Inosilicates: Amphiboles

Question 1

Amphiboles

Answer 1

Amphiboles are a group of minerals within the Inosilicate (Chain Silicate) class that have a double chain structure with 4 cation sites.

Question 2

Amphiboles General Formula

Answer 2

A(0-1) X2 Y5 Z8 O22 (OH,F)2

Question 3

Amphiboles 3 Subgroupings

Answer 3

Ca-poor
Ca-rich
Alkali (Na-rich)

Question 4

Hornblendes

Answer 4

General term for a grouping of 4 Ca-rich amphibole minerals that are widespread and have complete solid solution

Question 5

Amphibole Crystal System

Answer 5

Ca-poor (Mg-rich) - Orthorhombic

Rest of them (Ca-rich, Na-rich) - Monoclinic

Question 6

Amphibole structure

Answer 6

Amphiboles are characterised by a structure in which SiO4 tetrahedra form double chains twice the width of pyroxenes. A major feature in that there is an OH group at the center of a hexagonal ring formed by the tetrahedra. Amphiboles are hydrated and chemically flexible due to their structure.

Question 7

Actinolite/Tremolite (solid solution) in thin section

Answer 7

Weak coloured amphibole 
Colour: Colourless or pale green with weak pleochroism in actinolite (Fe-Rich) 
Pleochroism: None or weak
Form: Rhombic or long prismatic
Relief: High
Cleavage: 2 at 120 degrees
Birefringence: low to middle 2nd order
Extinction: Inclined around 20 degrees
Twinning: Frequent

Question 8

Glaucophane

Answer 8

Blue-violet amphibole
Colour: PALE BLUE 
Pleochroism: BLUE TO VIOLET
Form: Prismatic and rhombic
Relief: High
Cleavage: 2 at 120 degrees
Birefringence: 1st order violet
Extinction: Inclined 10 degrees
Twinning: Rare

Question 9

Amphiboles common characteristics in thin section

Answer 9

Pleochroic 
Form: Prismatic or rhombic
Relief: High
Cleavage: 2 intersecting at 120 degrees
Extinction: Inclined 10-30 degrees
Twinning: Often

Question 10

Hornblende in thin section

Answer 10

Colour: Green/Brown
Pleochroism: Strong
Form: Prismatic
Relief: High
Cleavage: 2 intersecting at 120 degrees
Birefringence: Mid Second order
Extinction: Inclined at 10-30 degrees
Twinning: Common

Question 11

Hornblende distinguishing features in thin section

Answer 11

Amphibole charachteristics, brown/green

Resembles Biotite but less perfect cleavage and had 2 rather than 1. Also, biotite has parallel extinction.

Question 12

Amphiboles vs Pyroxenes (9)

Answer 12

Amphiboles are more elongated
Amphiboles are diamond or hexagonal while pyroxenes are square hexagonal
Amphibole Cleavages are at 120 degrees while pyroxene at 90 degrees
Amphiboles are usually pleochroic, Pyroxenes are not
Amphibole Extinction Angles are Inclined, Pyroxenes are parallel or oblique
Exsolution lamellae are in Pyroxenes