Crystal Geochemistry

Question 1

What are the states of matter?

Answer 1

Solid, liquid, gas

Question 2

Three factors controlling crystal structures

Answer 2

1) Size of ions
2) Valences of ions
3) Bonding between atoms

Question 3

Coordination number

Answer 3

The number of anions that surround a particular cation in an ionic crystal

Question 4

Does the coordination number increase or decrease as the radii of anions and cations get closer?

Answer 4

Increase

Question 5

True or False: The size of an ion is influenced by the coordination number.

Answer 5

True

Question 6

Ionic substitution in crystals

Answer 6

Some of the ions of the major elements may be substituted by other minor/trace elements (not 100% pure)

Question 7

Why does a mineral that is composed of limited number of major elements contain so many trace elements?

Answer 7

Because most minerals are not composed purely of the major elements. So, they need minor/trace elements to substitute.

Question 8

Goldschmidt Rule #1

Answer 8

The ions of one element can replace those of another in crystals if their radii differ by less than about 15%

Question 9

Goldschmidt Rule #2

Answer 9

Ions whose charges differ by one unit substitute readily for one another provided electrical neutrality is maintained.

If charges differ by more than one, substitution is generally slight.

Question 10

Goldschmidt Rule #3

Answer 10

When two different ions occupy a crystal lattice, the ion with higher ionic potential forms a stronger bond with the anions surrounding the site

Question 11

Goldschmidt Rule #4 (Ringwood’s rule)

Answer 11

When the competing ions have different electronegativities and form bonds of different ionic character

Question 12

Camouflage

Answer 12

Substitution by a minor element when they have the same charge and similar radii

Question 13

Capture

Answer 13

Substitution by a minor element because the latter has a higher ionic potential

Question 14

Admission

Answer 14

Substitution by a minor element when the latter has a lower ionic potential (larger radii / lower charge)

Question 15

Coupled Substitution

Answer 15

When an ion is substituted by an ion of higher charge, another position must be occupied by one with a lower charge. Goal = electrical neutrality

Question 16

Incompatible elements

Answer 16

Trace elements that cannot find crystals to accommodate them. So they remain the melt of a magma system

Question 17

Distribution coefficient

Answer 17

Concentration of an element in the crystal vs that in the melt

Question 18

Compatible elements

Answer 18

Elements that can be accommodated by the Fe-Mg-Ca minerals

Question 19

Typical compatible elements

Answer 19

Mn, V, Co, Ni, Cr

Question 20

Lanthanide contraction

Answer 20

Decrease of radii of REE ions with increasing atomic number

Question 21

What does the REE pattern reflect?

Answer 21

The degree of enrichment of different REEs

Question 22

Eu anomaly

Answer 22

Abrupt change of Eu compared to neighbouring elements in the REE pattern

Question 23

What mineral is “U” compatible with?

Answer 23

Zircon

Question 24

The more positive the anomaly, the more _____

Answer 24

oxidizing

Question 25

The more negative the anomaly, the more _____

Answer 25

reducing

Question 26

Mineral geothermometer

Answer 26

Tool that measures the temperature in a mineral. The higher the T, the easier the substitution.

Question 27

Two examples of a mineral geothermometer

Answer 27

1) Sphalerite geothermometer
2) Arsenopyrite geothermometer

Question 28

Why is U concentrated in granitic rocks, while Cr in ultramafic rocks?

Answer 28

Most granitic rocks result from evolved magma… U is incompatible, so it will stay in the melt and become enriched in granite.
Cr is taken away early in crystallization = mafics

Question 29

How can the concentrations of trace elements in minerals be used to estimate the formation temperature of the minerals?

Answer 29

Geothermometers. The higher the temperature, the higher the substituion.