A 1:2 Mg phyllosilicate, Mg3Si4O10(OH)2, best known for its extreme lark of hardness; crystals are rarely seen. The is little variation in the composition. Small amounts of Al3+ may replace S4+ and Fe2+ or Mn2+ may replace Mg2+. The Fe-rich end-member, minnesotaite, Fe3Si4O10(OH)2, may be isostructural with talc and a complete solid solution probably exists between it and talc.

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