The substitution Fe3+-Al and the isosymmetric displacive phase transition in synthetic zoisite: A powder X-ray and infrared spectroscopy study

Abstract

The Fe 3+ -Al substitution in synthetic zoisite was studied in the system CFASH at 2.0 GPa and 750 °C (compositional range: 0.0–0.14 X ps ). The samples were characterized by powder X-ray diffraction, FTIR, and electron microprobe. Discontinuities in refined lattice parameters at ~0.05 X ps are attributed to two distinct and hitherto unknown modifications, zoisite I ( X ps ) and zoisite II (>0.05 X ps ). The following lattice parameters were derived: \[\begin{array}{ll}(zoisite I)&(zoisite II)\\\mathit{a}({\AA}) = {-}3.72 {\times} 10^{{-}2} {\times} \mathit{X}_{ps} + 16.1913&\mathit{a}({\AA}) = {-}8.26 {\times} 10^{{-}2} {\times} \mathit{X}_{ps} + 16.2061\\\mathit{b}({\AA}) = 6.43 {\times} 10^{{-}2} {\times} \mathit{X}_{ps} + 5.5488&\mathit{b}({\AA}) = 8.14 {\times} 10^{{-}2} {\times} \mathit{X}_{ps} + 5.5486\\\mathit{c}({\AA}) = 3.43 {\times} 10^{{-}2} {\times} \mathit{X}_{ps} + 10.0320&\mathit{c}({\AA}) = 1.18 {\times} 10^{{-}1} {\times} \mathit{X}_{ps} + 10.0263\\\mathit{V}({\AA}^{3}) = 11.4 {\times} \mathit{X}_{ps} + 901.3&\mathit{V}({\AA}^{3}) = 19.3 {\times} \mathit{X}_{ps} + 901.6\end{array}\] In both modifications, substitution of Fe 3+ expands the M3 octahedron, resulting in opposed rotations of the corner-linked T1 and T2 tetrahedra of the Si 2 O 7 group. The extent of rotation is limited and controls the maximum Fe 3+ content in zoisite I and II. With increasing Fe 3+ 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 Al 2 [M1,2] -O10-H···O2-Al 2 [M1,2] (Al,Fe 3+ ) [M3] , Al 2 [M1,2] -O10-H···O4-Al 2 [M1,2] Fe 3+[M3] , and Al 2 [M1,2] -O10-H···O4-Al 2 [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 X ps , confirming the two modifications of zoisite.