Motukoreaite from the Kłodawa Salt Dome, Central Poland

Motukoreaite, Mg6Al3(OH)18[Na(H2O)6][SO4]2·6H2O, is a poorly-known, rare layered double hydroxide mineral belonging to the wermlandite group in the hydrotalcite supergroup. It was found in abundant quantities (at least hundreds of thousands of Mt) in the roof part the Younger Potash unit of the Leine cyclothem on the Kłodawa Salt Dome in central Poland (Permian–Mesozoic basin of Central Europe), outside its known typical environments like altered basalts and basaltic tuffs, including those deposited in submarine volcanic areas. The mineral displays varying SO2–4, Na+ and H2O contents with negligible CO2–3, corresponding to the mean composition Mg5.75(Al3.20Fe3+0.04)∑3.25(OH)18.00{[Na(H2O)6]0.72□0.28}[(SO4)1.67(CO3)3.03(OH)0.57]·0.72H2O. It can be related to the partly dehydrated, hypothetical end-member Mg6Al3(OH)18[Na(H2O)6][(SO4)2]·6H2O. The Kł odawa motukoreaite represents the hexagonal, 3-layer polytype with the 3 × 3 superstructure in the xy plane, and unit cell a = b = 9.191(2)–9.199(2) Å, i.e. = 3 × 3.064–3.066 Å and c = 33.529(9)–33.562(7) Å, i.e. = 3 × 11.174–11.187 Å. The mineral was formed by alteration of clays delivered to the basin by an aeolian overprint and co-sedimented along with the evaporite sequence, initiated under the influence of Mg2+ - and SO2–4-bearing brines at temperatures not exceeding 160–200°C, released from the evaporites during diagenetic/metamorphic processes.