Parascorodite, FeAsO ₄ .2H ₂ O; a new mineral from Kank near Kutna Hora, Czech Republic

Abstract

Parascorodite, a new mineral from Kan k near Kutn Hora, Central Bohemia, Czech Republic, forms earthy white to white-yellow aggregates associated with scorodite, pitticite, bukovsky ite, kan kite, zy kaite, gypsum, and jarosite, wet chemical analysis gave (in wt%): As 2 O 5 44.45, P 2 O 5 0.84, SO 3 1.53, Fe 2 O 3 34.55, Al 2 O 3 0.17, H 2 O 17.81, totaling 99.95. The simplified chemical formula is FeAsO 4 2H 2 O. Selected area electron diffraction suggests hexagonal or trigonal symmetry. The extinction symbol is P-c-. Powder X-ray diffraction yielded unit-cell parameters a = 8.9327(5) , c = 9.9391(8) , V = 686.83 (8) 3 , Z = 6. Densities (measured and calculated, respectively) are D m = 3.213(3) g/cm 3 and D x = 3.212 g/cm 3 . SEM and TEM images showed that basal sections of parascorodite are hexagonal in shape; thicker prismatic crystals were also observed. Crystal size varies between 0.1 to 0.5 m. The strongest lines in the X-ray powder diffraction pattern are [d[I](hkl)]: 4.184(44)(012), 4.076(100)(111), 3.053(67)(202), 2.806(68)(211), 2.661(59)(113), 2.520(54)(212), 2.2891(44)(032). Refractive indexes could not have been measured due to extremely small crystallite size, n -(calc) = 1.797. The TG curve shows two weight losses: at 20-150 C (2.1 wt%, absorbed water) and at 150-620 C (15.5 wt%, molecular water), respectively. They correspond to the endothermic peaks on the DTA curve at 120 and 260 C. Strong exothermic reaction observed at 585 C reflects formation of the phase FeAsO 4 . Infrared absorption spectra of parascorodite are close to those of scorodite.