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Abstract

Carboxylic and dicarboxylic acids (glycolic, oxalic, malonic and succinic) have been extracted with tetrahydrofuran (THF) and H2O from large synthetic MgO crystals, crushed to a medium fine powder. The extracts were characterized by infrared spectroscopy and 1H-NMR. The THF extracts were derivatized with tert-butyldimethylsilyl (t-BDMS) for GC-MS analysis. A single crystal separated from the extract was used for an x-ray structure analysis, giving the monoclinic unit cell, space group P21/c with ao = 5.543 A, bo = 8.845 A, co = 5.086 A, and beta = 91.9 degrees, consistent with beta-succinic acid, HOOC(CH2)COOH. The amount of extracted acids is estimated to be of the order of 0.1 to 0.5 mg g-1 MgO. The MgO crystals from which these organic acids were extracted grew from the 2860 degrees C hot melt, saturated with CO/CO2 and H2O, thereby incorporating small amounts of the gaseous components to form a solid solution (ss) with MgO. Upon cooling, the ss becomes supersaturated, causing solute carbon and other solute species to segregate not only to the surface but also internally, to dislocations and subgrain boundaries. The organic acids extracted from the MgO crystals after crushing appear to derive from these segregated solutes that formed C-C, C-H and C-O bonds along dislocations and other defects in the MgO structure, leading to entities that can generically be described as (HxCyOz)n-. The processes underlying the formation of these precursors are fundamental in nature and expected to be operational in any minerals, preferentially those with dense structures, that crystallized in H2O-CO2-laden environments. This opens the possibility that common magmatic and metamorphic rocks when weathering at the surface of a tectonically active planet like Earth may be an important source of abiogenically formed complex organic compounds.

Organic protomolecule assembly in igneous minerals

“…Extraction experiments with crushed olivine crystals yielded longer chain-length fatty acids, C 6 to C 12 (1), suggesting that longer-chain C x OH y OO z entities had formed in the olivine matrix. The difference in chain length may reflect differences between the structures of MgO and olivine.…”

“…The CH bands are consistent with C x entities containing OCH 2 O and OCH 3 . These C x entities may represent ''protomolecules'' of those carboxylic, dicarboxylic, and fatty acids that have been extracted from MgO and olivine crystals (1,4). To understand how such protomolecules form, we review earlier work on the dissolution mechanism of H 2 O and CO 2 that has laid the foundation for the study presented here.…”

“…Today, the Earth captures between 3 ϫ 10 9 and 1 ϫ 10 The work presented here points at a source of organics that is different from all other sources discussed so far in the literature. If C x OH y OO z protomolecules form in mineral matrices, they will turn into organic molecules during weathering (1,4). The organic molecules thus liberated may be difficult to synthesize by any of the other reaction pathways given in Table 1.…”

“…Yet, our earlier research has shown that a suite of medium-to long-chain fatty acids, C 6 OC 12 , can be identified among the organics extracted from crushed olivine single crystals from the CO͞ CO 2 ͞H 2 O-laden high temperature, high pressure environment of the upper mantle (1). Freshly crushed olivine single crystals, when heated in vacuum, were found to release a range of organic molecules, including aromatic compounds (2).…”

Solid Solution Model for Interstellar Dust Grains and Their Organics

“…Crushing the MgO and olivine crystals makes polyatomic carbon-based molecules extractable by organic solvents. Although laboratory data are still sparse, crushed, laboratory-grown MgO crystals yielded predominantly short-chain carboxylic and dicarboxylic acids with 4 n 7, while crushed upper-mantle-derived olivine crystals yielded longer chain-length fatty acids with 6 n 12 (Freund et al 1999;Gupta & Freund 1998). The difference in mean chain length of the carboxylic acids is consistent with the difference in the CH intensities in Figure 1, i.e., average CH 2 /CH 3 ratios of 8:1 in MgO and 10-12:1 in olivine.…”