Synthesis and solubility studies of onoratoite have been undertaken to determine the role of this rare secondary phase in the immobilization of Sb and the conditions responsible for its formation in the supergene zone. Solubility studies were undertaken at 298.15 K. A value of ΔGfθ (Sb8O11Cl2, s, 298.15 K) = –2576 ±12 kJ mol–1 was derived. Calculations involving sénarmontite, Sb2O3, klebelsbergite, Sb4O4SO4(OH)2 and schafarzikite, FeSb2O4, show that onoratoite is a thermodynamically stable phase only at negligible activities of SO42–(aq) and low activities of Fe2+(aq), at low pH and very high activities of Cl–(aq). This explains why onoratoite is such a rare secondary phase and why it cannot exert any significant influence on the dispersion of Sb in the supergene environment.

Synthesis and solubility studies of brizziite, NaSbO3, have been undertaken to determine the possible role of this rare secondary phase in the immobilization of Sb under supergene conditions and the conditions responsible for its formation in the supergene zone. Solubility studies were undertaken at T = 298.15 K. A value of ΔGfө) (NaSbO3, s, 298.15 K) = –806.66 ± 1.4 kJ mol–1 was derived. Calculations involving tripuhyite, FeSbO4, byströmite, MgSb2O6, ordoñezite, ZnSb2O6 and rosiaite, PbSb2O6, show that brizziite is a thermodynamically stable phase only at negligible activities of Pb2+(aq) at high pH and high salinity. Calculations involving mopungite Na[Sb(OH)6] combined with reported mineral associations suggest that mopungite is the thermodynamically unstable precursor to brizziite and its presence in natural settings must be due to kinetic stability. This explains why brizziite is such a rare secondary phase and therefore why it cannot exert any significant influence on the dispersion of Sb in the supergene environment.

In order to clarify the roles that secondary minerals may have in determining the extent of dispersion of Sb in the supergene environment, syntheses and stability studies of the Sb(V) oxides byströmite, MgSb2O6, ordoñezite, ZnSb2O6 and rosiaite, PbSb2O6, have been undertaken. Solubilities in aqueous HNO3 were determined at 298.2 K and the data obtained used to calculate values of Δ at the same temperature. The derived Δ(s, 298.2 K) values for MgSb2O6 (–1554.1 ±3.6 kJ mol–1), ZnSb2O6 (–1257.0 ±2.6 kJ mol–1) and PbSb2O6 (–1154.2 ±2.6 kJ mol–1) have been used in subsequent calculations to determine their relative stabilities and relationships with other secondary Sb minerals.

Natural samples of the substituted basic Cu(II) chloride series, Cu4–xM x 2+(OH)6Cl2(M = Zn, Ni, or Mg) were investigated by single-crystal X-ray diffraction in order to elucidate compositional boundaries associated with paratacamite and its congeners. The compositional ranges examined are Cu3.65Zn0.35(OH)6Cl2 – Cu3.36Zn0.64(OH)6Cl2 and Cu3.61Ni0.39(OH)6Cl2 – Cu3.13Ni0.87(OH)6Cl2, along with a single Mg-bearing phase. The majority of samples studied have trigonal symmetry (R3̄m) analogous to that of herbertsmithite (Zn) and gillardite (Ni), with a ≈ 6.8, c ≈ 14.0 Å. Crystallographic variations for these samples caused by composition are compared with both published and new data for the R3̄m sub-cell of paratacamite, paratacamite-(Mg) and paratacamite-(Ni). The observed trends suggest that the composition of end-members associated with the paratacamite congeners depend upon the nature of the substituting cation.