Abhrak (Bhasma)

English Name

Biotite Mica

Definition

Abhraka is Biotite mica, a ferromagnesian silicate K (Mg Fe)3X(Si3AlO11) X (OH)2}. It is a black variety of mica group of minerals.

Synonyms

Samskata : Ghana, Vyoma, Abhra, Ni?candra, Vyomaka, Vajra, Vajr?bhra, Krsnabhra

Regional Name

Bangali - Abhraka, English - Mica, Gujarati - Abhraka, Hindi - Abhrak, Abarak, Kannada - Abhraka, Malayalam - Abrakam, Marathi - Abhraka, Tamil - Abragam, Karuppu Appirakam (S.F.I.), Telugu - Abbarakam, Urdu - Abrak Siyah (N.F.U.M.)

Broad Classification

A Phyllo-Silicate

Origin and occurrence:

Abhrak occurs in igneous rocks in pure form as well as in metamorphic rocks as schists and gneisses. It occurs in ore form in pegmatites but pure mineral in sheet-form can be made available through sorting in pegmatite bodies. It is found in all igneous terrains of the earth’s crust. In India, it occurs mostly in the mica belts of Rajasthan, Andhra Pradesh and Jharkhand. The main deposits in these states occur at Ajmer, Bhilwara, Tonk and Pali in Rajasthan, Kalichedu, Thalpur and Gudur in Andhra Pradesh, Koderma and Giridih in Jharkhand. Other known occurrences are in the states of Tamil Nadu, Karnataka, Kerala and West Bengal.

Optical properties

Anisotropic, Biaxial Negative, small 2V and strong birefringence. Refractive Index: ηα 1.565-1.625; ηβ 1.605- 1.696; ηγ 1.605-1.696 (Appendix-2).

Physical Properties

Nature: Platy (separable in thin layers), Colour: Greenish black, Streak: Greenish black, Cleavage: Perfect, Fracture: Uneven, Lustre: Splendent Tenacity: Flexible 2, Transparency: Translucent, Hardness: 2.5 to 3, Sp. Gr. : 2.6 to 3

Chemical Properties:

• Effect of Heat: Hold a piece of Abhrak by forceps and heat it over a burner flame in its outer zone (about 10000). It swells almost double in volume. Colour changes from black to silver moon while water is released.
• Solubility: Take about 1 g finely powdered (150 mesh) sample of Abhrak in 250 ml beaker. Add 50 ml sulphuric acid. Stir the solution. It decomposes leaving skeleton of silica (distinction from other micas which are not affected by sulphuric acid).
• Assay: Should contain not less than 50% silica (SiO2) when analysed by gravimetric method (Appendix-3.1.3).
• Heavy metals and Arsenic: Should not contain more than the stated limits for the following: - Lead = 45 ppm, Arsenic = 3 ppm, and Cadmium =2 ppm Appendix-3.2).
• Other Elements: May contain the following within ± 20% of the stated limits:- Iron = 6%, Aluminium = 5%, Magnesium = 9% and Potassium = 5% (Appendix-3.1 & 3.2).

?odhana

Shall not be used in the formulation without subjecting it to ?odhana. Abhrak is always used in the form of bhasma, the therapeutic uses, dose and other details of which are given in the monograph of bhasma.