The search for exsolved ferromagnesian olivines: A meteoritic survey

Abstract— Olivine grains from selected meteorites (the Springwater pallasite, the Lowicz mesosiderite, the ALH 84025 brachinite, the Krymka LL3 chondrite, and the Calcalong Creek lunar meteorite) and terrestrial rocks (San Carlos forsterite and Rockport fayalite) were studied by optical microscopy and high-precision electron microprobe analysis. Detailed microprobe traverses revealed regular igneous zoning in the Krymka and Calcalong Creek olivines. Traverses across the San Carlos forsterite grain are flat and display no chemical variations larger than the 2σ range of counting error (±0.2 mol% Fa). Traverses across olivine grains in the ALH 84025, Lowicz, and Springwater meteorites show regular patterns of periodic or wavy chemical variations well exceeding the 2σ uncertainty range. However, no lamellar structure was seen in backscattered electron images. It is suggested that the periodic chemical variations may be due to spinodal decomposition of primary, more or less homogeneous grains. I conclude that the absence of earlier reports of such variations simply means that olivine grains in equilibrated meteorites have not been examined closely enough to detect them.     

Amoeboid olivine aggregates and related objects in carbonaceous chondrites: records of nebular and asteroid processes

Amoeboid olivine aggregates (AOAs) are the most common type of refractory inclusions in CM, CR, CH, CV, CO, and ungrouped carbonaceous chondrites Acfer 094 and Adelaide; only one AOA was found in the CB_b chondrite Hammadah al Hamra 237 and none were observed in the CB_a chondrites Bencubbin, Gujba, and Weatherford. In primitive (unaltered and unmetamorphosed) carbonaceous chondrites, AOAs consist of forsterite (Fa_<2), Fe, Ni-metal (5-12 wt% Ni), and Ca, Al-rich inclusions (CAIs) composed of Al-diopside, spinel, anorthite, and very rare melilite. Melilite is typically replaced by a fine-grained mixture of spinel, Al-diopside, and ±anorthite; spinel is replaced by anorthite. About 10% of AOAs contain low-Ca pyroxene replacing forsterite. Forsterite and spinel are always ¹⁶O-rich (δ^17,18O∼−40‰ to −50‰), whereas melilite, anorthite, and diopside could be either similarly ¹⁶O-rich or ¹⁶O-depleted to varying degrees; the latter is common in AOAs from altered and metamorphosed carbonaceous chondrites such as some CVs and COs. Low-Ca pyroxene is either ¹⁶O-rich (δ^17,18O∼−40‰) or ¹⁶O-poor (δ^17,18O∼0‰). Most AOAs in CV chondrites have unfractionated (∼2-10×CI) rare-earth element patterns. AOAs have similar textures, mineralogy and oxygen isotopic compositions to those of forsterite-rich accretionary rims surrounding different types of CAIs (compact and fluffy Type A, Type B, and fine-grained, spinel-rich) in CV and CR chondrites. AOAs in primitive carbonaceous chondrites show no evidence for alteration and thermal metamorphism. Secondary minerals in AOAs from CR, CM, and CO, and CV chondrites are similar to those in chondrules, CAIs, and matrices of their host meteorites and include phyllosilicates, magnetite, carbonates, nepheline, sodalite, grossular, wollastonite, hedenbergite, andradite, and ferrous olivine.Our observations and a thermodynamic analysis suggest that AOAs and forsterite-rich accretionary rims formed in ¹⁶O-rich gaseous reservoirs, probably in the CAI-forming region(s), as aggregates of solar nebular condensates originally composed of forsterite, Fe, Ni-metal, and CAIs. Some of the CAIs were melted prior to aggregation into AOAs and experienced formation of Wark-Lovering rims. Before and possibly after the aggregation, melilite and spinel in CAIs reacted with SiO and Mg of the solar nebula gas enriched in ¹⁶O to form Al-diopside and anorthite. Forsterite in some AOAs reacted with ¹⁶O-enriched SiO gas to form low-Ca pyroxene. Some other AOAs were either reheated in ¹⁶O-poor gaseous reservoirs or coated by ¹⁶O-depleted pyroxene-rich dust and melted to varying degrees, possibly during chondrule formation. The most extensively melted AOAs experienced oxygen isotope exchange with ¹⁶O-poor nebular gas and may have been transformed into magnesian (Type I) chondrules. Secondary mineralization and at least some of the oxygen isotope exchange in AOAs from altered and metamorphosed chondrites must have resulted from alteration in the presence of aqueous solutions after aggregation and lithification of the chondrite parent asteroids.     

