Here are your brief notes on Amphioboles

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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.

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Chemical composition:

Hydrous silicates of Magnesium, Iron, Calcium, Aluminium as well as Alkali metals.

Physical properties:

1. Form:

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Some minerals of the group are distinctly orthorhombic, and monoclinic, others are triclinic in their crystallisation. [Accordingly each group shows different characteristics under micro­scope].

(a) Orthorhombic. (i) Anthophyllite (MgFe)7(Si40I1)(0H)2,.

(b) Monoclinic

2. Colour:

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Most of the members are greenish black in colour.

3. Lusture:

Vitreous.

4. Cleatage:

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2 sets, perfect, prismatic (110),

6. Sp. Gravity:

2 5 to 3 5.

7. Twinning:

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Contact twins are common.

Optical properties:

(i) Green colour and commonly pleochroic.

(ii) Ref-index. High.

(iii) Interference colour.

(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.

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