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 Compositional variation resulting from the substitution of one ion or ionic compound for another ion or ionic compound in an isostructural material. This results in a mineral structure with specific atomic sites occupied by two or more ions or ionic groups in variable proportions. Solid solutions can be complete (with probably exists between it and talc.