• Spratt, John

Abstract

<jats:title>Abstract</jats:title><jats:p>Diegogattaite (IMA2012-096), Na<jats:sub>2</jats:sub>CaCu<jats:sub>2</jats:sub>Si<jats:sub>8</jats:sub>O<jats:sub>20</jats:sub>·H<jats:sub>2</jats:sub>O, is a new mineral from the Wessels mine in the Kalahari manganese fields of South Africa. It occurs as a minor phase with other copper-bearing silicates, Cu-rich pectolite,sugilite, quartz, aegirine and undifferentiated Fe-Mn oxides. Diegogattaite is pale turquoise through teal blue. It is found as sub-mm sized grains in a main crystalline patch 3–4 mm in size, and is currently known from only one sample. The mineral is transparent with a vitreous lustreand may have a good cleavage on {001}. It is brittle, with an uneven fracture and a very pale-blue streak. It is non-fluorescent in short- and long-wave UV light and has an estimated Mohs hardness of ∼5–6. Diegogattaite is biaxial (–), α = 1.598(2), β = 1.627(2),γ = 1.632(2); 2V<jats:sub>meas</jats:sub> = 44.0(6)°, 2V<jats:sub>calc</jats:sub> = 44.5°; dispersion: strong <jats:italic>r &lt; v</jats:italic>, orientation: <jats:italic>X = b, Y</jats:italic> ≈ ⊥(001), <jats:italic>Z</jats:italic> ≈ <jats:italic>a</jats:italic>; pleochroism X colourless &lt;&lt; <jats:italic>Y ≈ Z</jats:italic> blue green. The calculated densityis 3.10 g/cm<jats:sup>3</jats:sup>. Electron-microprobe analysis gave: Na<jats:sub>2</jats:sub>O 8.07, CaO 7.3, CuO 20.5, FeO 0.36, SiO<jats:sub>262</jats:sub>.4, H<jats:sub>2</jats:sub>O<jats:sub>(calc)</jats:sub> 2.34, total 100.97 wt.%. A charge-balanced formula on the basis of 21 oxygen a.p.f.u. is: Na<jats:sub>2.00</jats:sub>Ca<jats:sub>1.00</jats:sub>Cu<jats:sub>1.98</jats:sub>Fe<jats:sub>0.04</jats:sub>Si<jats:sub>7.99</jats:sub>H<jats:sub>2</jats:sub>O<jats:sub>21</jats:sub>.Diegogattaite is monoclinic, space group C2/<jats:italic>m</jats:italic>, <jats:italic>a</jats:italic> = 12.2439(6) Å, <jats:italic>b</jats:italic> = 15.7514(4) Å, <jats:italic>c</jats:italic> = 10.6008(3) Å, β = 125.623(2)°, <jats:italic>V</jats:italic> = 1661.87(10) Å<jats:sup>3</jats:sup> and <jats:italic>Z</jats:italic> = 4. The five strongest lines in the X-ray powder pattern are[d<jats:sub>obs</jats:sub> in Å <jats:italic>(I<jats:sub>obs</jats:sub>)(hkl)</jats:italic>]: 4.25(75)(002,<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="png" xlink:href="S0026461X00012329_inline02.png" />22,220), 3.951(77)(040), 3.261(100)(<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="png" xlink:href="S0026461X00012329_inline01.png" />31,<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="png" xlink:href="S0026461X00012329_inline01.png" />13), 2.898(89)(042,<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="png" xlink:href="S0026461X00012329_inline04.png" />03,003), 2.332(66)(331,<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="png" xlink:href="S0026461X00012329_inline04.png" />43,<jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" mime-subtype="png" xlink:href="S0026461X00012329_inline02.png" />62,260,043). The crystal structure of diegogattaite was determined by single-crystal X-ray diffraction to finalagreement indices of <jats:italic>R</jats:italic><jats:sub>1</jats:sub> = 0.027, <jats:italic>wR</jats:italic><jats:sub>2</jats:sub> = 0.071 and GoF = 1.090. It represents a completely new silicate topology based upon a double-sheet of SiO<jats:sub>4</jats:sub> tetrahedra composed of connected 6<jats:sup>4</jats:sup>8<jats:sup>2</jats:sup> cages. The structure of diegogattaiteis related to those of synthetic nanoporous Na-Cu-Si-O-(OH)-H<jats:sub>2</jats:sub>O (CuSH) compounds, which are of interest to the solid-state chemistry community as potential ion-exchangers, catalysts and molecular sieves. The structure of diegogattaite forms a bridge between these structures andthose of the gillespite-group minerals, including wesselsite. The close spatial association of wesselsite and diegogattaite suggests a possible reaction between them that may point to a synthetic route for the production of novel alkaline-earth-based nanoporous copper silicates.</jats:p>

Topics

• impedance spectroscopy
• mineral
• dispersion
• compound
• grain
• phase
• x-ray diffraction
• Oxygen
• hardness
• copper
• Manganese
• space group