Abstract
Spectral features for natural stichtite at 1042, 1096, 1360, 1638, and 3482 cm−1 provide insights on mineral structure, with peaks consistent with OH− stretching modes bound to Mg or Cr, CO32− antisymmetric stretches and CO32− bound within the sample and molecular water. These Infrared (IR) data suggest natural stichtite forms at a pH of >12 with increased water and decreased carbonate in the interlayer due to a smaller interlayer distance and unit cell. Higher pH favors lower divalent cation purity and may explain observed ranges of non-end member compositions in stichtite from localities around the world, and across geologic time. This constrains stichtite formation to a range of very high pH conditions and is consistent with active serpentinizing fluid vents and some mine wastes. IR has clear application for the detection and quantification of stichtite under field and laboratory conditions within the detection limits of 5% stichtite within a serpentine host. The size and grade of terrestrial stichtite deposits, and resolution of remote sensing instruments, suggest remote IR detection of stichtite is possible, and remote IR detection for Earth and Mars is discussed.
Highlights
Stichtite (Mg6 Cr2 (OH)16 [CO3 ]·4H2 O) is a purple to pink magnesium-chromium hydroxycarbonate mineral that may contain a fossil stable isotope record of early earth serpentinizing conditions under which the Last Universal Common Ancestor (LUCA) is known to have thrived [1,2,3]
Three sets of stichtite-serpentine mineral separates from the Stichtite Hill locality were examined by x-ray diffraction (XRD)
While it is no surprise that stichtite would form at very high pH, given the known formation under serpentinizing conditions, e.g., [2,10,41,42], this does provide important supporting evidence for the conditions under which stichtite may form
Summary
Stichtite (Mg6 Cr2 (OH)16 [CO3 ]·4H2 O) is a purple to pink magnesium-chromium hydroxycarbonate mineral that may contain a fossil stable isotope record of early earth serpentinizing conditions under which the Last Universal Common Ancestor (LUCA) is known to have thrived [1,2,3]. Stichtite has a waxy to pearly luster and has never been reported as macroscopic crystals, but occurs as fine, platy to acicular aggregates of crystallites to crystals typically less than 2 mm in the longest dimension. Stichtite is believed to form during active serpentinization, based upon the cross-cutting relations and intergrowth textures [5,6,7]. Serpentinization occurs when water-rock reactions drive the alteration of low-silica ultramafic rocks to form serpentine, hydrotalcite minerals, and other reaction products
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