Pseudomalachite–cornwallite and kipushite–philipsburgite solid solutions: chemical composition and Raman spectroscopy

Abstract

A suite of 60 mineral samples with compositions intermediate between pseudomalachite, Cu 5 (PO 4 ) 2 (OH) 4 , and its arsenate analogue cornwallite, and between kipushite, (Cu,Zn) 6 (PO 4 ) 2 (OH) 6 ·H 2 O, and its arsenate analogue philipsburgite was investigated by electron-probe microanalysis, scanning electron microscopy, powder X-ray diffraction and micro-Raman spectroscopy. A wide range of compositions was found, covering 77.5 mol% and 75 mol% of the cornwallite–pseudomalachite and kipushite–philipsburgite solid solutions, respectively, demonstrating an extended P ↔ As substitution in these minerals. The analyses also reveal a Zn deficiency in the tetrahedral site of the kipushite–philipsburgite structure, leading to the crystal-chemical formula [4] (Zn,Cu) [6] Cu 5 (P 1−x As x O 4 ) 2 (OH) 6 ·H 2 O for this series. Raman spectra of the pseudomalachite–cornwallite solid solution are sensitive to the P/(As + P) ratio. The up-shift of a band assigned to the symmetric stretching vibrations of (AsO 4 ) group recorded in Raman spectra of the arsenate pseudomalachite is indicative of shortening As–O bonds and of bond-angle adjustment to accommodate the large As ions in the pseudomalachite structure. This feature is less evident in the arsenate kipushite, perhaps due its larger cell volume, where larger ions can be more easily accommodated in the anionic sublattice. Raman imaging and cluster analysis were applied to reveal chemical heterogeneity and structural disorder in the kipushite–philipsburgite solid solution.