Here are your brief notes on Garnets

This is a group of six minerals which are isomorphism in nature, and never occur pure as represented by their chemical composition.

1. Atomic structure:

These are neso-silicates, formed of independent Si0₄ tetrahedra.

2. Chemical composition:

The general formula for the garnets is R ₃ ¹¹ R2¹¹¹Si₃0₁₂, where R¹¹ can be Fe¹¹, Mg, Mn¹¹or Ca, and R¹¹¹ can be Fe¹¹¹, A1 or Cr.

3. Classification:

Mainly there are two groups as:

(i) Pyralspite:

Containing three members like:

(a) Pyrope. Mg₃Al₂(Si0₄)₃.

(b) Almandine. Fe₃Al₂(Si0₄)₃.

(c) Spessartite. Mn₃Al₂(Si0₄)₃.

(ii) Ugrandite:

It consisting of three members like:

(а) Uvarovite. Ca₃Cr₂(Si0₄)₃.

(b) Grossularite. Ca₃Al₂(Si0₄)₃.

(c) Andradite. Ca₃Fe₂(Si0₄)₃.

4. Physical properties:

(i) Garnets are externally cubic in form (Rhombododeca- hedron).

(ii) Any colour except blue; grossularite, uvarovite green, pyrope, almandite, and spessartite are deep red to black in colour, while andradite is black in colour.

(iii) Lusture. Vitreous to sub-vitreous.

(iv) Cleavage. None,

(v) Hardness. 7.

(vi) Sp. gravity. High.

(vii) Twinning. None.

5. Optical properties:

(i) Non-pleochroic colorless, sometimes pale pink in color- uvarovite is green in colour.

(ii) Ref index. Very high,

(iii) Under crossed nicol. Dark, i.e., isotropic.

6. Varieties:

(i) Melanite. Black andradite.

(ii) Schorlomite. Titaniferous andradite.

(iii) Demontoid. Clear-green andradite.

(iv) Hessonite. Also known as cinnamon stone is a yellow- brownish variety of grossularite.

(v) Topozolite. Yellow-andradite.

Occurrence:

Garnets are usually metamorphic minerals, but some also occur in igneous and sedimentary rocks. But everywhere they are characterized by their typical rhombododecahedron and trapezohedron forms.