Abstract
A heat treatment was performed on selected epidote and clinozoisite crystals to establish the nature of any changes in the optical and crystal-chemical properties and to identify a breakdown product using a wide spectrum of analytical methods. Natural samples were heated from 900 to 1200 °C under atmospheric pressure in ambient oxidation conditions for 12 h. Epidote and clinozoisite were stable at 900 °C; those heated at 1000 °C, 1100 °C, and 1200 °C exhibited signs of breakdown, with the development of cracks and fissures. The average chemical composition of epidote is Ca2.000Al2.211Fe0.742Si2.994O12(OH), while that of clinozoisite is Ca2.017A12.626Fe0.319Si3.002O12(OH). The breakdown products identified by electron microanalysis, powder X-ray diffraction, Raman spectroscopy, and high-resolution transmission electron microscopy were anorthite, pyroxene compositionally close to esseneite, and wollastonite. The decomposition of the epidote-clinozoisite solid solution is controlled by the following reaction: 4 epidote/clinozoisite → 2 pyroxene + 2 wollastonite + 4 anorthite + 2 H2O. Pyroxene likely contains a significant proportion of tetrahedral Fe3+ as documented by the Mössbauer spectroscopy. Moreover, the presence of hematite in the Mössbauer spectrum of the clinozoisite sample heated at 1200 °C can result from the following reaction: 4 epidote → pyroxene + 3 wollastonite + 4 anorthite + hematite + 2 H2O.
Highlights
IntroductionEpidote and clinozoisite are monoclinic sorosilicate minerals belonging to the epidote supergroup; their formulae can be written as A1 CaA2 CaM1 AlM2 AlM3 Fe3+ (Si2 O7 )(SiO4 )O(OH)
Introduction published maps and institutional affilEpidote and clinozoisite are monoclinic sorosilicate minerals belonging to the epidote supergroup; their formulae can be written as A1 CaA2 CaM1 AlM2 AlM3 Fe3+ (Si2 O7 )(SiO4 )O(OH)and A1 CaA2 CaM1 AlM2 AlM3 Al(Si2 O7 )(SiO4 )O(OH), respectively [1]
The epidote-clinozoisite solid solution stability was studied under ambient conditions, atmospheric pressure and without addition of any fluid
Summary
Epidote and clinozoisite are monoclinic sorosilicate minerals belonging to the epidote supergroup; their formulae can be written as A1 CaA2 CaM1 AlM2 AlM3 Fe3+ (Si2 O7 )(SiO4 )O(OH). A1 CaA2 CaM1 AlM2 AlM3 Al(Si2 O7 )(SiO4 )O(OH), respectively [1]. The A(1) and A(2) sites with a coordination number (CN) >6 are mainly occupied by Ca, while M(1), M(2), and. Several studies showed that Fe3+ preferentially occupies the M3 site [2,3,4], and that the amount of Fe3+ at the M1 and M2 sites is not very extensive [5]. The crystal structure and crystal chemistry of epidote-clinozoisite series at room or low temperature have been investigated extensively using several analytical methods iations
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