A new mineral species, segerstromite, ideally Ca3(As5+O4)2[As3+(OH)3]2, has been discovered at the Cobriza mine in the Sacramento district in the Copiapó Province, Chile. Crystals of segerstromite occur as tetrahedra, dodecahedra (up to 0.50 × 0.50 × 0.50 mm), or in blocky aggregates. Associated minerals include talmessite, vladimirite, and Sr-bearing hydroxylapatite. Similar to the associated minerals, segerstromite is a secondary mineral. The new mineral is colorless in transmitted light, transparent with a white streak and vitreous luster. It is brittle and has a Mohs hardness of ∼4.5. No cleavage, parting, or twinning was observed. The measured and calculated densities are 3.44(3) and 3.46 g/cm3, respectively. Optically, segerstromite is isotropic, with n = 1.731(5). It is insoluble in water or hydrochloric acid. An electron microprobe analysis yielded an empirical formula (based on 14 O apfu) Ca2.98(AsO4)2.00[As(OH)3]2.00.
Segerstromite is cubic, with space group I213 and unit-cell parameters a = 10.7627(2) Å, V = 1246.71(4) Å3, and Z = 4. Its crystal structure is constructed from three different polyhedral units: distorted CaO8 cubes, rigid As5+O4 arsenate tetrahedra, and neutral As3+(OH)3 arsenite triangular pyramids. The Ca-groups form layers of corrugated crankshaft chains that lie parallel to the cubic axes. These chains are linked by the isolated As5+O4 and As3+(OH)3 groups. Segerstromite is the first known crystalline compound that contains the As3+(OH)3 arsenite molecule, pointing to a new potential approach to remove highly toxic and mobile As3+(OH)3 from drinking water.