Maximum strontium content of piemontite formed by hydrothermal synthesis





In order to determine the maximum solubility of strontium in piemontite, hydrothermal syntheses of piemontite in the join Ca<sub>2</sub>Al<sub>2</sub>Mn<sup>3+</sup>Si<sub>3</sub>O<sub>12</sub>(OH)-Sr<sub>2</sub>Al<sub>2</sub>Mn<sup>3+</sup>Si<sub>3</sub>O<sub>12</sub>(OH) were attempted at pressures of 200-300 MPa, temperatures of 500-600°C, and <i>f</i><sub>O<sub>2</sub></sub> defined by Mn<sub>2</sub>O<sub>3</sub>-MnO<sub>2</sub> and CuO-Cu<sub>2</sub>O buffers. Ca-piemontite crystallized as almost a single phase from a starting material of Ca<sub>2</sub>Al<sub>2</sub>Mn<sup>3+</sup>Si<sub>3</sub>O<sub>12.5</sub>-oxide mixture with excess H<sub>2</sub>O. Sr-bearing piemontites crystallized from starting materials containing both Ca<sub>2</sub>Al<sub>2</sub>Mn<sup>3</sup><sup>+</sup>Si<sub>3</sub>O<sub>12</sub>(OH)- and Sr<sub>2</sub>Al<sub>2</sub>Mn<sup>3+</sup>Si<sub>3</sub>O<sub>12</sub>(OH)-components. They were accompanied by slawsonite (SrAl<sub>2</sub>Si<sub>2</sub>O<sub>8</sub>), bixbyite (Mn<sub>2</sub>O<sub>3</sub>) and other minor phases. Piemontite did not crystallize from Sr<sub>2</sub>Al<sub>2</sub>Mn<sup>3+</sup>Si<sub>3</sub>O<sub>12.5</sub>-oxide mixture with excess H<sub>2</sub>O at 300 MPa and 550°C; slawsonite+SrSiO<sub>3</sub>+SrSi<sub>2</sub>O<sub>5</sub>·H<sub>2</sub>O+Al<sub>2</sub>O<sub>3</sub>+bixbyite assemblage was formed instead.<br>    Synthesized piemontites had variable Sr contents between grains even in the same run product. But average Sr content of the synthesized piemontite increases with increasing Sr content in the starting material from Ca : Sr=2 : 0 to Ca : Sr=1 : 1. The Sr content of piemontite crystallized from starting material with Ca : Sr=1 : 1 reaches 19.65 wt% SrO, which corresponds to about one Sr<sup>2+</sup> atom per formula unit (O<sub>12</sub>(OH)), but piemontite containing more than one Sr atom per formula unit did not crystallize from starting material with Ca : Sr=0.8 : 1.2. The <i>a</i>- and <i>c</i>-dimensions and cell volume increase with increasing Sr content of the starting material from Ca : Sr=2 : 0 to 1 : 1, and reach the upper limit at Ca : Sr=1 : 1. These changes in cell parameters and chemical compositions of the synthesized Sr-bearing piemontite indicate that Sr substitutes for Ca up to one atom per formula unit.<br>    The results show that the maximum content of Sr in piemontite is limited to one atom per formula unit, and are in agreement with the published view that Sr<sup>2+</sup>-ions occupy only the ten-coordinated A(2) site in natural Sr-bearing piemontites.


  • Journal of mineralogical and petrological sciences

    Journal of mineralogical and petrological sciences 95(5), 84-94, 2000-06


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