Hydroxymcglassonite-(K), KSr4Si8O20(OH)·8H2O, the first Sr-bearing member of the apophyllite group, from the Wessels mine, Kalahari Manganese Field, South Africa

Abstract A new mineral species, hydroxymcglassonite-(K), ideally KSr4Si8O20(OH)·8H2O, has been found in the Wessels mine, Kalahari Manganese Field, Northern Cape Province, South Africa. It is granular (<0.05 mm), associated with meieranite, sugilite, aegirine, pectolite, and yuzuxiangite. The mineral is colorless, transparent with a white streak and a vitreous luster. It is brittle and has a Mohs hardness of 4.5–5.0; cleavage is perfect on {001} and no parting or twinning was observed. The measured and calculated densities are 2.60(3) and 2.614 g/cm3, respectively. Optically, hydroxymcglassonite-(K) is uniaxial (+), with ω = 1.555(5), ε = 1.567(5) (white light), and absorption O > E. Hydroxymcglassonite-(K) is insoluble in water or hydrochloric acid. An electron microprobe analysis yielded an empirical formula (based on 13 non-H cations pfu) K1.01(Sr2.99Ca1.03)Σ4.02Si7.99O20(OH)·8H2O, which can be simplified to K(Sr,Ca)4Si8O20(OH)·8H2O. Hydroxymcglassonite-(K) is tetragonal with space group P4/mnc and unit-cell parameters a = 9.0792(2), c = 16.1551(9) Å, V = 1331.70(9) Å3, and Z = 2. It is isostructural with hydroxyapophyllite-(K), KCa4Si8O20(OH)·8H2O, with Sr substituting for Ca. The crystal structure of hydroxy-mcglassonite-(K) is characterized by SiO4 tetrahedra sharing corners to form (Si8O20)8– sheets parallel to (001), which are connected by the K and B (= Sr + Ca) cations, as well as hydrogen bonding. The K cation is coordinated by eight H2O groups, and the average K–O distance of 2.941(3) Å is shorter than that of 2.950(3)–2.975(3) Å in hydroxyapophyllite-(K) or fluorapophyllite-(K). The B cation is sevenfold-coordinated (4O + 2H2O + OH), and the average B-O distance of 2.522(3) Å is noticeably longer than that of 2.422–2.435 Å in hydroxyapophyllite-(K) or fluorapophyllite-(K). The Raman spectra of hydroxymcglassonite-(K) and hydroxyapophyllite-(K) are very comparable, especially in the O-H stretching region. The discovery of hydroxymcglassonite-(K), the first Sr-bearing mineral of the apophyllite group, implies that more Sr-bearing members of the group may be found in nature or synthesized in laboratories, but the possibility for an incomplete solid solution between hydroxyapophyllite-(K) and hydroxymcglassonite-(K), due to the size difference between Sr2+ and Ca2+, cannot be ruled out.