Pickeringite from the Stone Town Nature Reserve in Ciężkowice (the Outer Carpathians, Poland)

Pickeringite, ideally MgAl2(SO4)4·22H2O, is a member of the halotrichite group minerals XAl2(SO4)4·22H2O that form extensive solid solutions along the joints of the X = Fe-Mg-Mn-Zn. The few comprehensive reports on natural halotrichites indicate their genesis to be mainly the low-pH oxidation of pyrite or other sulfides in the Al-rich environments of weathering rock-forming aluminosilicates. Pickeringite discussed here occurs within the efflorescences on sandstones from the Stone Town Nature Reserve in Ciężkowice (the Polish Outer Carpathians), being most probably the first find on such rocks in Poland. This paper presents mineralogical and geochemical characteristics of the pickeringite (based on SEM-EDS, XRPD, EPMA and RS methods) and suggests its possible origin. It belongs to the pickeringite–apjohnite (Mg-Mn joints) series and has the calculated formula Mg0.75Mn0.21Zn0.02Cu0.01Al2.02(S0.99 to 1.00O4)4·22H2O (based on 16O and 22H2O). The unit cell parameters refined for the monoclinic system space group P21/c are: a = 6.1981(28) Å, b= 24.2963(117) Å, c = 21.2517(184) Å and β = 100.304(65)°. The Raman spectra (SO4) bands are the intensive 994 cm−1 and a low-intensive 975 cm−1 (ν1), low-intensive 1081, 1123 and 1145 cm−1 (ν3), 524, 467 and 425cm−1 (ν2), 615 cm−1 (ν4), while those at 344 and 310 cm−1 are attributed to νg H2O and at 223 cm−1 to the lattice modes. Crystallization of pickeringite within the particular tor resulted from a certain set of conditions: climatic (e.g., season, temperature, humidity), physicochemical (e.g., pH, concentration), mineral (the presence of pyrite), and site-related (location and efflorescence protection). The sulfate ions could have been derived from oxidation of pyrite in the Ciężkowice sandstones and possibly are related to local mineral waters.