Paradimorphite, beta-As4S3, a vintage new mineral from Solfatara di Pozzuoli and Vesuvius, Napoli, Italy

The new mineral paradimorphite corresponds to the high temperature polymorph of As4S3, whose existence was supposed by Arcangelo Scacchi in 1850 in the fumaroles at the Solfatara di Pozzuoli, Campi Flegrei, near Napoli, Italy. Crystals of paradimorphite are orange yellow, transparent or semitransparent, with adamantine lustre. Habit is prismatic and observed forms are {110}, {101}, {111}, {100}, {010} and {001}. Tenacity is brittle, no distinct cleavage is observed and fracture is conchoidal. The mineral does not fluoresce in long- or shortwave ultraviolet light. No twinning is apparent. The streak is saffron yellow. Hardness (Mohs) = 1-2. The observed density is 3.510(3) g/cm(3), calculated density is 3.500 g/cm(3). The mineral is orthorhombic, space group Pnma, with a = 9.1577(7), b = 8.0332(6), c = 10.2005(8) angstrom, V = 750.41(10) angstrom(3) and Z = 4. The eight strongest powder X-ray diffraction lines are [d(obs) angstrom(I)(hkl)]: 6.299(48)(011), 5.186(100)(111), 4.174(31)(201), 3.133(34)(022), 3.116(58)(212), 2.980(41)(122), 1.846 (27)(413) and 1.808(23)(134). The structure was refined to R = 0.0229 for 979 reflections with I >2 sigma(I). Crystals of paradimorphite contain As4S3 molecules, of idealised C-3v symmetry, with the four arsenic atoms in a triangular pyramidal arrangement, with sulfur atom bridges on the three adjacent apical edges. Molecular dimensions and conformation are identical within standard uncertainties with those of the low-temperature polymorph dimorphite. No substantial differences, neither in the molecular packing nor in the molecular orientation, could be observed; minor differences being related to intermolecular distances only.