Hennomartinite, SrMn 2 3+ [Si 2 O 7 ](OH) 2 . H 2 O, is an orthorhombic (Cmcm, a = 6,245[1], b = 9.031[2]. c = 13,404[2] A, V = 756.0[1] A 3 , Z = 4) disilicate of the lawsonite structure type which occurs as anhedral poikiloblasts up to 1 mm, strongly intergrown with sugilite and serandite-pectolite. The mineral is translucent in thin section, pleochronic from yellowish brown to dark red-brown and has indices of refraction greater than 1,82. The calculated density is 3.68 g/cm 3 . The chemical composition was determined by electron microprobe analyses and IR-spectroscopy. The trivalent character of Mn is derived from stoichiometry and bond lengths obtained from a crystal structure analysis
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Armbruster et al. (1993) studied this question.