Structural relaxation and crystal field stabilization in Cr3+-containing oxides and silicates

The effect of crystal structure relaxation in oxygen-based Cr³⁺-containing minerals on the crystal field stabilization energy (CFSE) is considered. It is shown that the dependence of \textCFSE_{\textCr ³⁺} {\text{CFSE}}_{{{\text{Cr}}^{ 3+ } }} , which is found from optical absorption spectra, on the average interatomic distances is described by the power function with a negative exponent c \mathord

Fe2+, Mg-distribution among non-equivalent structural sites M1 and M2 in natural olivines: an optical spectroscopy study

Analysis of E||X-polarized optical absorption spectra of natural olivines of various origin in the range of electronic spin-allowed dd-transitions of Fe²⁺ evidences that in some crystals, there is a weak ordering of Fe²⁺ as in M1, as in M2 structural sites. The samples of three different depth facies seem showing a vague tendency of lowering of k _D -values from spinel-pyroxene (Sp-Px) through graphite-pyrope (G-Py) to diamond-pyrope (D-Py) facies, but the statistics are too poor (24 samples only) to be certain of it. Weak relations between Mg, Fe²⁺-distribution and iron content were found for the samples of Sp-Px- and G-Py-facies, while there is practically no one for those of the deepest D-Py facie.     

The first known natural occurrence of calcium monoaluminate,in a calcium‐aluminum‐rich inclusion from the CH chondrite Northwest Africa 470

Abstract— Natural calcium monoaluminate, CaAl₂O₄, has been found in a grossite‐rich calcium‐aluminum‐rich inclusion (CAI) from the CH chondrite Northwest Africa 470. The calcium monoaluminate occurs as colorless ～10 μm subhedral grains intergrown with grossite, perovskite, and melilite. Nebular condensation is the most likely origin for the precursor materials of this CAI, but calculations suggest that dust/gas ratios substantially enhanced over solar are required to stabilize CaAl₂O₄.     

Optical absorption of electronic Fe–Ti charge-transfer transition in natural andalusite: the thermal stability of the charge-transfer band

Differently colored natural Brazilian andalusite crystals heat-treated under reducing and oxidizing conditions were analyzed by optical spectroscopy. The intensity of a broad intense band at around 20,500 cm⁻¹ in the optical absorption spectra of all color zones of the sample is proportional to the product of Ti- and Fe-concentrations and herewith proves its attribution to electronic Fe²⁺/Ti⁴⁺ IVCT transition. The band is strictly E||c-polarized, causing an intense red coloration of the samples in this polarization. The polarization of the Fe²⁺/Ti⁴⁺ IVCT band in andalusite, E||c, shows that the electronic charge-transfer process takes place in Al–O octahedral groups that share edges with neighbors on either side, forming chains parallel to the c-axis of the andalusite structure. Under thermal treatments in air, the first noticeable change is some intensification of the band at 800°C. However, at higher temperatures its intensity decreases until it vanishes at 1,000°C in lightly colored zones and 1,100°C in darkly colored ones. Under annealing in reducing conditions at 700 and 800°C, the band also slightly increases and maintains its intensity at treatments at higher temperatures up to 1,000°C. These results demonstrate that weakening and disappearance of the Fe²⁺/Ti⁴⁺ IVCT band in spectra of andalusite under annealing in air is caused by oxidization of Fe²⁺ to Fe³⁺ in IVCT Fe²⁺/Ti⁴⁺-pairs. Some intensification of the band at 800°C is, most probably, due to thermally induced diffusion of Fe²⁺ and Ti⁴⁺ in the structure that leads to aggregation of “isolated” Ti⁴⁺ and Fe²⁺ ions into Fe²⁺–Ti⁴⁺-pairs. At higher temperatures, the competing process of Fe²⁺ → Fe³⁺ oxidation overcomes such “coupling” and the band continues to decrease. The different thermal stability of the band in lightly and darkly colored zones of the samples evidence some self-stabilization over an interaction of Fe²⁺/Ti⁴⁺-pairs involved in IVCT process.     

Using total organic carbon for the assessment of groundwater vulnerability in karst regions at regional scales

Groundwater vulnerability is the likelihood of contaminants reaching the groundwater system after introduction at some location above the aquifer. The objective of this work was to investigate the relationship between total organic carbon concentration in groundwater and the intrinsic vulnerability of groundwater in karst areas at the regional scale. For this purpose hydrochemical data from 17 springs draining a Dinantian pure bedded Limestone rock unit were analysed to investigate their relationship with known parameters of groundwater vulnerability (e.g. degree of karstification, type of recharge). Statistical methods that could be useful to investigate such relationships are suggested. Principal components analysis showed the potential to visualise the relationships among hydrochemical data based on estimates of the median value. Multiple correspondence analysis was applied to visualise the relationship between total organic carbon and potential sources of organic carbon on the surface, a novel approach which has not been used in similar studies. Moreover, multiple regression analysis was used to determine the significance of these relationships. The results showed that, taking into consideration differences in background concentration, total organic carbon was significantly related to groundwater vulnerability.     

