Andradite

Formula:  Ca₃Fe₂(SiO₄)₃

Origin of name:  For J.B. d'Andrada e Silva (1763-1838), Brazilian mineralogist, who described a variety

Crystal system:  Cubic

Crystal class:  4/m -3 2/m

Twinning:

Unit cell:   a = 12.06 Å

Colour:  Shades of yellowish green, green, brown, red-brown, black

Diaphaneity: Transparent to translucent

Luster:  Vitreous, adamantine to resinous

Habit:  Crystals usually dodecahedrons or trapezohedrons;  massive; granular

Hardness:  6.5 to 7

Specific gravity:  3.8 to 3.9

Cleavage:  None

Tenacity:  Brittle

Streak:  White

Synonyms/varieties:  Melanite (black Ti-rich variety), topazolite (yellow variety), demantoid (green gem variety)

Comments:  Series with grossular and with schorlomite. Member of the garnet group. Formed in skarns from contact metamorphosed impure limestones or calcic igneous rocks; in chlorite schists and serpentinites; alkalic igneous rocks, then typically titaniferous.