Leadhillite

Abstract: Lead (Pb) solubility is commonly limited by dissolution–precipitation reactions of secondary mineral phases in contaminated soils and water. In the research described here, Pb solubility and free Pb2+ ion activities were measured following the precipitation of Pb minerals from aqueous solutions containing sulfate or carbonate in a 1:5 mole ratio in the absence and presence of phosphate over the pH range 4.0–9.0. Using X-ray diffraction and Fourier-transform infrared spectroscopic analysis, we identified anglesite formed in sulfate-containing solutions at low pH. At higher pH, Pb carbonate and carbonate-sulfate minerals, hydrocerussite and leadhillite, were formed in preference to anglesite. Precipitates formed in the Pb-carbonate systems over the pH range of 6 to 9 were composed of cerussite and hydrocerussite, with the latter favored only at the highest pH investigated. The addition of phosphate into the Pb-sulfate and Pb-carbonate systems resulted in the precipitation of Pb3(PO4)2 and structurally related pyromorphite minerals and prevented Pb sulfate and carbonate mineral formation. Phosphate increased the efficiency of Pb removal from solution and decreased free Pb2+ ion activity, causing over 99.9% of Pb to be precipitated. Free Pb2+ ion activities measured using the ion-selective electrode revealed lower values than predicted from thermodynamic constants, indicating that the precipitated minerals may have lower KSP values than generally reported in thermodynamic databases. Conversely, dissolved Pb was frequently greater than predicted based on a speciation model using accepted thermodynamic constants for Pb ion-pair formation in solution. The tendency of the thermodynamic models to underestimate Pb solubility while overestimating free Pb2+ activity in these systems, at least in the higher pH range, indicates that soluble Pb ion-pair formation constants and KSP values need correction in the models.

Abstract: The crystal structure of macphersonite (Pb 4 SO 4 (CO 3 ) 2 (OH) 2 , Pcab, a = 9.242(2), b = 23.050(5), c = 10.383(2) Aa) from Leadhills, Scotland has been determined to an R = 0.053. The structure has many features in common with its polymorph leadhillite including three distinct types of layers. Layer A includes sulphate tetrahedra, Layer B is composed of Pb and OH, while Layer C is composed of Pb and CO 3 with topology identical to that in cerussite. In both macphersonite and leadhillite these layers are stacked along [010] as ...BABCCBABCC... The double CC layer is almost identical in the two structures and forms a structural backbone and occurs in other structures including hydrocerussite and plumbonacrite. The sulphate layer shows the greatest difference between the two structures and can be described by a pattern of up or down pointing tetrahedra. For macphersonite the sequence along [001] is ...UDUDUD... while in leadhillite the sequence along [010] is ...UDDUUDDU... This latter sequence effectively doubles b relative to the equivalent direction in macphersonite. Susannite, a third polymorph, may have yet another sequence of sulphates to give trigonal symmetry; by heating leadhillite, displacive movements of sulphate groups may occur with a conversion to susannite.

Abstract: The Raman spectrum of the basic carbonate-sulphate minerals known as leadhillite, susannite and caledonite have been measured and the spectra compared with the Raman spectra of cerussite, hydrocerussite and anglesite. Characteristic spectral patterns are observed for each mineral. The wavenumber position of the hydroxyl stretching bands is used to estimate the hydrogen bond distances in the minerals. The hydrogen bond distances for leadhillite polymorphs vary from 2.783 to 2.916 A. In comparison the estimated hydrogen bond distances for hydrocerussite are much longer with values of 2.961 and 3.127 A. The width of the hydroxyl stretching vibration provides an estimate of the variation of hydrogen bond distances for the OH groups in the mineral. The variation in bond length is greater for the longer hydrogen bonds. Characteristic sulphate and carbonate vibrations are also identified.

Abstract: Synopsis Mattheddleite, a new lead member of the apatite group with sulphur and silicon totally replacing phosphorus, occurs as tiny crystals ( a 9.963 and c 7.464 A (the cell volume is 642 A 3 ). The calculated density is 6.96 g/cm 3 . The strongest lines in the powder pattern are [ d , ( I ) ( hkl )]: 2.988 (100) (112, 211), 4.32 (40) (200), 4.13 (40) (111), 2.877 (40) (300), 3.26 (30) (210). Single crystal Weissenberg photographs are close to those of pyromorphite, space group P6 3 /m. Chemically, mattheddleite does not contain S and Si in the expected 1:1 ratio, and the ideal formula may be expressed as Pb 20 (SiO 4 ) 7 (SO 4 ) 4 Cl 4 . The infrared spectrum is very similar to that of hydroxyellestadite. Associated minerals are lanarkite, cerussite, hydrocerussite, caledonite, leadhillite, susannite, and macphersonite. The mineral is named after Matthew Forster Heddle (1828–1897), a famous Scottish mineralogist.