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 since 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}, {001}. Tenacity is brittle, no distinct cleavage is observed, fracture is conchoidal. The mineral does not fluoresce in long-or short-wave ultraviolet light. No twinning is apparent. The streak is saffron yellow. Hardness (Mohs) = 1-2. The observed density is 3.510(3) g/cm3, that calculated is 3.500 g/cm3. The mineral is orthorhombic, space group Pnma, with a = 9.1577(7), b = 8.0332(6), c = 10.2005(8) A, V = 750.41(10) A3 and Z = 4. The eight strongest X-ray powder diffraction lines are [dobs A(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?(I). Crystals of paradimorphite contain As4S3 molecules, of idealized C3v 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.