Formula: (Fe,Mg)4Al17(Si,Al)8O45(OH)3
Origin of name: from Greek for cross
Crystal system: monoclinic, pseudo-orthorhombic
Crystal class: 2/m
Twinning: common on {031} or {231} as 90° or 60° cruciform penetration twins
Unit cell: a = 7.87 Å, b = 16.58 Å, c = 5.64 Å, b = 90.2°
Colour: dark brown, reddish brown, yellowish brown, brownish black
Diaphaneity:
Luster: vitreous to resinous
Habit: crystals short prismatic, rough surfaces
Hardness: 7 to 7.5
Specific gravity: 3.65 to 3.83
Cleavage: 1; {010} distinct
Tenacity: subconchoidal to uneven, brittle
Streak: colourless, grayish
Synonyms/varieties: lusakite (Co-rich variety)
Comments:
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