Reference:
▪ Atencio, D., Chukanov, N.V., Nestola, F., Witzke, T., Coutinho, J.M.V., Zadov, A.E., Contreira Filho, R.R., Farber, G. (2012): Mejillonesite, a new acid sodium, magnesium phosphate mineral, from Mejillones, Antofagasta, Chile. American Mineralogist, 97, 19-25.

Abstract:
Mejillonesite, ideally NaMg2(PO3OH)(PO4)(OH)·H5O2, 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/cm3; the calculated density is 2.367 g/cm3. 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 > v, medium. The IR spectrum contains characteristic bands of the Zundel cation (H5O2+, or H+·2H2O) and the groups P-OH and OH−. The chemical composition is (by EDS, H2O by the Alimarin method, wt%): Na2O 9.19, MgO 26.82, P2O5 46.87, H2O 19, total 101.88. The empirical formula, based on 11 oxygen atoms, is Na0.93Mg2.08(PO3OH)1.00(PO4)1.06(OH)0.86 ·0.95H5O2. 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) Å3, Z = 8. The crystal structure of mejillonesite is based on a sheet (parallel to the b-c plane) formed by two types of MgO6 octahedra, isolated tetrahedra PO4 and PO3OH 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, NaFe53+(PO4)4(OH)4·4H2O and bakhchisaraitsevite, Na2Mg5(PO4)4·7H2O.