Jasonsmithite, a new phosphate mineral with a complex microporous framework, from the Foote mine, North Carolina, U.S.A

Abstract

Abstract Jasonsmithite (IMA2019-121), Mn42+ZnAl(PO4)4(OH)(H2O)7·3.5H2O, is a pegmatite-phosphate mineral from the Foote Lithium Company mine, Kings Mountain district, Cleveland County, North Carolina, U.S.A. It is interpreted as having formed by late-stage, low-temperature hydrothermal alteration. Crystals are colorless to light brown, slightly flattened prisms to about 1 mm in length with wedge-shaped terminations. The mineral is transparent with vitreous luster, white streak, Mohs hardness 2, brittle tenacity, irregular fracture, and perfect {001} cleavage. The density is 2.63(2) g/cm3. Jasonsmithite is biaxial (–), with α = 1.561(2), β = 1.580(2), γ = 1.581(2), measured in white light. The 2V is 25(5)° and dispersion is r < ν moderate. The optical orientation is Y = b, X ^ c = 18° in obtuse β. The Raman spectrum is dominated by vibrational modes of PO4 and ZnO4 tetrahedra, AlO6 and MnO6 octahedra, and OH groups. Electron microprobe analyses gave the empirical formula (Mn3.09Fe0.87)Σ3.96Zn1.05Al0.98(PO4)4(OH)(H2O)7·3.5H2O. The mineral is monoclinic, P21/c, a = 8.5822(3), b = 13.1770(6), c = 20.3040(14) Å, β = 98.485(7)°, V = 2271.0(2) Å3, and Z = 4. The structure (R1 = 0.0443 for 3685 I>2σI reflections) contains zigzag chains of edge-sharing MnO6 octahedra that corner-link to adjacent chains and to PO4 tetrahedra to form sheets, which are decorated by ZnO4 tetrahedra. The sheets are linked to one another via dimers of AlO6 octahedra, forming a framework with large channels containing H2O groups. With H2O groups removed, the framework has a void space of 70.2% per unit cell, and a framework density of 14.5 polyhedral atoms/1000 Å3, which would place jasonsmithite among the most porous minerals.