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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.
Highlights
Asbestos is a general term that collectively refers to six mineral species: chrysotile and five fibrous amphiboles of commercial interest[1,2,3]
We focus on the occurrence of tremolite that, along with hornblende and pargasite is among the most abundant amphibole phases reported from the Mid Ocean Ridges[8,9,10,11,12]
Sample SMA1971-214 has been collected from the surface of a normal fault in the Eastern Ridge-Transform Intersection of the Romanche Transform Fault recovered during the dive SMA1971 at 3978 m b.s.l.15 (Fig. 1 and Supplementary Fig. S1)
Summary
Asbestos is a general term that collectively refers to six mineral species: chrysotile and five fibrous amphiboles of commercial interest[1,2,3]. Actinolite, and anthophyllite asbestos occur in serpentinized basic to ultra-basic igneous rocks commonly found in ophiolitic complexes which represent the main host formations for these minerals[2,4,5]. Fibrous minerals are widespread in the oceanic lithosphere[8,9], the human exposition to mineral fibres has naturally confined the study of these species to the continental o ccurrences[2,3]. It is of great meaning exploring the compositional evolution and diversity of the oceanic and continental domains, being the oceanic lithosphere the main precursor of the continental deposits. Because tremolite is one of the six asbestos species, “tremolite asbestos” is defined as the fibrous variety of tremolite[3,13,14]
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