Molecular assembly in secondary minerals - Raman spectroscopy of the arthurite group species arthurite and whitmoreite

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Two members of the arthurite mineral group, arthurite and whitmoreite A(Fe3+) 2 (XO 4 ) 2 (O,OH) 2 .4(H 2 O) where A = Cu(II), Zn(II), Pb(II) or Fe(II) and X = As, P, S have been studied using Raman spectroscopy. The minerals are based upon the combination of a divalent cation such as Cu(II), Zn(II), Pb(II) or Fe(II) and the trivalent cation, Fe(III), with counterbalancing of hydroxyl, arsenate, phosphate and sulphate anions. Such minerals lend themselves to analysis by Raman spectroscopy. For arthurite Cu(Fe 3 + ) 2 (AsO 4 ,PO 4 ,SO 4 ) 2 (O,OH) 2 .4(H 2 O), arsenate and hydroxyl anions are observed as well as minor phosphate and significant carbonate anions. For whitmoreite FeFe 2 (PO 4 ) 2 (OH) 2 .4(H 2 O), hydroxyls and phosphate are the major anions with significant sulphate and some carbonate. Raman spectroscopic analysis of two whitmoreite samples from the same mineral field suggests that differentiation by concentration of the anions occurs in the molecular assembly of the anions as crystallisation occurs as a function of the equilibrium conditions. Changing solution conditions are reflected in the varying compositions in the lattice.