Nesosilicates: Olivines

Question 1

Olivine General Formula

Answer 1

M2SiO4

Question 2

4 Main types of Olivine

Answer 2

Forsterite (Fo) - Magnesium Mg2SiO4
Fayalite (Fa) - Iron Fe2SiO4
Tephroite (Te) - Manganese Mn2SiO4
Monticellite (Mo) - Calcium-Magnesium CaMgSiO4

Question 3

Nesosilicates

Answer 3

A group of minerals characterized by a structure of isolated SiO4 tetrahedra bonded by various cation combinations.
They have an absence of alkali elements in their compounds.
Notably dense, poor cleavage, high hardness, and high reliefs.

Question 4

Complete Solid Solutions in the Olivine Group

Answer 4

Fo-Fa

Fa-Te

Question 5

Forsterite (Fo)

Answer 5

Magnesium

Mg2SiO4

Question 6

Fayalite (Fa)

Answer 6

Ferric; Iron

Fe2SiO4

Question 7

Peridot

Answer 7

Semi-precious gem-quality Forsterite (Fo) Olivine

Question 8

Solid Solution

Answer 8

a mixture of two crystalline solids that coexist as a new crystalline solid, or crystal lattice; They can completely mix.

Question 9

Symmetry of Olivine crystallisation

Answer 9

Orthorhombic

Question 10

Forsterite melting point

Answer 10

1890 C due to it being MAGNESIUM rich

Question 11

Fayalite melting point

Answer 11

1200 C as it it IRON rich

Question 12

Serpentinization

Answer 12

Serpentinization is a process whereby ultramafic rock is changed, with the addition of water into the crystal structure of the minerals found within the rock.

Question 13

Serpentine polymorphs

Answer 13

Lizardite
Chrysotile (white asbestos)
Antigorite

Question 14

Olivine colour in thin section

Answer 14

Colorless, non-pleochroic

Question 15

Olivine form in thin section

Answer 15

anhedral in intrusive or metamorphic, euhedral in volcanics

Question 16

Olivine relief in thin section

Answer 16

High positive

Question 17

Olivine cleavage in thin section

Answer 17

None, but lots of irregular fracturing

Question 18

Olivine birefringence in thin section

Answer 18

Strong up to 2nd order reds.

Question 19

Olivine extinction in thin section

Answer 19

Extinction. Parallel to crystal edges.

Question 20

Olivine twinning in thin section

Answer 20

Rare, but zoning with iron-rich outer rim is common

Question 21

Olivine distinguishing features in thin section

Answer 21

Considerable fracturing, external zoning. Similar to Diopside, but diopside has oblique extinction and good cleavage

Question 22

Paragenisis

Answer 22

paragenesis, the sequence in which the minerals are formed in an ore deposit.

Question 23

Forsterite Olivine Paragenisis

Answer 23

Fo is common in mafic rocks e.g. basalt, gabbros. Fo is a major component of ultramafic rocks e.g. dunites and peridotites

Question 24

Fayalite Olivine Paragenisis

Answer 24

Rare, found in late-stage volcanics and thermally metamorphosed iron-rich sediments

Question 25

Most common Olivine

Answer 25

Forsterite, although Fayalite is also relatively common

Question 26

Olivine structure

Answer 26

• They are characterised by a structure of M2SiO4.
• Its structure is composed of isolated SiO4 tetrahedra that alternate pointing up and down in rows running parallel to the c-axis.
• Each level of tetrahedra is linked via octahedra which contain the cation sites
• The Oxygens lie in sheets arranged in hexagonal-close packing
• 2 interstitial sites, M1 and M2
• M2 is larger and forms a side branch to each chain
• M1 forms an edge sharing chain parallel to the c axis
• Olivines have orthorhombic crystal structure