Abstract
Chromatographic interaction between infiltrating solutions of racemic mixtures of enantiomers and enantiomorphic minerals with chiral excess has been proposed as a scenario for the emergence of biomolecular homochirality. Enantiomer separation is supposed to be produced by different partition coefficients of both enantiomers with regard to crystal faces or walls of capillary tubes in the enantiomorphic mineral. Besides quartz, nepheline is the only common magmatic mineral with enantiomorphic symmetry. It crystallizes from SiO2-undersaturated melts with low viscosity and is a promising candidate for chiral enrichment by autocatalytic secondary nucleation. Under liquidus conditions, the dynamic viscosity of silicate melts is mainly a function of polymerization. Melts with low concentrations of SiO2 (<55 wt%) and rather high concentrations of Na2O (>7 wt%) are only slightly polymerized and hence are characterized by low viscosities. Such melts can ascend, intrude or extrude by turbulent flow. Fourteen volcanic and subvolcanic samples from alkaline provinces in Africa and Sweden were chemically analyzed. Polished thin sections containing fresh nepheline phenocrysts were etched with 1% hydrofluoric acid at 20 °C for 15 to 25 min. Nepheline crystals suitable for a statistical evaluation of their etch figures were found in four samples. Crystals with chiral etch figures are mainly not twinned. Their chiral proportions in grain percentages of single crystals are close to parity in three samples. Only one sample shows a slight chiral excess (41.67% L-type vs. 58.33% D-type) but at a low level of significance (15 vs. 21 crystals, respectively).
Highlights
Extraterrestrial scenarios for chiral enrichment [1,2] have recently been favoured by the scientific community because of experimental evidence for enantiomeric excesses induced by circularly polarized light [3], and because of the fascinating challenge of the cometary mission Rosetta and its enantiomer-separating Cometary Sampling and Composition (COSAC) experiment onboard the Philae lander [4,5,6,7,8]
This pilot study has shown that nepheline enantiomorphs in unaltered volcanic rocks can be often identified by the shape of etch figures on section planes subparallel to the crystallographic c-axis
With regard to chiral proportions of nephelines originating from low-viscosity alkaline melts, the following conclusions can be drawn: 1
Summary
Extraterrestrial scenarios for chiral enrichment [1,2] have recently been favoured by the scientific community because of experimental evidence for enantiomeric excesses induced by circularly polarized light [3], and because of the fascinating challenge of the cometary mission Rosetta and its enantiomer-separating Cometary Sampling and Composition (COSAC) experiment onboard the Philae lander [4,5,6,7,8]. Temperature, and composition, the polymerization equilibrium of a silicate melt is realized when the distribution of oxygen species (O0 , O2− , O− ) minimizes the free energy of the solution [24] Such estimations were calculated for binary systems of SiO2 on the one hand and FeO, MnO, CaO, and Na2 O on the other hand [24,25]. Chiral enrichment by seeding [20] requires turbulence and unconstrained dispersion of crystalline nuclei within the entire magmatic volume This is only possible when the enantiomorphic crystalline species precipitates early from the melt (Figure 2a), and not late under eutectic conditions (Figure 2b).
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