Abstract: Cesanite occurs both as a solid vein (1 cm thick) and as a cavity-filling of an explosive breccia in core samples of the Cesano 1 geothermal well (Cesano area, Latium, Italy).
Cesanite is coloudess, medium to coarse grained, soft (H = 2 to 3) and light (ρmeas 2.786±0.002 g cm−3). It is uniaxial negative, ε = 1.564, ω = 1.570, with space group P63/m and cell parameters a = 9.442 (4), c = 6.903 (3) Å, for c/a = 0.730. Identifying spacings are 8.161, 2.822, 2.727,1.844 Å, in X-ray powder patterns strikingly similar to those of apatite. The chemical formula (microprobe analyses on two grains) is Ca1.53Sr0.03Na3.42K0.02[(Cl0.06F0.06OH0.44)(S2.99O12)]·0.44H2O, while the theoretical formula, derived from considerations on structural identity with apatite, is Ca2Na3[(OH)(SO4)3]. Cesanite is the end member of the apatite-wilkeite ellestadite series where [PO4]3− is entirely substituted by [SO4]2−, the charge balance being made up by Na+ substituting for Ca2+.