The synthetic rhomboclase (Rbc) and arsenic-substituted rhomboclase phases (As-Rbc) were colorless well-formed rhombic thin tabular crystals. For their dissolution at initial pH 2–10 and 25–45 °C, the constituents were preferentially dissolved in the order of H₅O₂⁺ ≫ AsO₄ > SO₄ > Fe for ~1 h, H₅O₂⁺ ≫ AsO₄ > Fe > SO₄ from ~1 h to 720 h and finally H₅O₂⁺ ≫ AsO₄ > SO₄ = Fe, suggesting an initial nonstoichiometric dissolution of As-Rbc to form Fe-rich outlayers and a final stoichiometric dissolution. The PHREEQC simulation indicated that the dissolved iron, sulfate and arsenate were present mainly as FeSO₄⁺/Fe(SO₄)₂^-/Fe³⁺, HSO₄⁻/FeSO₄⁺ and H₃AsO₄⁰/H₂AsO₄⁻, respectively. The SEM observation showed that the tops and edges of the crystals were dissolved and that the crystal surfaces were then gradually smoothed/rounded until all the edges and corners disappeared. For the dissolution at pH 2 and 25 °C for 720–2880 h, the ion-activity products [log IAP], which equal the solubility products [log K_sp] at equilibrium, and the Gibbs free energies of formation [ΔG_f^°] were estimated to be −8.80 ± 0.01 and −2706.44 ± 1.90 kJ/mol for the Rbc [(H₅O₂)_1.434Fe_0.855(SO₄)₂·2H₂O], respectively. For the As-Rbc [(H₅O₂)_y (Fe)_z [(AsO₄)_x (SO₄)_1-x]₂·2H₂O], the log IAP and ΔG_f^° values varied from −10.03 ± 0.03 to −8.89 ± 0.01 and from −2680.51 ± 2.05 kJ/mol to −2548.71 ± 1.94 kJ/mol, respectively. There was a strong negative correlation between the log IAP and the AsO₄/(AsO₄+SO₄) molar ratio of As-Rbc. The results of this study provide new interesting thermodynamic data for the most acidic sulfates Rbc and As-Rbc; these data can be applied in the geochemical modeling and prediction of acid mine drainage systems.

合成菱长石 (Rbc) 和砷取代的菱长石相 (As-Rbc) 是无色的、结构良好的菱形薄板状晶体。对于它们在初始 pH 值 2–10 和 25–45 °C 下的溶解，成分优先按照 H ₅ O ₂ ⁺ ≫ AsO ₄  > SO ₄  > Fe的顺序溶解约 1 小时，H ₅ O ₂ ⁺ ≫ AsO ₄  > Fe > SO ₄从 ~1 小时到 720 小时，最后是 H ₅ O ₂ ⁺ ≫ AsO ₄  > SO ₄= Fe，表明 As-Rbc 的初始非化学计量溶解形成富铁外层和最终化学计量溶解。PHREEQC 模拟表明，溶解的铁、硫酸盐和砷酸盐主要以 FeSO ₄ ⁺ /Fe(SO ₄ ) ₂ ^- /Fe ³⁺、HSO ₄ ^- /FeSO ₄ ⁺和 H ₃ AsO ₄ ⁰ /H ₂ AsO _{4 的形式存在}^-， 分别。SEM观察表明晶体的顶部和边缘被溶解，然后晶体表面逐渐平滑/圆化，直到所有的棱角消失。对于在 pH 2 和 25 °C 下溶解 720–2880 小时，离子活度积 [log IAP]，等于平衡时的溶解度积 [log K _sp ]，以及形成的吉布斯自由能 [Δ G _f对于 Rbc [(H ₅ O ₂ ) _1.434 Fe _0.855 (SO ₄ ) ₂ ·2H ₂ O]，^° ] 估计分别为 -8.80 ± 0.01 和 -2706.44 ± 1.90 kJ/mol 。对于 As-Rbc [(H ₅O ₂ ) _y (Fe) _z [(AsO ₄ ) _x (SO ₄ ) _1-x ] ₂ ·2H ₂ O]，log IAP 和 Δ G _f ^°值从 -10.03 ± 0.03 变化到 -8.89 ± 0.01 和分别从 -2680.51 ± 2.05 kJ/mol 到 -2548.71 ± 1.94 kJ/mol。log IAP 与 AsO ₄ /(AsO ₄ +SO ₄) As-Rbc 的摩尔比。这项研究的结果为大多数酸性硫酸盐 Rbc 和 As-Rbc 提供了新的有趣的热力学数据；这些数据可用于酸性矿山排水系统的地球化学建模和预测。