Warwickite from St. Lawrence County, New York: Mineral association, chemical composition, cation ordering, and splitting of the warwickite M1 site

Abstract

Warwickite has been discovered in the Edwards and Balmat #3 mines in the Balmat-Edwards mining district, St. Lawrence County, New York, located in the Adirondack Lowlands. The samples from the two mines are similar in chemistry and atomic arrangement but differ chemically from previously described samples; they are among the most Fe-poor samples described to date. The warwickite in the Edwards Mine sample occurs as 1–2 mm-diameter green crystals associated with pink spinel, forsterite, phlogopite, and pyrite in an impure dolomitic marble, whereas warwickite in the specimens from the Balmat #3 mine, approximately 10 km distant, occurs as brown to amber colored, slender, elongate, millimeter-size crystals in a calcitic marble in association with pink spinel, phlogopite, anhydrite, pyrite, and galena. Chemical analyses of the two specimens by electron microprobe show similar empirical formulas of (Mg1.43Ti0.36Al0.18Cr3+0.02Zr0.01)Σ2.00B0.98O4 (Edwards Mine) and (Mg1.39Ti0.40Al0.18Cr3+0.01Zr0.01Fe2+0.01)Σ2.00B0.94O4 (Balmat mine). The atomic arrangement of a specimen from each mine was determined, and the high-precision refinements provide new insight into the warwickite structure. The M1 site in warwickite is split into two sites to accommodate two occupants with differing bonding requirements; the M1 site contains Mg and the M1′ site hosts Ti, with the two sites being separated by approximately 0.2 Å. The optimized structural formula for both warwickite samples is similar to [M1(Mg0.84Al0.14Ti0.024+)2.74M1′(Ti0.914+Mn0.082+Mg0.01)1.30]Σ4.04M2(Mg0.86Al0.10Ti0.044+)4.00B4O16], demonstrating ordering of Mg at M1 and M2 and Ti at M1′. The site-splitting demonstrates how divalent Mg and tetravalent Ti can exist at a site in solid solution by ordering the two cations at split sites.