Forsterite Grains Research Articles

Two chondrule groups each with distinctive rims in Murchison recognized by cathodoluminescence

Abstract— Two groups of chondrules in the Murchison CM chondrite, which have previously been identified on the basis of FeO in the chondrule grains, are readily identified from cathodoluminescence (CL) and belong to those of the ordinary chondrite group A and B chondrules of Sears et al. (1992a). All chondrules are surrounded by fine‐grained rims containing forsterite with bright red CL, but on group A chondrules an outer thin rim grades into a much thicker rim, with a lower density of forsterite grains, which in turn grades into the central chondrule. Group B chondrules have only the thin outer rim with a high density of small forsterite grains. This is the first time an unequivocal correlation has been observed between chondrule rim thickness and the composition of the object on which the rim is located. We suggest that while all objects in the meteorite (group B chondrules, refractory inclusions, mineral and chondrule fragments, clasts) acquired a very thin rim during processing in a wet regolith, the thick rims on group A chondrules were formed by aqueous alteration of precursor metal‐ and sulfide‐rich rims which are a characteristic of group A chondrules in ordinary chondrites.

Oxygen isotopic composition of individual olivine grains from the Allende meteorite

The oxygen isotopic compositions of individual olivine grains from the Allende meteorite were determined by ion microprobe techniques. A variety of olivine grains were analysed: refractory forsterite grains (< 1 wt% FeO), cores of two isolated olivine grains (2–20 wt% FeO), their FeO-rich (> 26 wt%) rims, and twenty-eight individual matrix olivine grains (21–41 wt% FeO). In an oxygen three-isotope plot refractory forsterite grains ( δ 17 O ≈ −11%.; δ 18O ≈ −8‰ ) fall on the Allende mixing line but are significantly less enriched in 16O than spinel and pyroxene from Ca,Al-rich inclusions (CAIs), possibly indicating a later formation of forsteritic olivine. Cores of isolated olivine grains with somewhat higher FeO contents (IOGs) have oxygen isotopic compositions similar to those of the refractory forsterites( δ 17O ≈ −12‰ and δ 18O ≈ − 9‰ ). Both types of olivine grains have oxygen isotopic compositions significantly different from those of chondrules. This clearly demonstrates that these olivine grains are not the result of crystallization of chondrule melts. Fayalite-rich rims of the IOGs are less enriched in 16O ( δ 17O ≈ −3‰ ; δ 18O ≈ 0‰ ) than the forsteritic cores. Arguments are presented that this supports a condensation origin of the rims from a gas phase less enriched in 16O than the gas phase from which the forsteritic cores had formed. Matrix olivines have oxygen isotopic compositions similar to fayalite-rich rims. However, there are variations in the oxygen isotopic composition of matrix olivines that exceed analytical uncertainties, suggesting a more complex origin.

A New Method for Producing Ultrafine Grains of Forsterite and Periclase.

Periclase fine grains have been formed by evaporating magnesium in mixture gases of Ar (80%) and O2 (20%) at 13 kPa by heating at 1000°C. Forsterite fine grains have been produced by flushing SiO powders into the flame during MgO grain formation. Periclase and metal magnesium grains were also produced in atmosphere.

On the relationship between isolated and chondrule olivine grains in the carbonaceous chondrite ALHA77307

The origin of isolated olivine grains in carbonaceous chondrites has been suggested to be either by direct condensation or by fragmentation of chondrules. In an attempt to resolve this debate, isolated olivine grains in the carbonaceous chondrite ALHA77307 have been studied in detail. Zoning characteristics and minor element compositions of olivines from all isolated and chondrule occurrences are described. The data show that a strong relationship exists between isolated olivines and the chondrule population. Chondrule olivine compositions and zoning properties are consistent with olivine crystallization from the molten chondrules. Because properties of isolated grains are very similar to chondrule olivines, an igneous origin for most, if not all, isolated grains is proposed. Large isolated forsterite grains may be derived from fragmentation of “macroporphyritic” type I chondrules which contain one or very few large olivine grains. Relic forsterite grains in type II chondrules, and complex isolated olivine grains with forsteritic cores and fayalite-rich rims, provide evidence that fragmentation and recycling of chondrules and their components was a common and efficient process in the chondrule-forming region.

