Cairncrossite is a new phyllosilicate species found in manganese ore on dumps of the Wessels Mine, Kalahari Manganese Field, South Africa. Associated minerals are richterite, sugilite, lizardite and fibrous pectolite. It occurs as radiating platy micaceous aggregates of up to 1 cm in size. Cairncrossite is colourless, appearing white, and the crystals are translucent to transparent with a white streak and vitreous to pearly lustre. The crystals are sectile before brittle fracture, with a Mohs hardness of 3. A perfect cleavage parallel (001) is observed. The calculated density is 2.486 g cm−3. The mineral is biaxial positive with nα = 1.518(2), nβ = 1.522(2), nγ = 1.546(2), 2Vobs = 33.9(6)° (2Vcalc = 44.97°) at 589.3 nm and 24°C. The orientation of the indicatrix is Z ^ c* = 10°. The dispersion is weak (r < v) and no pleochroism is observed. An intense light-blue fluorescence is emitted under shortwave UV radiation. Cairncrossite is triclinic, space group P1¯, a = 9.6265(5), b = 9.6391(5), c = 15.6534(10) Å, α = 100.89(1), β = 91.27(1), γ = 119.73(1)°, V = 1227.08(13) Å3, Z = 1. The strongest lines in the Gandolfi X-ray powder-diffraction pattern [d in Å(I)(hkl)] are 15.230 (100)(001), 8.290 (15)(1–10), 5.080(25)(003), 3.807(30)(004), and 3.045(20)(005). The chemical composition obtained by electron-microprobe analysis is Na2O 3.06, K2O 0.11, CaO 18.61, SiO2 54.91, SrO 11.75, total 88.44 wt%. The relevant empirical formula, based on 16 Si atoms per formula unit (apfu) and TGA data is: Sr1.99K0.02Ca5.81Na1.73 Si16O55.84H30.33. Taking variable sodium contents into account, the idealized structural formula is Sr2Ca7–xNa2x(Si4O10)4(OH)2(H2O)15–x with 0 ≤ x ≤ 1, and the simplified formula for sodium-rich crystals is SrCa3Na(Si4O10)2(OH)(H2O)7 with Z = 2. The structure of cairncrossite was refined on single-crystal X-ray data (MoKα radiation) to R1 = 0.047. Cairncrossite belongs to the gyrolite and reyerite mineral groups, it is characterized by sheets consisting of edge-sharing CaO6 octahedra, which are corner-linked on both sides to silicate layers. These units are intercalated by layers formed by SrO8 polyhedra, which are arranged in pairs via a common edge, and further bound to disordered NaO6 polyhedra. A complex system of hydrogen bonds strengthens the linkage to adjacent silicate layers. Cairncrossite exhibits a two-phase endothermic weight loss of the H2O molecules in the range 25–400°C; however, the mineral shows a nearly complete rehydration capability up to 400°C. The new mineral is named in honour of Bruce Cairncross, Professor and Head of the Department of Geology, University of Johannesburg.

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