On the structure of epidote

Structural Breakdown of Natural Epidote and Clinozoisite in High-T and Low-P Conditions and Characterization of Its Products

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.

Epidotes of the Borrowdale Volcanic rocks of central Borrowdale

Epidote and clinozoisite are widely distributed in the andesitic lavas and tuffs of the Borrowdale Volcanic Series. Four main types of occurrence may be distinguished, namely quartz-epidote vein fillings; ‘Shap type’ veins, bordered by pink reaction zones rich in SiO2, Na2O, and K2O; autometasomatized lavas and tuffs; and tuffs subjected to alkali metasomatism. Epidote alone formed in the quartz-epidote veins; clinozoisite is found with epidote in the other types of occurrence.

Epidote and clinozoisite formed in vesicles at pressures of 10–280 kg/cm2, at temperatures of 300–550° C; epidote associated with pre-Bala mineral veins formed in the range 130–550° C at 1000 kg/cm2; epidote around the Caledonian granites probably formed up to 600° C at about 2000 kg/cm2. Clinozoisite crystallized together with epidote in the vesicles, demonstrating a gap in the clinozoisite-epidote solid solution series. Early formed epidote in vesicles is more strongly coloured than late epidote, although the compositions of the earliest and latest fractions are identical.

Properties of the Al–Fe–Mn epidotes

Natural Al-Fe-Mn epidotes are divided into Al-Fe and Al-Fe-Mn series.

In the Al-Fe series,a, b, andcincrease steadily over the range 0 to 0·9 Fe3+per formula unit, and more slowly at higher Fe contents. Index variation diagrams show the reverse trend, with indices increasing moderately fast over the range 0 to 0·85 Fe, and faster at higher Fe contents.

In the Al-Fe-Mn series,bincreases almost linearly over the range of (Fe, Mn) contents, with Mn causing a larger increase than Fe, whilstaandcincrease over the range 0 to 0·9 (Fe, Mn), and then decrease. This curious behaviour is accounted for by the application of crystal field theory. The indices increase linearly over the range 0·8 to 1·5 (Fe, Mn), the optic axial angle depending on (Fe, Mn) content and the ratio Mn/(Fe, Mn).

Thermal and mechanical properties (conductivity, thermal expansion, specific heat, compressibility, and hardness), electromagnetic properties (dielectric constants, conductivity, magnetic susceptibility), and thermodynamic data (S) are also summarized, and estimates given where experimental data are not available.

Structural interpretation of polarized absorption spectra of the Al-Fe-Mn-Cr epidotes

Measurements have been made of the magnetic susceptibility and of the polarized absorption spectra of epidotes of the Al-Fe, Al-Fe-Mn, and Al-Cr series. Magnetic susceptibility measurements at room temperature by the Guoy method indicate that Fe3+, Mn3+, and Cr3+are in ‘high-spin’ electronic states in the epidote structure, and the electronic transitions observed in the absorption spectra have been assigned on this basis. The polarized spectra, measured over the range 4000 to 22 000 Å by a polarizing microscope using a universal stage technique, showed each epidote group to be distinctly pleochroic. This pleochroism correlates with the presence of ions in distorted coordination sites.

Extinction coefficients of Fe3+in epidote are higher than those for other Fe (III) compounds, indicating that Fe3+ions occupy the non-centrosymmetric (Al, Fe) site. The low value (13200 cm−1at 0·155 Fe3+and 12700 cm−1at 0·915 Fe3+) of the crystal-field splitting parameter, Δ, implies that the site is easily expanded to accommodate ions larger than Al3+There is also evidence that increasing Fe3+content causes distortion of the (Al,Fe)O6octahedron, suggesting a possible mechanism for the zoisite ⇌ clinozoisite transition.

The spectra of Al-Mu-Fe epidotes have been interpreted in terms of intense contributions from Mn3+ions in the non-centrosymmetric (Al, Fe) site, and weak contributions from Mn3+ions in the centrosymmetrie AlOH site. Values of Δ vary from 13730 cm−1(0·082 Mn3+) to 13400 cm−1(0·747 Mn3+). Both (Al, Fc) and AlOH sites have strong tetragonal distortions (c/a~ 0·95), leading to the observed large Jahn-Teller splitting of 12000 cm−1.

The Al-Cr epidote spectra show weak absorption bands, attributed to Cr3+ions in a centrosymmetric site of approximately orthorhombic symmetry (i.e. the AlO site). The value of Δ is about 16300 cm−1.

Review of all available data on the distribution of Fe3+, Mn3+, and Cr3+between the sites gives the following site preferences: Fe3+, Al > AlO > AlOH; Mn3+, Al ≥ AlOH ≫ AlO ; Cr3+, AlO ≫ Al, AlOH.

Theoretical and experimental study of the vibrational spectra of (para)symplesite and hörnesite

Arsenate water-bearing minerals, hörnesite (Alšar, Macedonia) and symplesite (Laubach, Germany), were studied by vibrational (IR and Raman) spectroscopy and X-ray powder diffraction. The observed vibrational spectra in both the high (1100-600 cm(-1)) and low (600-450 cm(-1)) wavenumber regions of AsO4 and H2O vibrations could be used to discriminate the two studied minerals. Spectral differences are especially pronounced in the bending and stretching regions of the H2O vibrations in the IR spectra. The observed bands in IR and Raman spectra were tentatively assigned. To support the assignment, IR spectra were theoretically simulated. These calculations were performed using the crystal structure of parasymplesite (no structural information of symplesite has been published so far) and hörnesite using a 3D periodic plane-wave pseudopotential density functional theory approach applying various combinations of exchange-correlation functionals. In this article, we report on the first experimental study of the vibrational spectra of the very rare symplesite mineral.

Crystal field effect on EPR and optical absorption properties of natural green zoisite

In this study the electron paramagnetic resonance (EPR) and optical absorption (OA) of natural crystal of zoisite were investigated after γ ((60)Co) irradiation and high temperature annealing. EPR measurements show that the zoisite from Tefilo Otoni MG Brazil contain Cr(3+), Fe(3+) and Mn(2+) ions and occupy distorted Al(3+) octahedral and tetrahedral sites which are subjected to the action of a strong crystal field in axial direction. Absorption bands which in principle give rise to sets of EPR lines between 500 and 2500 G were found using the deconvolution method. The application of high doses of gamma ray and high temperature annealing has shown no significant effects on EPR and OA spectra. Spin-allowed, spin-forbidden and crystal field parameters were calculated for 3d(3) configuration and interpreted using the spin Hamiltonian formalism containing axial and rhombic terms in low symmetries.

Ordering of V 2+ , Mn 2+ , and Fe 3+ Ions in Zoisite, Ca 2 Al 3 Si 3 O 12 (OH)

The presence of very small amounts of Mn(2+), V(2+), and Fe(3+) ions in zoisite can be easily detected by the electron paramagnetic resonance technique at room temperature. The Mn(2+) and Fe(3+) ions are completely ordered and are probably located in the Ca(1)- and Al(II)-sites, respectively, whereas the V(2+) ions probably occupy both Ca(1)- and Ca(2)-sites, with a preference for the Ca(1)-site.