Scandium arsenate is an inorganic compound of scandium and the arsenate ion, with the chemical formula ScAsO4. It forms hydrated phases, including the well-characterized dihydrate ScAsO4·2H2O. The dihydrate occurs naturally as the rare mineral bonacinaite.
Preparation Scandium arsenate can be precipitated from aqueous solutions containing scandium(III) and arsenate ions.
A hydrated product can be obtained by reacting scandium nitrate with diammonium hydrogen arsenate.
The dihydrate can also be prepared by reaction of scandium hydroxide with arsenic acid at room temperature.
A hydrated phase of approximate composition ScAsO4·2.5H2O has been reported to precipitate from solutions over the approximate pH range 2.0–7.1.
Properties Hydrates and crystal structure The best-characterized hydrated phase is scandium arsenate dihydrate, ScAsO4·2H2O.
It crystallizes in the monoclinic crystal system, space group P21/n (No. 14), with the metavariscite structure type.
For natural bonacinaite, representative single-crystal lattice parameters are a = 5.533 Å, b = 10.409 Å, c = 9.036 Å, β = 91.94° with four formula units per unit cell.
Its framework is built from corner-sharing AsO4 tetrahedra and ScO4(H2O)2 octahedra. Hydrogen bonding between coordinated water molecules and framework oxygen atoms reinforces the three-dimensional structure.
The ideal dihydrate is isostructural with other members of the metavariscite group.
Natural occurrence ScAsO4·2H2O occurs naturally as bonacinaite, the first known scandium arsenate mineral.
The mineral was discovered at the Varenche Mine in the Aosta Valley, Italy, where it occurs as a low-temperature hydrothermal mineral. It forms colorless to faintly violet, transparent crystals and has a calculated density of about 2.82 g/cm3.
Natural bonacinaite commonly contains partial substitution of Al for Sc and P for As; its ideal end-member formula is nevertheless ScAsO4·2H2O.
Thermal behaviour Hydrated scandium arsenate loses water stepwise on heating.
The approximately 2.5-hydrate loses about half a water molecule between roughly 50 and 130 °C to form ScAsO4·2H2O.
Further dehydration occurs between approximately 250 and 370 °C, ultimately giving anhydrous ScAsO4.
Earlier comparative work found the dihydrate to be isomorphous with scandium phosphate dihydrate and somewhat more soluble and less thermally stable than the corresponding phosphate.
Solubility Scandium arsenate is very sparingly soluble in water.
A concentration-based solubility-product value of pKsp,c = 26.72 ± 0.27 has been reported.
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