Parkinsonite, (Pb,Mo, □)8O8Cl2, a new mineral from Merehead Quarry, Somerset

Abstract

AbstractParkinsonite, ideally (Pb,Mo,□)8O8Cl2, is a new mineral from the Merehead Quarry, Cranmore, Somerset, England. It occurs as compact clusters or patches of red to purplish red bladed crystals, which have an adamantine lustre and a perfect {001} cleavage and occupy fractures and cavities in carbonate vughs in veins of manganese and iron oxide and hydroxide minerals. Associated minerals are mendipite, diaboleite, chloroxiphite, wulfenite, cerussite and hydrocerussite. Discrete crystals were not found; intergrown crystalline aggregates are the usual form of occurrence. The maximum grain size is about 300 × 100 µm, but most grains are appreciably smaller. Parkinsonite was synthesized using high purity chemicals. The measured density of the synthetic material is 7.32 g/cm3; the calculated density is 7.39 g/cm3, the difference being due to minor impurity and slight porosity in the synthetic sample. Parkinsonite is translucent. Reflectance spectra were obtained in air and in oil. Refractive indices calculated from these (at 589 nm) are for Ro, 2.58, and Re', 2.42, i.e. uniaxial negative. VHN50 is 113–133 from which the calculated Mobs hardness is 2–2.5.X-ray studies show that parkinsonite is tetragonal with space group I4/mmm, I4̄2m, I4̄m2, I4/mm, or I422 and a 3.9922(3), c 22.514(2) Å. It has a cell volume of 358.82(5) Å3 with Z = 1. The strongest six lines of the X-ray powder diffraction pattern are [d in Å (I) (hkl)] 2.823, 2.813(100) (110,008); 5.63(85) (004); 2.251(33) (116, 0.0.10); 2.988(27) (105); 3.750(15) (006); 1.994(11) (200,118). Averaged electron microprobe analyses give the empirical formula Pb6.34Mo0.89□0.77O8.02Cl1.98 on the basis of 10 atoms [O + Cl]. The name is for Reginald F. D. Parkinson, mineral collector of Somerset, UK, who first found the mineral.