Alforsite is a barium phosphate chloride mineral with formula: Ba<sub>5</sub>(PO<sub>4</sub>)<sub>3</sub>Cl. It was discovered in 1981, and named to honor geologist John T. Alfors (1930–2005) of the California Geological Survey for his work in the area where it was discovered.

Alforsite is a hexagonal colorless crystal in the chemical class phosphates and the group apatite. It is found in certain parts of central California, primarily Fresno, Mariposa, and Tulare Counties. It has also been found in Baja California, Mexico.

Alforsite is a constituent of the apatite group of minerals. It crystallizes in the hexagonal crystal system with a point group of 6/m and space group P6<sub>3</sub>/m. It occurs as colorless grains that are hard to distinguish from fluoroapatite, as they both display low birefringence and high relief.

Composition

Alforsite's ideal chemical formula is Ba<sub>5</sub>(PO<sub>3</sub>)<sub>4</sub>Cl. Chemical analysis of the composition of alfrosite was demonstrated by the use of an Ortex current digitizer and an electron microbe. Synthetic barium chlorapatite, strontianite, and fluorapatite were used as standards for the elements they contain. The chemical analysis revealed that traces of Mn, S, Si, and Pb were present and provided computational formulas of (Ba<sub>4.68</sub>Sr<sub>0.19</sub>Ca<sub>0.13</sub>)(P<sub>2.98</sub>Si<sub>0.01</sub>)O<sub>11.96</sub>(Cl<sub>0.99</sub>F<sub>0.05</sub>) and (Ba<sub>4.05</sub>Ca<sub>0.75</sub>Sr<sub>0.24</sub>Pb<sub>0.03</sub>)(P<sub>2.94</sub>Si<sub>0.01</sub>)O<sub>11.93</sub>(Cl<sub>0.93</sub>F<sub>0.14</sub>).

Structure

Alforsite exhibits Lau symmetry 6/m. It is concluded that the space group is P63/m because every single apatite compound possess that space group. The parameters of the unit cell are a=10.25&nbsp;Å and c=7.64&nbsp;Å.

Physical properties

Alforsite occurs as colorless, minuscule subhedral grains with a diameter less than 0.05&nbsp;mm. However, on rare occasions, it can crystallize to a diameter up to 0.2&nbsp;mm. This can pose problems and confusion because alforsite bears an almost exact resemblance to other barium silicates, which exemplifies its group Greek name, apatao, meaning, “I am misleading”. The only way to differentiate alforsite from other barium minerals is through microchemical analysis (Walstrom and Leising, 2005). This misleading mineral emulates the physical and structural features of apatite and is analogously similar to chlorapatite, providing alfrosite with the alternate name, barium chlorapatite. Alforsite has a low birefringence and a high relief resembling fluorapatite in thin-section.

References

  • Euromin
  • California Geological Survey
  • John T. Alfors obituary (CGS)