Kazushige Tomeoka

Indicators of aqueous alteration in CM carbonaceous chondrites: Microtextures of a layered mineral containing Fe, S, O and Ni

A petrographic and transmission electron microscopy study of the Mighei, Murchison. and Murray CM carbonaceous chondrites shows that much of the CM matrix material was probably produced by aqueous alteration of olivine, pyroxene, sulfide, and metal. The amount of CM matrix appears to be proportional to the degree of alteration, as suggested by McSween (1979), and microtextures of PCP (“poorly characterized phase”) provide evidence of the progressive alteration. n nPCP is divided into two major types; one occurs in chondrules and aggregates and consists largely of an Fe-Ni-S-O phase (Type-I), and the other occurs in matrix and consists of the Fe-Ni-S-O phase and cronstedtite in various proportions (Type-II). Microtextures of PCP suggest that it resulted from a three-stage alteration process. n1. n(1) Type-I was produced by alteration of kamacite in chondrules and aggregates, presumably early, in the parent body regolith. n n2. n(2) As the alteration advanced, olivine and pyroxene were converted to serpentine. Type-I PCP separated from chondrules and aggregates (into the matrix) during regolith gardening. Simultaneously, the Fe-Ni-S-O phase reacted with Si (released by alteration of olivine and pyroxene), producing well-formed platy cronstedtite and coherent intergrowths of the Fe-Ni-S-O phase and cronstedtite. The Fe-Ni-S-O phase also recrystallized into platy and pod-like crystals. Fe. S. Ni, Cr. and P were leached out of Type-I PCP and were deposited as small grains of Fe-Ni Sulfides, magnetite, chromite. and a mineral (unidentified) containing Fe. Ni, Cr, and P. As a result, PCP came to consist primarily of the Fe-Ni-S-O phase and cronstedtite, i.e., Type-II PCP. n n3. n(3) During continued alteration, the well formed crystals of the Fe-Ni-S-O phase, cronstedtite, and their intergrowths in Type-II PCP were replaced by poorly formed fibers. n n n n nIn comparison to other CM chondrites, Mighei. Murchison, and Murray are relatively unaltered. Their matrices retain abundant amounts of the Fe-Ni-S-O phase and cronstedtite, commonly as distinctive PCP grains, which account for a large proportion of the Fe in these meteorite matrices. In more altered CM chondrites, much of the Fe-Ni-S-O phase was probably consumed to produce cronstedtite, magnetite, and sulfides. With further alteration, cronstedtite itself reacted with the serpentine to form ferroan serpentine. Thus the CM matrix was increasingly enriched in Mg with alteration, and PCP was increasingly degraded and intimately mixed with the magnesian phyllosilicates.

Matrix mineralogy of the Orgueil CI carbonaceous chondrite

A petrographic and transmission electron microscopic study of the Orgueil CI carbonaceous chondrite shows that the matrix consists mainly of Fe-bearing, Mg-rich serpentine and smectite (saponite) as well as a poorly crystallized Fe-rich material that contains minor, variable S and Ni; the Fe-rich material is probably ferrihydrite (a ferric hydroxide). The ferrihydrite occurs in small particles (<80 A in diameter), and the S and Ni are probably adsorbed on its surfaces. The serpentine and saponite occur in poorly crystallized, fine crystallites (“fine phyllosilicates”) that are intimately associated with the ferrihydrite. Coarsely crystallized phyllosilicates (“coarse phyllosilicates”) occur in clusters relatively free of ferrihydrite. The ferrihydrite is a major matrix constitutent and probably accounts for most of the Fe3+ as well as the superparamagnetic fraction in the Orgueil matrix. n nThe textures suggest that the fine phyllosilicates resulted from alteration of the coarse phyllosilicates. The alteration probably involved a substantial amount of water and coincided with the period of sulfate vein formation. During the aqueous alteration magnetite grains, disaggregated from “framboids”, and Fe-(Ni) sulfides were replaced by ferrihydrite. Simultaneously, aqueous solutions reacted with the coarse phyllosilicates and precipitated ferrihydrite, producing intimate, pervasive mixtures with the fine phyllosilicates. n nThe matrix mineralogy of the Orgueil chondrite suggests that Orgueil experienced considerably different alteration conditions from the CM chondrites. The mineralogical and petrological differences between CI and CM chondrites can not simply be explained as the result of different degrees of aqueous alteration.

Transmission electron microscopy of the “LOW-CA” hydrated interplanetary dust particle

Transmission electron microscopy of a hydrated interplanetary dust particle indicates that it consists largely of a poorly crystalline phyllosilicate containing Fe, Mg and Al with an interlayer spacing of 10 to 12Aand so is distinct from the major phyllosilicate in CI and CM carbonaceous chondrites. The silicate is probably an Fe- and Mg-rich smectite or mica. Submicron, spherical to euhedral pyrrhotite and pentlandite are prominent. Unusual, low-Ni ( < 3 at.% Ni) pentlandite is also common and typically occurs as rectangular platelets. Unlike many chondritic interplanetary dust particles, olivine is rare and pyroxene was not observed. Other less abundant phases are magnetite, chromite, and an unidentified phase containing Fe, Mg, Si, Ca, and Mn. This particle differs from a hydrated micrometeorite described previously by Brownlee [1], indicating there are mineralogically different subsets of hydrated interplanetary dust particles. Despite gross similarities in mineralogy between the particle and the carbonaceous chondrites, they show appreciable differences in detail.