Mineralogy

Question 1

What causes Isochromes?

Answer 1

Minerals that are thick or have high birefringence.This is because retardation and interference color depends on Thickneed and birefringence.

Question 2

What is the retardation of an uniaxial or Biaxial mineral when light follows an optic axis?

Answer 2

Zero retardation

Question 3

Name 3 common polymorphs of serpentine

Mg₃Si₂O₅(OH)₄

Answer 3

• Lizardite
• Antigorite (after Antigorio in Italy
• Chrysolite (from the greek word for golden abd fiber)

Question 4

Give give 2 examples of Order-Disorder rnxs in Dolomite and Calcite

Answer 4

• Cation disorder in Dolomite
• Anion disorder in Calcite

Question 5

What importance has the Order-disorder rxn of Calcite and Dolomite?

Answer 5

• Carbonates can carry C into the Earth
• Important in the C-cycle, CCS

Question 6

What is a Polymorph?

Answer 6

• Same Chemical Composition but different structures (e.g., stishvite (tetragonal), low quartz (trigonal), etc; water, ice, & steam; calcite &aragonite)

Question 7

What is a Isomorph?

Answer 7

• Same structure but different Chemical Composition (e.g.,CaCO₃ Calcite, MgCO₃ magnesite, both hexagonal))

Question 8

Waht is a Solid solution?

Answer 8

Is a homogeneous mineral of variablre composition (ie. a continuous range of chemistries) and communly the same structure (e.g. olivine (forsterite-fayalite), plagioclase (albite-anorthite)

It is possible by isomoerphos substitution of one element in the crystal structure to occur

Question 9

in terms of symmetry

What is order-disorder?

Answer 9

• Can be considerd as a variety of polymorphism. The two structures are related, but the ordered form (low temp) has a lower symmetry than the disorder form (high temp).

Question 11

what are two types of phase transitions?

Answer 11

• Displacive = change in symmetry, no breaking of bonds, rapid transitions (dashed lines)
• Reconstructive = breaking & reforming of chemical bonds new structural arrangement, sluggish (solid lines)

Question 12

What mineral has the highest refractive index and density in SiO₂?

Answer 12

• Stishovite

It is formed under great pressure so has a more dense crystal structure. Good property.

Question 13

what are the 2 most abundant minerals in the Earth’s crust?

Answer 13

Next to Feldspar, Quartz is the most abundant mineral in the Earth’s crust

Question 14

ne… and Si-

With what minerals Quartz incompatible with?

Answer 14

Incompatible with nepheline and other Si-poor minerals (e.g. sodalite, cancrinite) are therefore absent from undersaturated igneous rock)

Question 15

What Silicate minerals are very rare in nature and associated with meteor impact craters?

Answer 15

coesite and stishovite

Can also be brought up by Kimbelites to the surfice.

Question 16

What minerals can occur in association extrusive or volcanic equivalents of granites and related rocks? ?

Answer 16

tridymite and cristobalite

Rapid cooling preserves these polymorphs

Question 17

Give 6 distinguishing features of Quartz

Answer 17

• Lack of color, cleavage, visible twinning
• Low relief, weak birefringence
• Uniaxial interference figure
• Positive Optic sign
• Lack of alteration
• Undulatory extinction in strained rocks

Question 18

What is the relation between Temperature and Pressure with density in polymorphs?

Answer 18

• High-temperature polymorphs have low densities
• High-pressure polymorphs have higher densities

Question 19

why micas have cleavages?

Answer 19

The basal cleavage in Micas occur because there is only weak K bonding between the structural sheets of the mica structure. Movement , or cleavage can occur easily along those planes of weak bonds.

Question 21

Give at least 3 distinguish features of Micas

Answer 21

• Platy morphology
• Perfecrt basal cleavage (gama aprox. perp. to their perfect 001 cleavage)
• Flexible sheets
• Negative Biaxial
• Coloured Micas are strongly Pleochroic (biotite)
• Muscuvite & phlogopite: absence of defects and inclusions
• Biotite: colored, inclusions and pleochroic halos, Higer refractive index than other micas.

Question 22

Which mica has the highest 2V?

Answer 22

Biotite (highest), than muscavite and Phlogopite (lowest) last