Occurrence and characterization of tremolite asbestos from the Mid Atlantic Ridge

Our plain-English summary of Occurrence and characterization of tremolite asbestos from the Mid Atlantic Ridge, by D. Di Giuseppe, N. Perchiazzi, D. Brunelli, et al (Scientific Reports, 2021).

The study

• Authors: D. Di Giuseppe, N. Perchiazzi, D. Brunelli, T. Giovanardi, L. Nodari, G. Della Ventura, D. Malferrari, M. Maia, A.F. Gualtieri
• Journal: Scientific Reports
• Published: 2021;11(1):6285
• Cited by: 7 articles, as counted by NIH iCite
• Read it: PubMed · Publisher · Free full text

Abstract

Tremolite is one of the most common amphibole species and, in the fibrous form (i.e., characterized by crystals/particles consisting of fibres with length > 5 µm, width < 3 µm and aspect ratio > 3), one of the six asbestos minerals. Until now the attention of crystallographers has focused only on samples from continental environment. Here we report the first chemical and structural data of a tremolite asbestos found along the Mid Atlantic Ridge (MAR) at the eastern intersection of the Romanche Transform Fault (Equatorial MAR). Tremolite is associated with chlorite and lizardite and was formed through the green shale facies lower than zeolite in a predominantly fluid system. MAR tremolite asbestos shows very slight deviations from the ideal crystal structure of tremolite. Differences in cation site partitioning were found with respect to tremolite asbestos from ophiolitic complexes, attributed to the different chemical-physical conditions during the mineral formation. In particular, oceanic tremolite asbestos is enriched in Al and Na, forming a trend clearly distinct from the continental tremolites.

In plain English

This was a laboratory study of the mineral make-up of a single rock sample, not a study of people or health. The sample was collected from the surface of a seafloor fault on the Mid Atlantic Ridge at the eastern end of the Romanche Transform Fault, during a manned submersible dive at 3978 m below sea level on a 2019 French-Italian expedition. The authors described it as the first detailed characterization of tremolite asbestos from the oceanic lithosphere. The sample was mostly tremolite. Elongated prismatic particles and cleavage fragments made up most of it, and a smaller part was fibrous. Measured fibres were 5.12 to 93.4 μm long and 0.17 to 3.03 μm wide. Under the fibre definition the authors used, a fraction of the fibres qualified as tremolite asbestos. Compared with tremolite from continental sites, this oceanic tremolite contained more aluminium and sodium and followed a different compositional trend. The authors concluded that it formed in a shear-driven, fluid-dominated environment.

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: Mineralogical and crystal-chemical characterization of one natural rock sample, using microscopy (optical, SEM, TEM), electron probe chemical analysis, X-ray single-crystal diffraction, Mössbauer, Raman and FTIR spectroscopy, and thermal analysis. No human or animal data.
• Who was studied: One rock sample (SMA1971-214) taken from the surface of a normal fault at the eastern Ridge-Transform Intersection of the Romanche Transform Fault, Equatorial Mid Atlantic Ridge, during dive SMA1971 at 3978 m below sea level, on the SMARTIES expedition (July to August 2019).
• Exposure: No human exposure was studied. The material examined was naturally occurring tremolite, including a fibrous fraction classed as tremolite asbestos, found together with chlorite and lizardite.
• Main results:
  • The sample was mainly tremolite with two habits: elongated prismatic, including elongate mineral particles and cleavage fragments (the major part), and fibrous (the subordinate part).
  • Observed fibres had lengths from 5.12 to 93.4 μm (average 22.3 μm) and widths from 0.17 to 3.03 μm (average 1.17 μm). All had a length-to-width ratio above 3 (average 24.6).
  • Under the authors' fibre definition (length over 5 µm, width under 3 µm, length-to-width ratio over 3), a fraction of the tremolite fibres in the sample could be classified as tremolite asbestos.
  • Chemical analysis gave average MgO of 22.44 wt% and CaO of 12.10 wt%. Al2O3 ranged from 2.78 to 4.27 wt% (average 3.73 wt%).
  • Chlorite-based geothermometers gave temperature estimates of 202 plus or minus 10 °C and 206 plus or minus 10 °C. The authors suggested these may represent minimum crystallization temperatures for the tremolite.
  • FeO in oceanic tremolite was always above 1.4 wt%, while in continental tremolite it often fell below the detection limit. Continental and oceanic tremolites plotted on different compositional trends.
  • Among the samples compared (one from Gouverneur Mining District, New York, and two from the Italian Alps), this sample had the highest aluminium content in the T(1) site (0.48).
• The authors' conclusion: The authors concluded that the Mid Atlantic Ridge tremolite asbestos follows compositional trends different from both tremolite asbestos and prismatic tremolite from continental settings, and that it formed in a shear-driven, fluid-dominated environment.
• Limitations the authors state:
  • Mössbauer results could not be rigorously tied to the tremolite crystal sites, because the contributions of tremolite and accessory minerals could not be separated.
  • The literature lacks detailed crystallographic descriptions, which limited strict comparison with continental tremolites.
  • The upper formation temperature of the tremolite could not be well constrained, because no other high-temperature mineral was present.
  • The influence of sea water on the mineral's composition is poorly constrained.

Topics

Medical terms in this study, explained in our Lexicon: Tremolite, Amphibole Group, Aspect Ratio (Length-to-diameter ratio), Asbestos, Chlorite, Lizardite, Zeolite, Crystal Structure.

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