Abstract
Two new minerals, nigelcookite, ideally PbFe2+2V3+2(PO4)3(OH)3 and plumbojohntomaite, ideally PbFe2+2Fe3+2(PO4)3(OH)3 have been found in the Yushui Cu deposit, South China. They occur in the bedded/massive Cu ore, and often occur in association with chalcopyrite, galena, sphalerite, nolanite, pyrite, anhydrite, apatite, fluorite, xenotime-(Y), and quartz. They are both new members of the bjarebyite group, exhibiting the general formula XM12M22(PO4)3(OH)3. Nigelcookite occurs as spearhead-shaped or platy crystals ranging from 5 to 100 μm in size, and plumbojohntomaite occurs as platy crystals ranging from 5 to 80 μm in size. Both minerals are translucent to transparent with vitreous luster but they differ in color. Nigelcookite ranges from dark brown to greenish brown while plumbojohntomaite varies from dark green to olive green. Calculated on the basis of 8 cations, the empirical formulas are Pb0.98(Fe2+1.69Mn0.24Cu0.04Zn0.03)Σ2.00(V1.31Fe3+0.63Al0.06)Σ2.00(PO4)3.02(OH)3 and Pb1.03(Fe2+1.91Mn0.02Cu0.04Zn0.03)Σ2.00(Fe3+1.95V0.05)Σ2.00(PO4)2.97(OH)3 for nigelcookite and plumbojohntomaite, respectively. Both of the two new minerals are monoclinic, with space group P21/m (#11) and Z = 2. The unit-cell parameters are a = 9.1159(5) Å, b = 12.2328(7) Å, c = 5.0092(3) Å, β = 100.708(6)°, V = 548.86(6) Å3 and Z = 2 for nigelcookite, and a = 9.0999(4) Å, b = 12.1911(6) Å, c = 5.0176(2) Å, β = 100.671(4)°, V = 547.02(4) Å3 and Z = 2 for plumbojohntomaite.
Nigelcookite (IMA 2023-113) and plumbojohntomaite (IMA 2023-119) are approved by the International Mineralogical Association Commission on New Minerals, Nomenclature and Classification. They are isostructural with other minerals of the bjarebyite group, while nigelcookite is the member of the group to have V3+ as the dominant ion at the M2 site. The negative correlation between V and Fe contents indicates isomorphic substitution between V3+ and Fe3+ in nigelcookite and plumbojohntomaite. Given the Fe3+ occurrence in plumbojohntomaite, its redox formation condition is more oxidized than nigelcookite.
