Raygrantite, Pb₁₀Zn(SO₄)₆(SiO₄)₂(OH)₂, a new mineral isostructural with iranite, from the big horn mountains, Maricopa county, Arizona, USA

A new mineral species, raygrantite, ideally Pb₁₀Zn(SO₄)₆(SiO₄)₂(OH)₂, has been found in the Big Horn Mountains, Maricopa County, Arizona, USA. Associated minerals are galena, anglesite, cerussite, lanarkite, leadhillite, mattheddleite, alamosite, hydrocerussite, caledonite, and diaboleite. Raygrantite crystals are bladed with striations parallel to the elongated direction (the c axis). Twinning (fish-tail type) is pervasive on (1 2 1). The mineral is colorless, transparent with white streak, and has a vitreous luster. It is brittle and has a Mohs hardness of ~3; cleavage is good on {120} and no parting was observed. The calculated density is 6.374 g/cm³. Optically, raygrantite is biaxial (+), with n_α = 1.915(7), n_β = 1.981(7), n_γ = 2.068(9), 2V_meas = 76(2)°, and 2V_calc = 85°. It is insoluble in water, acetone, or hydrochloric acid. An electron microprobe analysis yielded the empirical formula Pb²⁺_9.81Zn²⁺_0.93 (S_1.00O₄)₆(Si_1.05O₄)₂(OH)₂. Raygrantite is a new member of the iranite mineral group. It is triclinic, with space group P1 and unit-cell parameters a 9.3175(4), b 11.1973(5), c 10.8318(5) A, α 120.374(2), β 90.511(2), γ 56.471(2)°, and V 753.13(6) A³. Its crystal structure, refined to R₁=0.031, is characterized by slabs that lie parallel to (120) of SO₄ and SiO₄ tetrahedra with ZnO₄(OH)₂ octahedra, held together by Pb²⁺ cations displaying a wide range of Pb-O bond distances. The discovery of raygrantite indicates that, in addition to complete OH-F and Cu-Zn substitutions, there is also a complete substitution between (CrO₄)^2- and (SO₄)^2- in the iranite group of minerals, pointing to the possible existence of a number of other (SO₄)^2--bearing iranite-type phases yet to be found or synthesized.