Stanevansite, Mg(C2H3O3)2·2H2O, A New Hydrous Glycolate Mineral, from the Santa Catalina Mountains, Tucson, Arizona, USA

Abstract

Abstract A new organic mineral species, stanevansite, ideally Mg(C2H3O3)2·2H2O, was discovered from the western end of Pusch Ridge in the Santa Catalina Mountains (32° 21′ 42″ N, 110° 57′ 30″ W, at the elevation of 975 m), north of Tucson, Arizona, USA. It occurs as sprays of bladed crystals (up to 0.40 × 0.07 × 0.03 mm). Associated minerals include lazaraskeite, chrysocolla, malachite, wulfenite, mimetite, phosphohedyphane, cerussite, hematite, calcite, microcline, phlogopite, and quartz. Stanevansite crystals are colorless in transmitted light, transparent with white streak and vitreous luster. They are brittle and have a Mohs hardness of ∼1½; cleavage is perfect on {100}. Twinning is common on (100). The measured and calculated densities are 1.69(5) and 1.682 g/cm3, respectively. Optically, stanevansite is biaxial (+), with α = 1.539(5), β = 1.545(5), γ = 1.558(5), 2Vmeas. = 62(2)°, 2Vcal. = 69°. It is insoluble in water, but slowly dissolves in hydrochloric acid. An electron-microprobe analysis yielded an empirical formula, based on 8 O apfu and Σ(Mg + Zn) = 1 apfu, of (Mg0.95Zn0.05)Σ1.00(C2H3O3)2·2H2O, which can be simplified as (Mg,Zn)(C2H3O3)2·2H2O. Stanevansite is monoclinic with space group P21/c and unit-cell parameters a = 11.4927(2), b = 5.85470(10), c = 12.4711(2) Å, β = 91.1610(10)°, V = 838.96(2) Å3, and Z = 4. It is isostructural with several synthetic glycolate compounds having the general chemical formula M2+(C2H3O3)2·2H2O, where M2+ = Co2+, Mn, Zn, and Mg. The crystal structure of stanevansite is characterized by the mononuclear complex [Mg(C2H3O3)2(H2O)2], with such complexes being connected to one another by hydrogen bonds to form a three-dimensional supramolecular architecture. In a [Mg(C2H3O3)2(H2O)2] complex, Mg is octahedrally coordinated by two chelating glycolate ligands and two H2O molecules. Stanevansite represents the first hydrous glycolate mineral and is believed to have formed through the interaction of fluids containing glycolic acid (C2H4O3) derived from decaying plant materials or bacterial activities with Mg produced by the alteration of primary and secondary minerals. Its discovery, together with other glycolate minerals documented recently, namely lazaraskeite Cu(C2H3O3)2, jimkrieghite Ca(C2H3O3)2, and lianbinite (NH4)(C2H3O3)(C2H4O3), not only implies that more glycolate minerals may be found in nature, but also suggests that glycolate minerals may serve as a potential reservoir for biologically fixed carbon.