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 AsO₄^3– 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 CO₃^2– 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.