Macphersonite, a new mineral from Leadhills, Scotland, and Saint-Prix, France— a polymorph of leadhillite and susannite

Abstract

Macphersonite is white, resinous to adamantine, hardness (Mohs) 2½−3, density 6.50–6.55 gm/cm³ and possesses a perfect cleavage on {010}. Optically it is negative with 2V_α 35–36°, α = 1.87, β = 2.00 and γ = 2.01, α = b, γ = c, and γ = a, dispersion r > v. Polysynthetic twinning, with either coarse or fine lamellae, is common, as are contact twins. Crystals are orthorhombic, tabular on b with a 10.37, b 23.10 and c 9.25 Å, cell volume 2215.8 ų and space group Pcab; Z is 8 formula units. The seven strongest lines in the X-ray powder pattern are 3.274 (50) 052; 3.234 (100) 251; 2.654 (90) 351,203; 2.598 (30) 172, 400; 2.310 (30) 004, 371, 0.10.0; 2.182 (30) 263; and 2.033 (30) 234, 452, 154, 1.10.2. Electronprobe-microanalysis-determined chemistry leads to the empirical formula (Pb_4.08, Cu_0.10, Cd_0.07)_Σ4.25S_0.90 C_2.18O_10.55(OH)_1.58 which yields the ideal formula Pb₄(SO₄)(CO₃)₂(OH)₂ and hence macphersonite is a polymorph of leadhillite and susannite. The infrared spectrum shows basic similarities to leadhillite and susannite spectra with additional diagnostic absorption bands. Macphersonite shows an identical thermogravimetric behaviour to that of leadhillite. It is associated, in varying combinations, with leadhillite, susannite, cerussite, caledonite, pyromorphite, scotlandite, galena, and the ‘lead hydroxyapatite’ of Temple (1955). The new mineral mimics leadhillite.