FTIR spectroscopic study of natural andalusite showing electronic Fe–Ti charge-transfer processes: zoning and thermal evolution of OH-vibration bands

We investigated a natural Brazilian Fe–Ti-containing andalusite and its thermal behavior by polarized infrared and optical spectroscopy. Polarized infrared spectra of the Brazilian andalusite and their evolution at thermal annealing in air clearly evidence that there are several types of OH-groups in the structure. Optical spectra and their evolution with temperature indicate that the incorporated iron (about 0.43 wt% calculated as FeO) is in the ferrous and ferric state. Incorporation of ferrous iron in the Al-sites of andalusite is discussed as a possible incorporation mechanism for hydrogen. The weakening and disappearance of the Fe²⁺/Ti⁴⁺ IVCT band in the andalusite spectra under annealing in air is caused by oxidization of Fe²⁺ to Fe³⁺ in Fe²⁺/Ti⁴⁺ IVCT pairs. The process of oxidation is accompanied by a rearrangement of the hydroxyl groups and dehydration of the sample, especially vivid at the final stage of the thermal annealing at 1,200 °C. During thermal annealing, structural hydroxyls of different types apparently transform into each other: the most distinct are the hydroxyls causing the doublet at 3,516 and 3,527 cm^?1 (i.e., H bonded to O1) which seem to transform into the hydroxyls causing the line at 3,461 cm^?1 (i.e., H bonded to O2). The infrared spectra scanned across differently colored zones of the crystal clearly show that some amount of hydroxyls is related to Fe²⁺/Ti⁴⁺ IVCT pairs which are the cause of the red-to-black coloration of the sample in E||c-polarized illumination: it is evident that in a part of the hydroxyl groups, OH-vector changes orientation aligning directly along crystallographic a-axis due to some kind of interaction with Fe²⁺/Ti⁴⁺ IVCT pairs.     

Spectroscopic studies of synthetic and natural ringwoodite, γ-(Mg,Fe)<Subscript>2</Subscript>SiO<Subscript>4</Subscript>

Synthetic ringwoodite γ-(Mg_1?x Fe _x )₂SiO₄ of 0.4 ≤ x ≤ 1.0 compositions and variously colored micro-grains of natural ringwoodite in shock metamorphism veins of thin sections of two S6-type chondrites were studied by means of microprobe analysis, TEM and optical absorption spectroscopy. Three synthetic samples were studied in addition with Mössbauer spectroscopy. The Mössbauer spectra consist of two doublets caused by ^VIFe²⁺ and ^VIFe³⁺, with IS and QS parameters close to those established elsewhere (e.g., O’Neill et al. in Am Mineral 78:456–460, 1993). The Fe³⁺/Fe_total ratio evaluated by curve resolution of the spectra, ranges from 0.04 to 0.1. Optical absorption spectra of all synthetic samples studied are qualitatively very similar as they are directly related to the iron content. They differ mostly in the intensity of the observed absorption features. The spectra consist of a very strong high-energy absorption edge and a series of absorption bands of different width and intensity. The three strongest and broadest absorptions of them are attributed to splitting of electronic spin-allowed ⁵ T _2g → ⁵ E _g transitions of ^VIFe²⁺ and intervalence charge-transfer (IVCT) transition between ferrous and ferric ions in adjacent octahedral sites of the ringwoodite structure. The spin-allowed bands at ca. 8,000 and 11,500 cm^?1 weakly depend on temperature, whilst the Fe²⁺/Fe³⁺ IVCT band at ~16,400 cm^?1 displays very strong temperature dependence: i.e., with increasing temperature it decreases and practically disappears at about 497 K, a behavior typical for bands of this type. With increasing pressure the absorption edge shifts to lower energies while the spin-allowed bands shift to higher energy and strongly decreases in intensity. The IVCT band also strongly weakens and vanishes at about 9 GPa. We assigned this effect to pressure-induced reduction of Fe³⁺ in ringwoodite. By analogy with synthetic samples three broad bands in spectra of natural (meteoritic) blue ringwoodite are assigned to electronic spin-allowed transitions of ^VIFe²⁺ (the bands at ~8,600 and ~12,700 cm^?1) and Fe²⁺/Fe³⁺ IVCT transition (~18,100 cm^?1), respectively. Spectra of colorless ringwoodite of the same composition consist of a single broad band at ca. 12,000 cm^?1. It is assumed that such ringwoodite grains are inverse (Fe, Mg)₂SiO₄-spinels and that the single band is caused by the split spin-allowed ⁵ E → ⁵ T ₂ transition of ^IVFe²⁺. Ringwoodite of intermediate color variations between dark-blue and colorless are assumed to be partly inversed ringwoodite. No glassy material between the grain boundaries in the natural colored ringwoodite aggregates was found in our samples and disprove the cause of the coloration to be due to light scattering effect (Lingemann and Stöffler in Lunar Planet Sci 29(1308), 1998).