Thermodynamic properties, crystal structure and phase relations of pushcharovskite [Cu(AsO₃OH)(H₂O) ⋅ 0.5H₂O], geminite [Cu(AsO₃OH)(H₂O)] and liroconite [Cu₂Al(AsO₄)(OH)₄ ⋅ 4H₂O]

<p>The phases pushcharovskite, geminite and liroconite were synthesized or acquired and characterized by powder X-ray diffraction, infrared spectroscopy, electron microprobe analysis, thermogravimetric analysis and optical emission spectrometry, as needed. Their thermodynamic properties were determined by a combination of acid-solution calorimetry and relaxation calorimetry, resulting in Gibbs free energies of formation (<span class="inline-formula">Δ_fG^o</span>, all values in kilojoules per mole) of <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M5" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>-</mo><mn mathvariant="normal">1036.4</mn><mo>±</mo><mn mathvariant="normal">3.8</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="70pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="a45b48a98adf8eac63c9c2113d74835c"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ejm-32-285-2020-ie00001.svg" width="70pt" height="10pt" src="ejm-32-285-2020-ie00001.png"/></svg:svg></span></span> (pushcharovskite, <span class="inline-formula">Cu(AsO₃OH)(H₂O)⋅0.5H₂O</span>) and <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M7" display="inline" overflow="scroll" dspmath="mathml"><mrow><mo>-</mo><mn mathvariant="normal">926.7</mn><mo>±</mo><mn mathvariant="normal">3.2</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="64pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="6eddae8aedd075fb5241de6d041289bf"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ejm-32-285-2020-ie00002.svg" width="64pt" height="10pt" src="ejm-32-285-2020-ie00002.png"/></svg:svg></span></span> (geminite, <span class="inline-formula">Cu(AsO₃OH)(H₂O)</span>). For the natural liroconite (<span class="inline-formula">Cu₂Al[(AsO₄)_0.83(PO₄)_0.17](OH)₄⋅4H₂O</span>), <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M10" display="inline" overflow="scroll" dspmath="mathml"><mrow><mrow class="chem"><msub><mi mathvariant="normal">Δ</mi><mi mathvariant="normal">f</mi></msub><msup><mi mathvariant="normal">G</mi><mi mathvariant="normal">o</mi></msup></mrow><mo>=</mo><mo>-</mo><mn mathvariant="normal">2996.3</mn><mo>±</mo><mn mathvariant="normal">9.2</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="107pt" height="14pt" class="svg-formula" dspmath="mathimg" md5hash="b980a747480b9cf6b6220146315ae9e4"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="ejm-32-285-2020-ie00003.svg" width="107pt" height="14pt" src="ejm-32-285-2020-ie00003.png"/></svg:svg></span></span>&thinsp;kJ&thinsp;mol<span class="inline-formula">⁻¹</span>. The estimated <span class="inline-formula">Δ_fG^o</span> for the endmember <span class="inline-formula">Cu₂Al(AsO₄)(OH)₄⋅4H₂O</span> is <span class="inline-formula">−2931.6</span>&thinsp;kJ&thinsp;mol<span class="inline-formula">⁻¹</span>. The crystal structure of liroconite was refined (<span class="inline-formula"><i>R</i>₁=1.96</span>&thinsp;% for 962 reflections with <span class="inline-formula"><i>I</i>&gt;3<i>σ</i>(<i>I</i>)</span>) by single-crystal X-ray diffraction and the positions of H atoms, not known previously, were determined. Liroconite is a rare mineral, except for several localities, notably Wheal Gorland in England. Thermodynamic modelling showed that liroconite will be preferred over olivenite if the Al(III) concentration in the fluid reaches levels needed for saturation with X-ray amorphous <span class="inline-formula">Al(OH)₃</span>. We assume that such fluids are responsible for the liroconite formation during contemporaneous oxidation of primary Cu–As ores and pervasive kaolinization of the host peraluminous granites. pH had to be kept in mildly acidic (5–6), and the activities of dissolved silica were too low to form dioptase. The main stage with abundant liroconite formation was preceded by an acidic episode with scorodite and pharmacosiderite and followed by a late neutral to mildly basic episode with copper carbonates. Geminite and pushcharovskite, on the other hand, are minerals typical for very acidic solutions. At the studied site in Jáchymov (Czech Republic), extremely acidic water precipitates arsenolite; sulfate is removed by formation of gypsum. Geminite associates with other acidic minerals, such as slavkovite, yvonite and minerals of the lindackerite group. Pushcharovskite is metastable with respect to geminite and probably converts quickly to geminite under field conditions.</p>