Disordered biopyriboles, amphibole, and talc in the Allende meteorite: products of nebular or parent body aqueous alteration?

Our plain-English summary of Disordered biopyriboles, amphibole, and talc in the Allende meteorite: products of nebular or parent body aqueous alteration?, by A.J. Brearley (Science, 1997).

The study

• Author: A.J. Brearley
• Journal: Science
• Published: 1997;276(5315):1103-5
• Cited by: 8 articles, as counted by NIH iCite
• Read it: PubMed · Publisher

Abstract

Transmission electron microscope observations of the Allende carbonaceous chondrite provided evidence of widespread hydrous phases replacing enstatite in chondrules. Calcic amphibole and talc occur in thin (less than 0.3 micrometer) crosscutting veins and as alteration products of primary chondrule glass in contraction cracks within the enstatite. In addition, talc and disordered biopyriboles were found replacing enstatite grains along cracks and fractures. Although rare hydrous phases have been reported in calcium- and aluminum-rich inclusions in the Allende meteorite, these observations suggest that aqueous fluids played a much more significant role in the mineralogical and geochemical evolution of Allende than has previously been thought.

In plain English

This article is a mineralogy study of the Allende meteorite, a carbonaceous chondrite. It does not study people, animals, or asbestos-related disease. The author used transmission electron microscopy to examine seven pyroxene-rich chondrules from a single thin section of the meteorite. In all seven chondrules, hydrous (water-bearing) minerals had formed in place of the mineral clinoenstatite, although the degree of alteration was minor. Thin crosscutting veins, described as less than 0.3 micrometer wide and filled with fibrous minerals, contained talc and calcic amphibole with minor anthophyllite. Talc and amphibole also occurred in contraction cracks. Highly disordered biopyriboles, chain structures three to eight chains wide, were found together with talc. Amphibole also occurred as inclusions inside iron-rich olivine. From this, the author reported that the veins formed before or at the same time as that olivine. The author wrote that a model of alteration in the early solar nebula appeared inconsistent with the observations. The author concluded that aqueous fluids played a much more significant role in the meteorite's history than had previously been thought.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Laboratory mineralogical study of meteorite samples by transmission electron microscopy (no human or animal subjects)
• Who was studied: Seven porphyritic pyroxene chondrules from a single thin section of the Allende CV3 carbonaceous chondrite (a meteorite); no people were studied
• Exposure: No human or animal exposure was studied. The minerals described are those found in the meteorite: talc, calcic amphibole (composition close to magnesiohornblende), minor anthophyllite, and disordered biopyriboles. Chrysotile is mentioned only in citing earlier experimental data on its dehydration to talc.
• Main results:
  • In all seven chondrules, hydrous phases were present, restricted to clinoenstatite crystals. The degree of alteration was minor, and the alteration was identical in all the chondrules.
  • Thin veins (described in the article's abstract as less than 0.3 micrometer) of fibrous minerals crosscut the clinoenstatite. The vein minerals were talc with calcic amphibole and minor anthophyllite.
  • Regions of highly disordered biopyriboles were found associated with talc. They consisted of clinoenstatite interlayered with chains three to eight chains wide. Only a single lamella of an ordered biopyribole (jimthompsonite) was observed.
  • Amphibole occurred as inclusions within iron-rich olivine. The author reported that the veins and amphibole inclusions formed before or at the same time as that olivine.
  • The author wrote that a nebular (early solar system gas) model of alteration appeared inconsistent with the observations.
• The authors' conclusion: The author concluded that the observations suggest aqueous fluids played a much more significant role in the mineralogical and geochemical evolution of the Allende meteorite than previously thought.
• Limitations the authors state:
  • The phase relations could not be analyzed rigorously because the mineral assemblage is not at thermodynamic equilibrium.
  • The conditions of the proposed initial episode of aqueous alteration are uncertain.

Topics

The National Library of Medicine indexes this study under subjects our site covers: Anthophyllite, Talc and vermiculite.

Medical terms in this study, explained in our Lexicon: Amphibole Group, Talc, Calcic Amphibole.

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