Desautelsite, a new mineral of the pyroaurite group

Desautelsite, $Mg6Mn23+(CO3)(OH)16⋅4H2O$⁠, is the trivalent manganese analog of pyroaurite. The mineral occurs at two localities in San Benito County, California, and at the type locality, the Cedar Hill quarry, Lancaster County, Pennsylvania. At both localities it is associated with altered serpentines. Desautelsite is hexagonal, space group R3m or R3m with a = 3.114(1) and c = 23.39(2)A, V = 196.4A³, Z = 3/8. The strongest lines in the X-ray diffraction pattern (d, in A; intensity; hkl) are: 7.76 100 0003, 3.88 60 0006, 2.332 60 0115, 1.981 60 0118 and 2.622 50 0112.

Desautelsite is bright orange in color and is the last mineral to form in the paragenesis. Optically, it is uniaxial (—) with ɛ = 1.547 and ω = 1.569. The density is 2.13 g/cm³, in excellent agreement with the theoretical value of 2.10 g/cm³. Morphologically, desautelsite forms simple hexagonal crystals, tabular along [0001], and shows the forms {0001} and {1010}. Microchemical tests provide a ready means for distinguishing between desautelsite and pyroaurite.