Raman Microscopy Study of Tyrolite: A Multi-Anion Arsenate Mineral

Abstract

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 v4 modes around 433 and 480 cm−1 plus a broad band around 840 cm−1 as the v3 overlapping with the v1. The v2 mode is observed as a single band around 355 cm−1. The CO32– v1 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 v4 modes are assigned to the 717 and 755 cm−1 bands. The v3 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.