Lazaraskeite, Cu(C2H3O3)2, the first organic mineral containing glycolate, from the Santa Catalina Mountains, Tucson, Arizona, U.S.A.

Abstract

Abstract A new organic mineral species, lazaraskeite, ideally Cu(C2H3O3)2 with two polytypes M1 and M2, was discovered in the high elevation of the Santa Catalina Mountains, north of Tucson, Arizona, U.S.A. Both lazaraskeite-M1 and -M2 occur as euhedral individual crystals (up to 0.20 × 0.20 × 0.80 mm) or aggregates, with the former being more equant crystals and the latter bladed crystals elongated along the c axis. Associated minerals include chrysocolla, malachite, wulfenite, mimetite, hydroxylpyromorphite, hematite, microcline, muscovite, and quartz. Both polytypes are greenish-blue in transmitted light, transparent with white streak, and a vitreous luster. They are brittle and have a Mohs hardness of ~2; cleavage is perfect on {101}. No parting or twinning was observed. The measured and calculated densities are 2.12(2) and 2.138 g/cm3, respectively, for lazaraskeite-M1 and 2.10(2) and 2.086 g/cm3 for lazaraskeite-M2. Optically, lazaraskeite-M1 is biaxial (−), with α = 1.595(3), β = 1.629(8), γ = 1.645(5), 2Vmeas = 69(2)°, 2Vcal = 67°. Lazaraskeite-M2 is also biaxial (−), with α = 1.520(5), β = 1.578(6), γ = 1.610(5), 2Vmeas = 73(2)°, 2Vcal = 70°. Lazaraskeite is insoluble in water or acetone. An electron microprobe analysis for Cu and an Elemental Combustion System equipped with mass spectrometry for C yielded an empirical formula, based on 6 O apfu, Cu1.01(C1.99H2.99O3)2 for lazaraskeite-M1 and Cu1.01(C1.98H3.00O3)2 for lazaraskeite-M2. The measured δ13C ‰ values are –37.7(1) and –37.8(1) for lazaraskeite-M1 and -M2, respectively. Both lazaraskeite-M1 and -M2 are monoclinic with the same space group P21/n. The unit-cell parameters are a = 5.1049(2), b = 8.6742(4), c = 7.7566(3) Å, β = 106.834(2)°, V = 328.75(2) Å3 for M1 and a = 5.1977(3), b = 7.4338(4), c = 8.8091(4) Å, β = 101.418(2)°, V = 333.64(3) Å3 for M2. Lazaraskeite-M1 is the natural analog of synthetic bis(glycolato)copper(II), Cu(C2H3O3)2. Its crystal structure is characterized by layers made of octahedrally coordinated Cu2+ cations and glycolate (C2H3O3)– anionic groups. These layers, parallel to (101), are linked together by the strong hydrogen bonds (O-H···O = 2.58 Å). The CuO6 octahedron is highly distorted, with four equatorial Cu-O bonds between 1.92 and 1.94 Å and two axial bonds at 2.54 Å. Lazaraskeite-M2 has the same topology as lazaraskeite-M1 and possesses all structural features of the low-temperature phase transformed from lazaraskeite-M1 at 220 K (Yoneyama et al. 2013). The major differences between the two polytypes of lazaraskeite include: (1) M1 has b > c, with β = 106.8°, whereas M2 has b < c, with β = 101.4°; (2) the CuO6 octahedron in M1 is more elongated and distorted than in M2; and (3) there is a relative change in the molecular orientation between the two structures. Lazaraskeite represents the first organic mineral that contains glycolate. Not only does its discovery imply that more glycolate minerals may be found, but also suggests that glycolate minerals may serve as a potential storage for biologically fixed carbon.