Raman Microscopy Study of Tyrolite: A Multi-Anion Arsenate Mineral
The Raman spectrum of tyrolite, CaCu5(AsO4)2(CO3)(OH) 4.6H2O, from Brixlegg, Tyrol, Austria, is reported. Comparison with copper hydroxy-arsenate
and basic carbonates was used to achieve assignments of the observed bands. The AsO43- group is characterized by two upsilon modes around 433 and 480 cm -1 plus a broad band around 840 cm -1
as the upsilon overlapping with the upsilon. The upsilon mode is observed as a single band around 355 cm -1. The CO32- upsilon mode is observed around 1035 and 1088 cm -1, although this assignment
is difficult because of the in-plane OH bending vibrations at similar frequencies. Two upsilon4 modes are assigned to the 717 and 755 cm - 1 bands. The upsilon3 mode is present as
three bands at 1431, 1463, and 1498 cm - 1. A large split caused by bridging carbonates may explain the band at 1370 cm -1. The H2O bending region shows two bands at 1635 and 1667 cm -1 together
with stretching modes around 3204 and 3303 cm -1, the first associated with adsorbed H2O, while the second indicates more strongly bonded H2O. Three bands around 3534, 3438, and 3379
cm -1 are assigned to OH stretching modes of the OH groups in the crystal structure. The 202, 262, 301, 524, and 534 cm -1 bands are assigned to Cu-OH bending and stretching modes, whereas the bands around
179, 202, and 217 cm -1 are ascribed to O-(Ca, Cu)-O(H) with the O(H) at much greater distance from the cation. The bands around 503, 570, and 598 cm -1 are ascribed to the Cu-O stretching modes. Index
Headings: Arsenate; Carbonate; Hydroxyl groups; Raman microscopy; Tyrolite.
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