Revision of the geometrical and optical crystallography of roselite—(Ca, Co, Mg)3As2O8·2H2O from Schneeberg, Saxony, leads to results that differ in many important respects from the classical data of Schrauf (1874). Roselite proves to be monoclinic, prismatic; a:b:c = 0.8780:1:0.4398, β = 100°53′ forms: c{001}, b{010}, a{100}, j{120}. k{350}, l{230}, m{110}, n{210}, o{012}, p{111}, q{111}, r{238}, s{122}, t{454}; cleavage {010}; twin plane (100). Pale rose crystals show: X (pale rose):[001] = +0° to 1°; Y (paler rose) = [010]; Z (palest rose); indices (Na): nX = 1.694, nY = 1.704, nZ = 1.719; positive; 2V = 75°; r<v. Dark rose crystals are zoned; X (deep rose) = [010]; Y(pale rose): [001] = + 12° to 20°; Z (paler rose); indices (Na): nX = 1.725, nY = 1.728, nZ = 1.735; positive; 2V = 60°; r<v. As now described, roselite is homeomorphous with brandtite—Ca2MnAs2O8 · 2H2O, as described by Aminoff (1919).

Roselite is another case in which the lattice with the highest pseudo-symmetry (pseudo-orthorhombic) is a multiple lattice of the proper crystal lattice. In all such cases the Rule of Highest Pseudo-Symmetry gives unsatisfactory morphological elements and abnormal form symbols.

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