Original paper

Thermodynamics, stability, crystal structure, and phase relations among euchroite, Cu2 (AsO4)(OH)·3H2O, and related minerals

Majzlan, Juraj; Števko, Martin; Dachs, Edgar; Benisek, Artur; Plášil, Jakub; Sejkora, Jiří

Abstract

Euchroite, ideally Cu2 (AsO4)(OH)· 3H2 O, is a supergene copper arsenate mineral, known from a number of oxidation zones in a form of emerald-green crystals. Here, we investigated thermodynamics, crystal structure, and parageneses of euchroite from the Svätodušná Cu-deposit near the village of L'bietová (Slovakia). The actual chemical composition of the studied sample is Cu2 [(AsO4)0.98 (PO4)0.02](OH)· 3H2O. A single-crystal study added the positions of H atoms to the previously known structural model of euchroite. All of the (H2 O) groups in the structure are non-transformer, with their O atoms four-fold coordinated. The structural formula of euchroite can be written as [[6] Cu2 2+(AsO4)(OH)(H2 [4]O)3]. The measured thermodynamic properties of Cu2 [(AsO4)0.98 (PO4)0.02](OH)·3H2 O are ΔfH° = 1884.7 ± 4.7 kJ mol-1, S° = 276.6 ± 3.3 J mol-1 K1, and ΔfG° = 1555.7 ± 4.8 kJ mol-1. The equilibrium constant for the reaction Cu2 (AsO4 )0.98 (PO4)0.02 (OH)· 3H2O + 3H+ = 2Cu2+0.98H2 AsO4 - + 0.02H2 PO4 -+ 4H2 O is log K sp = 4.19 ± 0.87. We estimated the properties of the end-member euchroite Cu2 (AsO4)(OH)·3H2 O with log K sp = 4.19. Euchroite is metastable with respect to olivenite, and numerous observations document this metastability. Yet, there are many samples that persist for centuries without change. Observations in the field show that there are also pseudomorphs of cornwallite after euchroite or complex pseudomorphs of cornwallite + cornubite + olivenite after euchroite. Euchroite at the site studied is younger than chalcophyllite and strashimirite; the temporal relationship to parnauite is more complex, as euchroite can be older or younger than parnauite. Our results suggest that euchroite is one of the less-stable copper arsenates, but the energetic margin to the stable phases in the CuO– As2 O5 –H2 O system is small.

Keywords

euchroitecrystal chemistrythermodynamicsoxidation zoneparagenesis