Formula: Fe3(PO4)2.8H2O
Origin of name: for J.G. Vivian, English mineralogist
Crystal system: Monoclinic
Crystal class: 2/m
Twinning: Translation gliding on {010}, lamellar
Unit cell: a = 10.03 Å, b = 13.45 Å, c = 4.71 Å, b = 102.6°
Colour: colourless, green to dark purplish or bluish black
Diaphaneity: Transparent to translucent to opaque
Luster: Vitreous, pearly on cleavage, dull when earthy
Habit: Crystals short to long prismatic, elongated on c-axis, often flattened, also equant or tabular, massive, bladed or fibrous, earthy
Hardness: 1.5 to 2
Specific gravity: 2.6-2.7
Cleavage: 3; {010} perfect, {-100} and {100} trace
Tenacity: Sectile, fibrous, thin lamellae flexible
Streak: Colourless to bluish white, altering to dark blue, brown
Synonyms/varieties: Blue iron earth
Comments: darkens on exposure to light
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