Jimkrieghite, Ca(C2H3O3)2, a New Glycolate Mineral, from the Santa Catalina Mountains, Tucson, Arizona, USA

Abstract

Listen

Translate article

ABSTRACT A new organic mineral species, jimkrieghite, ideally Ca(C2H3O3)2, was discovered from the western end of Pusch Ridge in the Santa Catalina Mountains, north of Tucson, Arizona, USA. It occurs as pseudo-cubic or thick-tabular crystals (up to 0.12 × 0.12 × 0.10 mm) associated with calcite, cerussite, chrysocolla, hematite, lazaraskeite, malachite, microcline, mimetite, phlogopite, phosphohedyphane, quartz, stanevansite, and wulfenite. Jimkrieghite is colorless and transparent with a white streak and vitreous luster. It is brittle and has a Mohs hardness of ∼2½; cleavage is perfect on {100}. No twinning was observed. The measured and calculated densities are 1.85(5) and 1.879 g/cm3, respectively. Optically, jimkrieghite is biaxial (−), with α =1.539(5), β = 1.579(5), γ = 1.595(5), 2Vmeas. = 68(3)°, 2Vcal. = 63°. Calcium is the only metal cation detected by electron microprobe, and the chemical formula was derived from the structural data showing that jimkrieghite is an anhydrous calcium glycolate. Jimkrieghite is orthorhombic with space group Pbca and unit-cell parameters a = 9.01716(12), b = 9.70761(11), c = 15.3554(2) Å, V = 1344.13(3) Å3, and Z = 8. It is isostructural with the orthorhombic Pbca form of synthetic Cd(C2H3O3)2. In the structure of jimkrieghite, edge-sharing dimers of [CaO8] polyhedra link with one another by corner sharing to form a polyhedral layer parallel to (001). Two different (C2H3O3)− glycolate anions form linkages between the [CaO8] polyhedra. One links polyhedra only within the same layer, whereas the other links polyhedra in adjacent layers, thereby creating a three-dimensional framework. The eight O atoms coordinated to a Ca atom are from five (C2H3O3)− groups, three being bidentately bonded and two monodentately bonded, with the Ca–O bond lengths ranging from 2.369 to 2.868 Å. The discovery of jimkrieghite, together with other glycolate minerals documented thus far, namely lazaraskeite Cu(C2H3O3)2, stanevansite Mg(C2H3O3)2·2H2O, lianbinite (NH4)(C2H3O3)(C2H4O3), and glecklerite Na(C2H3O3), implies that glycolate minerals may be rather widespread in nature, thus serving as a potential reservoir for biologically fixed carbon.