Chemical composition: Al2Si05. Minor amounts of Fe, Mn, Cr may be present.
(i) Form: Long blade-shaped, sometimes form parallel or radiating aggregates.
(ii) Colour: Blue, darker towards center of crystal, occasionally white, gray or green.
(iii) Lustre: Vitreous to pearly.
(iv) Streak: White.
(v) Hardness: 5 parallel to length of crystal and 7 perpendicular to length.
(vi) Cleavage: Perfect two sets.
(vii) Fracture: Uneven.
(viii) Specific gravity: 3.6 – 3.7.
(ix) Diaphaneity: Transparent to translucent.
Use: Manufacture of spark plugs and refractory porcelains. 4.3.25. Pyrite
Chemical composition: FeS2. Ni and Co replace Fe; in addition Cu, V, Mo, Cr, W, Au and Ti may be present in the crystal lattice.
(i) Form: Commonly occurs as cubes (often striated), pyritohedra or octahedra. Also occur as massive, granular, reniform, globular and stalactitic.
(ii) Colour: Blue, bronze-yellow to brass-yellow.
(iii) Lustre: Metallic to splendent.
Streak: Greenish or brownish-black.
(v) Hardness: 6-7.
(vi) Cleavage: Imperfect.
(vii) Fracture: Conchoidal to uneven.
(viii) Specific gravity: 4.8 – 5.1.
(ix) Diaphaneity: Opaque.
Use: Pyrite is mined for the gold and copper associated with it. It is the source of iron metal in countries where oxide ores are not available. It is also the chief source of sulphur from which H, S04 and copperas (FeS04) are manufactured. Copperas is used in dyeing, manufacture of inks, as preservative of wood and as a disinfectant.