Inosilicates: Pyroxenes

Question 1

Inosilicates

Answer 1

A group of minerals whose defining characteristic is their chain arrangement of silica (SiO4) tetrahedra.

Question 2

Single Chain Inosilicates

Answer 2

Pyroxenes

Question 3

Double chain Inosilicates

Answer 3

Amphiboles

Question 4

Pyroxenes General Formula

Answer 4

X Y Si2 O6

Question 5

2 main solid solution minerals

Answer 5

Augite (Au) - Diopside (Di) CaMg, Hedenbergite (He) CaFe

Hypersthene (Hy) - Enstatite (En) Mg2, Ferrosilite (Fs) Fe2

Question 6

Augite (Au)

Answer 6

Solid solution of:
Diopside (Di) CaMg
Hedenbergite (He) CaFe

A common Clinopyroxene

Question 7

Hypersthene (Hy)

Answer 7

Solid solution of:
Enstatite (En) Mg2,
Ferrosilite (Fs) Fe2

A common Orthopyroxene

Question 8

Enstatite (En)

Answer 8

Magnesium Pyroxene

Mg2 Si2 O6

Question 9

Ferrosilite (Fs)

Answer 9

Iron Pyroxene

Fe2 Si2 O6

Question 10

Diopside (Di)

Answer 10

Calcium Magnesium Pyroxene

Ca Mg Si2 O6

Question 11

Hedenbergite (He)

Answer 11

Calcium Iron Pyroxene

Ca Fe Si2 O6

Question 12

3 rare pyroxenes

Answer 12

Aegirine - Na Fe Si2 O6
Jadite - Na Al Si2 O6
Omphacite

Question 13

Aegirine

Answer 13

Na Fe Si2 O6

Question 14

Jadeite

Answer 14

Na Al Si2 O6

Question 15

Structure of Pyroxenes

Answer 15

Pyroxenes are formed by series of SiO4 tetrahedra sharing a common oxygen on two corners forming a single chain parallel to the c-axis. These chains are liked laterally with octahedron layers (100 plane) which contain the cation

Question 16

Orthopyroxene minerals composition

Answer 16

Orthopyroxenes contain a solid solution of En-Fs
Enstatite-Ferrosilite
Hypersphene is the most common orthopyroxene mineral formed

Question 17

Clinopyroxenes mineral composition

Answer 17

Contain Calcium

Diopsite + Hedenbergite With some En-Fs

Question 18

Exsolution

Answer 18

process through which an initially homogeneous solid solution separates into at least two different crystalline minerals without the addition or removal of any materials. In most cases, it occurs upon cooling below the temperature of mutual solubility or stability of the solution.

Question 19

Miscibility

Answer 19

Miscibility is the property of two substances to mix in all proportions, forming a homogeneous mixture.

Question 20

Orthopyroxene Crystal System

Answer 20

Orthorhombic

Question 21

Clinopyroxene crystal system

Answer 21

Monoclinic

Question 22

Orthopyroxene colour in thin sections

Answer 22

Pale, subtle pinkish to greenish pleochroism in some

Question 23

Clinopyroxene colour in thin sections

Answer 23

Colourless or grey/pale green/pale brown: darker if Fe rich

Question 24

Pyroxene form in thin sections

Answer 24

Subhedral stubby prisms, basal sections 4-8 sided

Question 25

Pyroxene relief in thin sections

Answer 25

High positive

Question 26

Pyroxene cleavage in thin sections

Answer 26

2 good cleavages at 90 degrees to one another parallel to prism faces

Question 27

Orthopyroxene birefringence in thin sections

Answer 27

Weak First order yellow

Question 28

Clinopyroxene birefringence in thin sections

Answer 28

Strong 2nd order green and red

Question 29

Orthopyroxene extinction in thin sections

Answer 29

PARALLEL

Question 30

Clinopyroxene extinction in thin sections

Answer 30

OBLIQUE

Question 31

Orthopyroxene twinning in thin sections

Answer 31

rare

Question 32

Clinopyroxene twinning in thin sections

Answer 32

Common

Question 33

Clinopyroxene distinguishing features in thin sections

Answer 33

2 good cleavages at ~90o, high birefringence, twinning and oblique extinction.

Question 34

Orthopyroxene distinguishing features in thin sections

Answer 34

2 good cleavages at 90, low birefringence, parallel extinction.

Question 35

Paragenisis of pyroxenes

Answer 35

Ultrabasic rocks often with olivine