LEADHILLITE

leadhillite, a rare mineral consisting of basic lead sulphato-carbonate, pb4so4(co3)2(oh)2. crystals have usually the form of six-sided plates (fig. 1) or sometimes of acute rhombohedra (fig. 2); they have a perfect basal cleavage (parallel to p in fig. 1) on which the lustre is strongly pearly; they are usually white and translucent. the hardness is 2.5 and the sp. gr. 6.26-6.44. the crystallographic and optical characters point to the existence of three distinct kinds of leadhillite, which are, however, identical in external appearance and may even occur intergrown together in the same crystal: (a) monoclinic with an optic axial angle of 20°; (b) rhombohedral (fig. 2) and optically uniaxial; (c) orthorhombic (fig. 1) with an optic axial angle of 723⁄4°. the first of these is the more common kind, and the second has long been known under the name susannite. the fact that the published analyses of leadhillite vary somewhat from the formula given above suggests that these three kinds may also be chemically distinct. [illustration: fig. 1.] [illustration: fig. 2.] leadhillite is a mineral of secondary origin, occurring with cerussite, anglesite, &c., in the oxidized portions of lead-bearing lodes; it has also been found in weathered lead slags left by the romans. it has been found most abundantly in the susanna mine at leadhills in scotland (hence the names leadhillite and susannite). good crystals have also been found at red gill in cumberland and at granby in missouri. crystals from sardinia have been called maxite. (l. j. s.)