Minor elements in forsterites of Orgueil (C1), Alais (C1) and two interplanetary dust particles compared to C2‐C3‐UOC forsterites

Abstract— The rare Mg‐rich silicate fraction of the C1 meteorites, Orgueil and Alais, is dominated by minute (&lt; 30 μm) forsterite. Twenty three forsterite grains of these meteorites as well as large forsterites in two chondritic porous interplanetary dust particles (IDPs) are characterized by levels of MnO generally, but not always, higher than found in forsterites of C2, C3 and unequilibrated ordinary chondrites (UOC). Forsterite in Orgueil contains 900 to 6200 ppmw MnO while Alais forsterite has less than 2000 ppmw MnO suggesting that the forsterites in the two meteorites are chemically distinct. Alais forsterite shows lower Cr and Al relative to Orgueil forsterite. The C1 forsterites do not show Fe‐poor (FeO &lt; 0.3), refractory‐rich (Al, Ca, Ti, V) compositions which are relatively common in the C2‐C3‐UOC meteorites suggesting that the most primitive forsterite compositions are not present in these C1 meteorites. While minor elements in forsterite can not distinguish unambiguously between C1 and C2‐C3‐UOC sources, the high Mn levels in some IDP forsterites are similar to some C1 forsterites suggesting a possible relation between the forsterites of these two extraterrestrial samples.

FeO‐rich rims and veins in Allende forsterite: Evidence for high temperature condensation at oxidizing conditions

Abstract— Forsteritic olivine grains in the Allende meteorite are commonly rimmed by FeO‐rich olivine. New evidence is presented in this paper that the fayalitic rims formed by condensation from a gas and not by thermal equilibration of forsterite with FeO‐rich metal or FeO‐rich olivine in the interior of a parent body. A similar origin is inferred for fayalitic veins within forsterite crystals.A good correlation of FeO with MnO was observed along profiles from forsteritic cores to fayalite‐rich rims, excluding oxidation of metal as a source of rim FeO. The fayalite content of the rim is generally lower than that of the adjacent matrix olivines excluding formation of FeO‐rich rims by equilibration with present matrix. This is also supported by systematic differences in Cr between rim and matrix olivines.Chromites, associated with FeO‐rich olivine, were found at the boundary between forsterite and fayalite‐rich rim, within the rim itself and in fayalitic veins. Condensation of chromite from a gas phase is likely for the latter two occurrences since no other suitable source of Cr is available. Chromite at the forsterite‐fayalite interface was formed by diffusion of Cr from forsterite. Increasingly oxidizing conditions, apparently enabled formation of chromite.The steep compositional gradient between forsterite and fayalite provides severe constraints on the thermal history of forsterite grains. The width of oxidized zones (halos) around metal inclusions in the interior of forsterite grains are not compatible with this steep gradient. Their development requires longer diffusion times than would be allowed by the rims, indicating that these forsterite grains must have had an independent history of oxidation prior to formation of the rim.Condensation calculations indicate that FeO‐rich olivine can be formed by condensation from a gas with enhanced oxygen fugacity. Increasing the oxygen fugacity would provide conditions allowing the thermodynamic stability of chromite.High temperatures and oxidizing conditions must have prevailed for some time in that part of the solar nebula where the Allende meteorite formed.

Compositions of isolated forsterites in Ornans (C3O)

Luminescing forsterite with FeO between 0.25 and 2.0 wt% occurs as cores of isolated grains and within an Fe-rich porphyritic chondrule in the Ornans (C3O) carbonaceous chondrite. The color variation of the cathodoluminescence depends on the relative intensity of a red emission caused by Cr and a blue emission most intense when transition metal impurities are at the lowest levels. For Omans forsterite, the blue emission is quenched by Fe at 0.75 wt% and the red at 2.0 wt%. Compositional profiles from core to rim of five isolated forsterites show details of Al, Ca, Sc, Ti, V, Cr, Mn and Fe zoning with respect to position and cathodoluminescence color. FeO shows normal zoning within the core of the five grains, reaching values of 0.75 wt%, at which point rapid enrichment in FeO occurs, reaching FeO values of 20–30 wt% at the grain edge. Titanium and Al are linearly correlated but show erratic changes within each grain; TiO 2 ranges from 500 to 600 ppmw in the core to 100 ppmw in the rim; Al 2O 3 varies from 0.6 wt% in the core to below 0.1 wt% in the rim; CaO is near constant at 0.70 wt% within the blue luminescing core and below 0.4 wt% in the rim; Sc 2O 3 is present in two grains up to 130 ppmw; V 2O 3 shows 500 ppmw at the core and 100 ppmw at the rim. Evidence is presented for growth of euhedral forsterite either from a liquid or gas, subsequent fracturing, precipitation of Fe-metal, and growth of progressively more Fe-rich rims. Diffusion has affected these grains to give halos around metal and diffuse CL boundaries in some crystallographic directions. Some forsterite grains have been incorporated into an Fe-rich assemblage to form chondrules which retain evidence of their origin in the form of these relic grains.

