• journal_title：American Mineralogist
• Contributor：Daniel Atencio ; Nikita V. Chukanov ; Fabrizio Nestola ; Thomas Witzke ; José M.V. Coutinho ; Aleksandr E. Zadov ; Reynaldo R. Contreira Filho ; Gunnar Färber
• Publisher：Mineralogical Society of America
• Date：2012-
• Format：text/html
• Language：en
• Identifier：10.2138/am.2012.3867
• journal_abbrev：American Mineralogist
• issn：0003-004X
• volume：97
• issue：1
• firstpage：19
• section：Articles

Mejillonesite, ideally NaMg<sub>2sub>(PO<sub>3sub>OH)(PO<sub>4sub>)(OH)·H<sub>5sub>O<sub>2sub>, 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<sup>3sup>; the calculated density is 2.367 g/cm<sup>3sup>. 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 = <strong>astrong>, Z = elongation direction. The mineral is non-pleochroic. Dispersion is r > v, medium. The IR spectrum contains characteristic bands of the Zundel cation (H<sub>5sub>O<sub>2sub><sup>+sup>, or H<sup>+sup>·2H<sub>2sub>O) and the groups P-OH and OH<sup>−sup>. The chemical composition is (by EDS, H<sub>2sub>O by the Alimarin method, wt%): Na<sub>2sub>O 9.19, MgO 26.82, P<sub>2sub>O<sub>5sub> 46.87, H<sub>2sub>O 19, total 101.88. The empirical formula, based on 11 oxygen atoms, is Na<sub>0.93sub>Mg<sub>2.08sub>(PO<sub>3sub>OH)<sub>1.00sub>(PO<sub>4sub>)<sub>1.06sub>(OH)<sub>0.86sub> ·0.95H<sub>5sub>O<sub>2sub>. 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<sup>3sup>, Z = 8. The crystal structure of mejillonesite is based on a sheet (parallel to the <strong>b-cstrong> plane) formed by two types of MgO<sub>6sub> octahedra, isolated tetrahedra PO<sub>4sub> and PO<sub>3sub>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<sub>5sub><sup>3+sup>(PO<sub>4sub>)<sub>4sub>(OH)<sub>4sub>·4H<sub>2sub>O and bakhchisaraitsevite, Na<sub>2sub>Mg<sub>5sub>(PO<sub>4sub>)<sub>4sub>·7H<sub>2sub>O.