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1 GeoForschungsZentrum Potsdam, 14473 Potsdam, Germany
2 Technische Universität Berlin, Fachgebiet Petrologie, EB 15, 10623 Berlin, Germany
Correspondence: * E-mail: alieb{at}gfz-potsdam.de
The Fe3+-Al substitution in synthetic zoisite was studied in the system CFASH at 2.0 GPa and 750 °C (compositional range: 0.0–0.14 Xps). The samples were characterized by powder X-ray diffraction, FTIR, and electron microprobe. Discontinuities in refined lattice parameters at ~0.05 Xps are attributed to two distinct and hitherto unknown modifications, zoisite I (<0.05 Xps) and zoisite II (>0.05 Xps). The following lattice parameters were derived:
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In both modifications, substitution of Fe3+ expands the M3 octahedron, resulting in opposed rotations of the corner-linked T1 and T2 tetrahedra of the Si2O7 group. The extent of rotation is limited and controls the maximum Fe3+ content in zoisite I and II. With increasing Fe3+ content, zoisite I transforms to zoisite II and zoisite II to clinozoisite. The transformation from zoisite I to II can be classified as a substitutionally induced isosymmetric displacive phase transition.
Four significant IR bands were observed at ~3250, ~3195, ~3155, and ~2170 cm–1. The first three bands are attributed to the configurations Al2[M1,2]-O10-H···O2-Al2[M1,2](Al,Fe3+)[M3], Al2[M1,2]-O10-H···O4-Al2[M1,2]Fe3+[M3], and Al2[M1,2]-O10-H···O4-Al2[M1,2]Al[M3]. O10-H···O2 is bifurcated between the two symmetrically arranged O2 and O2' atoms. The band at ~2170 cm–1 is interpreted as the first overtone of the bending vibration of O10-H···O2. In analogy with the results from powder XRD the IR bands show discontinuities at ~0.05 Xps, confirming the two modifications of zoisite.
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