Cathodoluminescence zoning and minor elements in forsterites from the Murchison (C2) and Allende (C3V) carbonaceous chondrites

Although Mg-rich olivine occurs as isolated grains and in aggregates of carbonaceous meteorites1, its chemical features have generally been neglected in favour of the more esoteric Ca/Al-rich inclusions with their challenging mineralogy and isotope systematics. Because cathodoluminescence has proved useful in revealing textures and compositional varieties for some minerals in meteorites, we have now applied the technique to look for textural features of olivine in carbonaceous meteorites relevant to the unresolved dispute over its origin, whether from a vapour or a liquid2–9. Our cathodoluminescence photographs of forsterite grains in Murchison (C2) and Allende (C3) meteorites reveal a blue core (inclusion-free) with planar boundaries to a red or dark rim (inclusions of glass, metal and silicate). We have also performed high-precision electron microprobe analyses revealing in these forsterites unusually large amounts of the ‘minor’ elements Al, Ti and Ca in the blue cores (and sharp chemical changes at the blue/red boundary), suggesting formation by crystallization at high temperatures from a source rich in these metals. Following conclusions drawn from previous analyses of olivine in meteorites (achondrites10, Niger (I) C27, Belgica 7904 (C2)11), we believe the minor element signature should be able to characterize olivines in micrometeorites and in deep-sea particles (see accompanying paper12).

Thermal Transformation of Chrysotile Studied by High Resolution Electron Microscopy

AbstractThermal transformation of chrysotile from Uruaçu District, state of Goiás, Brazil, heated in dry conditions at temperatures from 600°C to 1300°C was studied by high resolution electron microscopy and selected area electron diffraction (SAD). Up to 600°C, no morphological or SAD pattern changes were observed. At 600°C, the fibrils were still crystalline with the characteristics of the clinochrysotile. In addition, a new fringe system of 10–15 Å spacings appeared sporadically parallel to the 7.3 Å fringes of chrysotile. Areas of these extra fringes seem to constitute favorable sites for the nucleation of forsterite. At 650°C, forsterite nuclei appeared inside the nearly amorphous fibrils in the shape of patches consisting of flaky crystallites. At 700°C the chrysotile structure had disappeared; the new spots present in the SAD pattern were indexed as those of forsterite. Between 800–900°C the crystallinity of the patches was clearly demonstrated. From the lattice images in the patches, topotactic relations between chrysotile and forsterite were analyzed. At 1000°C very tiny grains of enstatite were formed mixed with forsterite grains. The SAD pattern is complex due to the coexistence of forsterite, enstatite, and silica-rich amorphous areas. From 1100°C to 1300°C the tridimensional growth of enstatite was promoted. The present results support the topotactic relations between chrysotile and forsterite found by X-ray analysis although differences up to several degrees may exist when these phases are observed microscopically. Evidence suggesting a topotactic growth between forsterite and enstatite was also obtained.

On the nature and origin of isolated olivine grains in carbonaceous chondrites

Many carbonaceous chondrites contain discrete olivine fragments that have been considered to be primitive material, i.e. direct condensates from the solar nebula or pre-solar system material. Olivine occurring in chondrules and as isolated grains in C3(0) chondrites has been characterized chemically and petrographically. Type I chondrules contain homogeneous forsterite grains that exhibit a negative correlation between FeO and CaO. Type II chondrules contain zoned fayalite olivines in which FeO is positively correlated with CaO and MnO. The isolated olivines in C3(0) chondrites form two compositional populations identical to olivines in the two types of porphyritic olivine chondrules in the same meteorites. Isolated olivines contain trapped melt inclusions similar in composition to glassy mesostasis between olivines in chondrules. Such glasses can be produced by fractional crystallization of olivine and minor spinel in the parent chondrule melts if plagioclase does not nucleate. The isolated olivine grains are apparently clastic fragments of chondrules. Some similarities between olivines in C3(0), C2, and Cl chondrites may suggest that olivine grains in all these meteorites crystallized from chondrule melts.