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 (ρ_meas 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 Ca_1.53Sr_0.03Na_3.42K_0.02[(Cl_0.06F_0.06OH_0.44)(S_2.99O₁₂)]·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⁺.