Cuprodongchuanite, Pb₄CuZn₂(PO₄)₂(PO₄)₂(OH)₂: a new mineral from Yunnan province, China, and the Jahn-Teller effect in dongchuanite group minerals

Cuprodongchuanite (IMA2021-065), Pb 4 CuZn 2 (PO 4 ) 2 (PO 4 ) 2 (OH) 2 , is a new mineral from Sanguozhuang Village, Yunnan Province, China.It occurs as striking apple green crust-like radial or spherical aggregates of microscopic blade-like crystals on hemimorphite.Cuprodongchuanite is brittle with an irregular fracture, a Mohs hardness of 2-3, and perfect cleavage on {011}.The calculated density is 6.10 g/cm 3 .The empirical chemical formula is (Pb 4.06 Ca 0.02 ) 4.08 (Cu 1.07 Cd 0.01 ) 1.08 Zn 2.03 [(P 0.74 As 0.20 S 0.01 Si 0.01 ) 0.96 O 4 ] 2 (PO 4 ) 2 (OH) 2 based on 18 O atoms per formula unit.Cuprodongchuanite is triclinic, space group P-1(#2), with unit-cell parameters.a = 4.7405(3) , b = 8.5023(5) , c = 10.2897(6), = 97.206(5), = 101.877(5), = 91.786(5), V = 401.95(4) 3 , and Z = 1.Cuprodongchuanite's name combines the prefix "cupro" (for copper) with the root "dongchuanite".As a member of the dongchuanite group, cuprodongchuanite features a dongchuanite-type structure with the general formula A 4 VI B IV B 2 (X1O 4 ) 2 (X2O 4 ) 2 (OH) 2 .In addition to providing a detailed characterisation of cuprodongchuanite, this study delves deeper into exploring the Jahn-Teller effect of Cu 2+ ions at the VI B site, an important factor in the mineral's crystallography.Employing the oct parameter, the study quantitatively links the degree of octahedral distortion to the Cu-Zn ratio.This method provides a refined understanding of the dongchuanite-type structure and offers an example for assessing similar mineral compositions where cations exhibit closely related properties.