term for the phenomenon where a mineral displays different colors when viewed along different crystallographic axes?

Accepted Answer

Pleochroism

Answer

Fluorescence

Answer

Adularescence

Answer

Labradorescence

Question 2

Spinel crystallizes in the cubic system and commonly forms which crystal habit?

Accepted Answer

Octahedra

Answer

Prismatic columns

Answer

Tabular plates

Answer

Rhombohedra

Question 3

Which silicate structural group do garnets belong to?

Accepted Answer

Nesosilicates (isolated tetrahedra)

Answer

Tectosilicates (framework)

Answer

Cyclosilicates (rings)

Answer

Phyllosilicates (sheets)

Question 4

What causes the adularescence (moonstone glow) in orthoclase feldspar?

Accepted Answer

Light scattering from alternating albite and orthoclase layers

Answer

Fluorescence from trace uranium

Answer

Inclusions of fine rutile needles

Answer

Surface oxidation creating an iridescent film

Question 5

A gemstone with a specific gravity of 3.52 and Mohs hardness of 10 is most likely which mineral?

Accepted Answer

Diamond

Answer

Corundum

Answer

Spinel

Answer

Moissanite

Question 6

What type of twinning commonly occurs in chrysoberyl and produces a pseudohexagonal crystal shape?

Accepted Answer

Cyclic (trilingual) twinning

Answer

Polysynthetic twinning

Answer

Contact twinning on the basal plane

Answer

Penetration twinning along c-axis

Question 7

Which property best explains why diamond has exceptional thermal conductivity despite being an electrical insulator?

Accepted Answer

Its strong, rigid covalent carbon lattice with minimal phonon scattering

Answer

Its metallic bonding network

Answer

High electron mobility from delocalized pi bonds

Answer

Presence of trace metallic inclusions