La brassite, MgHAsO4 • 4H2O, une nouvelle espèce minérale

Abstract

The study of artificial crystals allows a complete description of a new-mineral species, observed as early as 1958, under the shape of crusts and white, cryptocristalline and pulverulent coatings, on specimens from various origins : Jachymov (Bohemia) (type-deposit), Bieber (Saxony), Neurode (Silesia) and Wittichen (Baden). The name of brassite is in honour of Réjane Brasse. Brassite is orthorhombic Pbca a = 7.472, b = 10.891, c = 16.585 Å, Z = 8, dmes. = 2.28, dₓ = 2.326. Main powder diagram lines : 4.97(10) 4.14(6) 3.88(5) 3.45(5) 3.20(8) 3.07(7) 2.32(5). Artificial crystals are colourless, tabular on (001), which is an excellent cleavage, rectangular and bevel edged by : (011) (021) and (102). Biaxial (+), 2V very large, ng = 1.562 on c, nm = 1.546 on a, nv = 1.531 on b. The chemical analysis of the Jachymov brassite corresponds well to the MgHAsO₄ • 4H₂O composition of the artificial product. Thermic analysis show that deshydration begins at 85° C and leads to the formation of a hemihydrate at around 135° C, then to the pyroarseniate at a temperature as high as 250° C with exothermic crystallization at around 570° C. The roesslerite deshydrates itself spontaneously into brassite, which remains stable in normal laboratory conditions. This type of formation certainly occurs in nature, but on the other hand, the direct crystallization of brassite seems to be exceptional, perhaps because of the abundance of carbonates increasing the pH of solutions in deposits rich in arsenic. Type material will be preservated at the Ecole nationale supérieure des mines de Paris.