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Mejillonesite, ideally NaMg 2 (PO 3 OH)(PO 4 )(OH)· H 5 O 2 , is a new mineral approved by the CNMNC (IMA 2010-068). It occurs as isolated crystal aggregates in thin zones in fine-grained opal-zeolite aggregate on the north slope of Cerro Mejillones, Antofagasta, Chile. Closely associated minerals are bobierrite, opal, clinoptilolite-Na, clinoptilolite-K, and gypsum. Mejillonesite forms orthorhombic, prismatic, and elongated thick tabular crystals up to 6 mm long, usually intergrown in radiating aggregates. The dominant form is pinacoid {100}. Prisms {hk0}, {h0l}, and {0kl} are also observed. The crystals are colorless, their streak is white, and the luster is vitreous. The mineral is transparent. It is non-fluorescent under ultraviolet light. Mohs’ hardness is 4, tenacity is brittle. Cleavage is perfect on {100}, good on {010} and {001}, and fracture is stepped. The measured density is 2.36(1) g/cm 3 ; the calculated density is 2.367 g/cm 3 . Mejillonesite is biaxial (-), α = 1.507(2), β = 1.531(2), γ = 1.531(2), 2V (meas) = 15(10)°, 2V (calc) = 0° (589 nm). Orientation is X = a, Z = elongation direction. The mineral is non-pleochroic. Dispersion is r &gt; v, medium. The IR spectrum contains characteristic bands of the Zundel cation (H 5 O 2  + , or H + ·2H 2 O) and the groups P-OH and OH−. The chemical composition is (by EDS, H 2 O by the Alimarin method, wt%): Na 2 O 9.19, MgO 26.82, P 2 O 5  46.87, H 2 O 19, total 101.88. The empirical formula, based on 11 oxygen atoms, is Na 0.93 Mg 2.08 (PO 3 OH) 1.00 (PO 4 ) 1.06 (OH) 0.86  · 0.95H 5 O 2 . The strongest eight X-ray powder-diffraction lines [d in Å(I)(hkl)] are: 8.095(100)(200), 6.846(9) (210), 6.470(8)(111), 3.317(5)(302), 2.959(5)(132), 2.706(12)(113), 2.157(19)(333), and 2.153(9) (622). The crystal structure was solved on a single crystal (R = 0.055) and gave the following data: orthorhombic, Pbca, a = 16.295(1), b = 13.009(2), c = 8.434(1) Å, V = 1787.9(4) Å 3 , Z = 8. The crystal structure of mejillonesite is based on a sheet (parallel to the b-c plane) formed by two types of MgO 6  octahedra, isolated tetrahedra PO 4  and PO 3 OH whose apical vertices have different orientation with respect to the sheet. The sheets are connected by interlayer, 5-coordinated sodium ions, proton hydration complexes, and hydroxyl groups. The structure of mejillonesite is related to that of angarfite, NaFe 5  3+ (PO 4 ) 4 (OH) 4 ·4H 2 O and bakhchisaraitsevite, Na 2 Mg 5` (PO 4 ) 4 ·7H 2 O.

Mejillonesite, a new acid sodium, magnesium phosphate mineral, from Mejillones, Antofagasta, Chile