Whiteite-(MnMnMn), a new jahnsite-group mineral species from the Foote mine, North Carolina, USA, and chemical pressure effects in jahnsite-group minerals.

Abstract

Whiteite-(MnMnMn), Mn 2+ Mn 2+ Mn 2+ 2 Al 2 (PO 4 ) 4 (OH) 2 •8H 2 O, is a new whiteite-subgroup member of the jahnsite group from the Foote Lithium Company mine, Kings Mountain district, Cleveland County, North Carolina, USA. It was found in small vugs of partially oxidised pegmatite minerals on the East dump of the mine, in association with eosphorite, hureaulite, fairfieldite, mangangordonite, whiteite-(CaMnMn) and jasonsmithite. It occurs as sugary aggregates of blade-like crystals up to 0.1 mm long and as epitaxial overgrowths on whiteite-(CaMnMn). The crystals are colourless to very pale brown, with a vitreous lustre and a white streak. The blades are flattened on {001} and elongated along [010], with poor cleavage on {001}. The calculated density is 2.82 g•cm -3 . Optically it is biaxial (-) with α = 1.599(2), β = 1.605(2), γ = 1.609(2) (white light); 2V (calc.) = 78.2°, having no observable dispersion or pleochroism, and with orientation X = b. Electron microprobe analyses and structure refinement gave the empirical formula (Mn 2+ 0.59 Ca 0.38 Na 0.03 ) Σ1.00 Mn 1.00 (Mn 2+ 1.04 Fe 3+ 0.58 Fe 2+ 0.23 Zn 0.16 Mg 0.08 ) Σ2.09 Al 2.04 (PO 4 ) 3.89 (OH) 3.18 (H 2 O) 7.26 . Whiteite-(MnMnMn) is monoclinic, P2/a, a = 15.024(3) Å, b = 6.9470(14) Å, c = 9.999(2) Å, β = 110.71(3)°, V = 976.2(4) Å 3 and Z = 2. The crystal structure was refined using synchrotron single-crystal data to wR obs = 0.057 for 2014 reflections with I > 3σ(I). Site occupancy refinements confirm the ordering of dominant Mn in the X, M1 and M2 sites of the general jahnsitegroup formula XM1(M2) 2 (M3) 2 (H 2 O) 8 (OH) 2 (PO 4 ) 4 . A review of published crystallochemical data for jahnsite-group minerals shows a consistent chemical pressure effect in these minerals, manifested as a contraction of the unit-cell parameter, a, as the mean size of the X and M1 site cations increases.This is analogous to negative thermal expansion, but with increasing cation size, rather than heating, inducing octahedral rotations that result in an anisotropic contraction of the unit cell.

Whiteite-(CaMnFe), a new jahnsite-group mineral from the Hagendorf-Süd pegmatite, Oberpfalz, Bavaria

“…Clear relationships exist between the unit-cell parameters and cation assignments in specific sites. The unit-cell a parameter is inversely related to the size of the cations occupying the M1 and X sites, as previously discussed by Grey et al (2020Grey et al ( , 2021. This is seen in Table 6 from the progressive increase in the a parameter with a decrease in the mean <X-O> oct bond length.…”