CGC Crystal Structure & Mineralogy 2 — Questions and Answers
Question 1: Which crystal system does quartz belong to?
- Cubic
- Orthorhombic
- Trigonal (Correct answer)
- Monoclinic
Correct answer: Trigonal
Quartz belongs to the trigonal crystal system (a subdivision of hexagonal), forming characteristic six-sided prisms with pyramidal terminations.
Question 2: What term describes two distinct minerals that have the same chemical composition but different crystal structures?
- Isomorphs
- Polymorphs (Correct answer)
- Allotropes
- Pseudomorphs
Correct answer: Polymorphs
Polymorphs are minerals with identical chemical compositions but different crystal structures; diamond and graphite are classic examples, both being pure carbon.
Question 3: Which type of luster is described as resembling the surface of a pearl or the inside of a shell?
- Vitreous
- Adamantine
- Pearly (Correct answer)
- Silky
Correct answer: Pearly
Pearly luster results from light reflection off internal cleavage planes and produces a soft, iridescent sheen similar to the surface of a pearl.
Question 4: Alexandrite is a variety of which mineral known for its dramatic color-change property?
- Spinel
- Chrysoberyl (Correct answer)
- Tourmaline
- Garnet
Correct answer: Chrysoberyl
Alexandrite is a color-change variety of chrysoberyl (BeAl2O4) that appears green in daylight and red under incandescent light due to chromium absorption.
Question 5: What is the cleavage quality of diamond described as?
- Poor in one direction
- Perfect in four directions (octahedral) (Correct answer)
- Imperfect in two directions
- None
Correct answer: Perfect in four directions (octahedral)
Diamond has perfect octahedral cleavage in four directions parallel to the faces of the octahedron, which diamond cutters exploit when cleaving rough stones.
Question 6: The mineral group that includes jadeite and spodumene belongs to which family?
- Pyroxenes (Correct answer)
- Amphiboles
- Feldspars
- Garnets
Correct answer: Pyroxenes
Jadeite (NaAlSi2O6) and spodumene (LiAlSi2O6) are both pyroxene group minerals, single-chain inosilicates characterized by two cleavage directions at ~87°/93°.
Question 7: Which optical phenomenon causes the star effect (asterism) seen in star rubies and star sapphires?
- Dispersion of light by the crystal lattice
- Reflection from needle-like rutile inclusions oriented along crystal axes (Correct answer)
- Interference from twinning planes
- Diffraction from surface etch pits
Correct answer: Reflection from needle-like rutile inclusions oriented along crystal axes
Asterism in corundum is caused by reflection of light from oriented needle-like inclusions of rutile (TiO2) that align along the three crystallographic axes of the hexagonal crystal.
CGC Crystal Structure & Mineralogy 2
Which crystal system does quartz belong to?