Mineralogical, studies of the nitrate deposits of Chile. II. Darapskite, Na₃(NO₃)(SO₄)·H₂O¹

Darapskite, Na₃(NO₃)(SO₄)·H₂O, though occurring only sparsely at four other localities, is widespread in the nitrate deposits of Chile. It occurs as euhedral crystals in cavities and as platy to granular material admixed with other saline minerals. The abundance of darapskite here and its scarcity elsewhere results from the exceptionally high concentration of nitrate in the deposits and can be explained by the phase relations in the system NaNO₃-NaCl-Na₂SO₄-H₂O, which includes the principal saline components of the nitrate ore.

Crystals from Oficina Alemania are monoclinic, P2₁/m; a = 10.564±0.002 Å, b = 6.913 ± 0.001 Å, c = 5.1890±0.0009 Å, β = 102° 47.8′±0.8′, Z = 2, ρ(meas) 2.202 g/cm³. X-ray powder diffraction data for darapskite, presented for the first time, show the following strong lines: 10.29 Å (100) (100); 3.456 Å (35) (020); 2.865 Å (35) (220) and (021); 2.594 Å (30) (102); 4.13 Å (25) (210); and 3.522 Å (25) (211). The line d = 10.29 Å is extremely sensitive and appears in mixtures of saline minerals that contain as little as $13$ of a percent darapskite. Comparison of physical, optical, crystallographic, and X-ray data confirms published data and demonstrates very close similarity between natural and synthetic darapskite.