Formula: (Fe,Fe,Mg)2-3(Si,Al)2O5(OH)4
Origin of name: For Pierre Berthier, French chemist/geologist (1782-1861)
Crystal system: Monoclinic (-1M), hexagonal (-1T)
Crystal class: 2/m (-1M), 3m (-1T)
Twinning:
Unit cell: -1M: a = 5.41 Å, b = 9.33 Å, c = 7.28 Å, b = 104.5; -1T: a = 5.415 Å, c = 7.114 Å
Colour: Green
Diaphaneity: Transparent
Luster:
Habit: Microcrystalline oolitic aggregates
Hardness: 1
Specific gravity: 3.03
Cleavage: None
Tenacity:
Streak:
Synonyms/varieties:
Comments: 1M and 1T polytypes. Kaolinite-serpentine group. Commonly found in unmetamorphosed marine sediments, in lateritic and polar soils.
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