Oxidation processes and thermal stability of actinolite

Abstract

Understanding the thermal behaviour of iron-containing amphiboles (AB₂C₅T₈O₂₂W₂, C₅ = M(1)₂ M(2)₂ M(3)) at atomic-level scale may have important implications in several fields, including metamorphic petrology, geophysics, and environmental sciences. Here, the thermally induced oxidation and decomposition of actinolite are studied by in situ high-temperature Raman spectroscopy and complementary thermogravimetric/mass-spectrometry analyses as well as X-ray diffraction of the products of amphibole decomposition. The effect of ^CFe²⁺ on dehydrogenation/dehydroxylation is followed by comparing the results on actinolite with those for tremolite. We show that mobile charge carriers, namely polarons (conduction electrons coupled to FeO₆ phonons) and H⁺ cations, exist in actinolite at elevated temperatures ~ 1150–1250 K. The temperature-induced actinolite breakdown is a multistep process, involving (i) delocalization of e⁻ from ^CFe²⁺ as well as of H⁺ from hydroxyl groups shared by Fe-containing M(1)M(1)M(3) species, which, however, remain in the crystal bulk; (ii) dehydrogenation and ejection of e⁻ between 1250 and 1350 K, where actinolite can be considered as “oxo-actinolite”, as H⁺ also from hydroxyl groups next to ^M(1,3)(MgMgMg) configurations become delocalized and mostly remain in the crystal bulk; (iii) complete dehydroxylation and consequent structure collapse above 1350 K, forming an Fe³⁺-bearing defect-rich augitic pyroxene. The dehydrogenation of tremolite occurs at 1400 K, triggering immediately a disintegration of the silicate double-chain into single SiO₄-chains and followed by a rearrangement of the amphibole octahedral strips and ^BCa²⁺ cations into pyroxene-type octahedral sheets at 1450 K. The result of tremolite decomposition is also a single-phase defect-rich clinopyroxene with an intermediate composition on the diopside–clinoenstatite join.