Mineralogy & Crystallography

Question 1

apfu

Answer 1

atoms per formula unit (subscript number)

Question 2

Diamond

Answer 2

formula: C
- one carbon atom per formula unit

Question 3

Quartz

Answer 3

SiO2

Question 4

Pyrite

Answer 4

FeS2

Question 5

Solid solution

Answer 5

Minerals whose chemical composition is a homogenous mixture of two mineral species
Ie., components are variable

Eg.
(Mg,Fe)2SiO4 –> Peridot formula
All Mg and Fe sum to 2; either Mg or Fe could occupy up to 2 apfu

Question 6

Charge neutrality

Answer 6

All compounds are charge neutral
- every positive charge must be balanced by a negative charge
- anions (-) and cations (+)

Eg.
SiO2 –> Quartz
Si cation, valence +4
Two O anions, valence -2
Si + 2O = SiO2
(+4). + 2(-2) = 0
(+4). + (-4) = 0

Question 7

Major elements

Answer 7

• fundamental in mineral’s crystal structure
• major impact on resulting bulk properties
• always part of written chemical formula

Question 8

Minor elements

Answer 8

• present in smaller amounts
• commonly replace major elements in a mineral
• sometimes part of written chemical formula

eg.,
(Fe,Mg)2SiO4 –> iron-dominated olivine (dominant element is listed first)
- Mg considered a minor element

Question 9

Trace elements

Answer 9

• found only in very small amounts
• either a replacement for one of the major elements in a crystal structure, or occupying “holes” in a crystal structure that are big enough for them to hide in
• not usually included in written chemical formula

Question 10

Element substitutions

Answer 10

• minor and trace elements commonly substitute for major elements
• can occur when two elements have similar charge and ionic radius (Goldilocks Principle)
  –> Fe and Mg have +2 charge and similar ionic radius (Mg ~0.72Å; Fe ~0.61Å)
• dominant element is listed first in the formula brackets
  –> (Fe,Mg)2SiO4 is the Fe end-member of olivine (fayalite), where minor element Mg substitutes for major element Fe
  –> (Mg,Fe)2SiO4 is the Mg end-member of olivine (forsterite), where minor element Fe subs for major element Mg

Question 11

Goldilocks Principle

Answer 11

An element can substitute for another if the valence charge and ionic radius are similar
- can apply to trace elements as well

Not too big, not too small, but just right

Question 12

Simple substitution

Answer 12

One elements substitutes for just one other element

Question 13

Coupled substitution

Answer 13

More than two elements are involved in substitution; occurs to ensure charge neutrality

Eg.
Al2O3 –> Corundum
Al (+3 charge) can be replaced by a charge-balanced amount of Fe (+2) and Ti (+4)
–> net charge is +3 (half of a combination of Fe and Ti), allowing Fe and Ti to enter crystal structure of corundum (producing sapphire!)

Question 14

Ionic radius

Answer 14

The radius of a sphere about the same size as the cation or anion

The apparent size of an atom depends on charge and kind of bond
- difficult to define exact boundary of an atom
- ions participating in ionic bonding behave like spheres; average size given by ionic radius

Question 15

What are the 10 main physical properties of minerals?

Answer 15

• colour
• luster
• diagostic streak
• cleavage
• fracture
• tenacity
• hardness
• refractive index
• specific gravity
• fluorescence

Question 16

What are the six crystal systems?

Answer 16

• cubic, tetragonal, hexagonal & triagonal, monoclinic, orthorhombic, triclinic

Question 17

Cubic crystal system

Question 18

Crystal habits

Question 19

Polyhedral method for constructing atomic models of crystal structures

Question 20

What mineralogical characteristics of ruby and spinel could be used to tell them apart?

Question 21

What is a solid solution? Give an example of a common mineral that displays solid solution in its chemical formula.