These are hydrous ferromagnesian silicate minerals and along with pyroxene constitutes about l/5th of all the known rocks.
Atomic structure:
Inosilicates characterised by double chain structure.
Chemical composition:
ADVERTISEMENTS:
Hydrous silicates of Magnesium, Iron, Calcium, Aluminium as well as Alkali metals.
Physical properties:
1. Form:
Some minerals of the group are distinctly orthorhombic, and monoclinic, others are triclinic in their crystallisation. [Accordingly each group shows different characteristics under microscope].
ADVERTISEMENTS:
(a) Orthorhombic. (i) Anthophyllite (MgFe)7(Si40I1)(0H)2,.
(b) Monoclinic
2. Colour:
Most of the members are greenish black in colour.
ADVERTISEMENTS:
3. Lusture:
Vitreous.
4. Cleatage:
2 sets, perfect, prismatic (110),
ADVERTISEMENTS:
6. Sp. Gravity:
2 5 to 3 5.
7. Twinning:
Contact twins are common.
ADVERTISEMENTS:
Optical properties:
(i) Green colour and commonly pleochroic.
(ii) Ref-index. High.
(iii) Interference colour.
ADVERTISEMENTS:
(iv) Extinction.
Orthoamphiboles parallel.
Clinoamphiboles 10° to 20°.
Varieties:
(i) Smargdite:
Foliated acttonolite.
(ii) Uralite:
Secondary tremolite-actinolite.
(iii) Asbestos:
(a) Amosite-Fibrous anthophyllite.
(b) Actinolite-Asbestos proper.
(c) Crocidolite-Fibrous riebeckitc.
(d) Chrysotile-Fibrous serpentine.
(iv) Nephrite:
Fine grained tremolite-actinolite.
Occurrence:
Mostly occur in metamorphic rocks and also in igneous rocks. Crocidolite occurs in highly siliceous metamorphic rocks.