Abstract
In order to determine the maximum solubility of strontium in piemontite, hydrothermal syntheses of piemontite in the join Ca2Al2Mn3+Si3O12(OH)-Sr2Al2Mn3+Si3O12(OH) were attempted at pressures of 200-300 MPa, temperatures of 500-600°C, and fO2 defined by Mn2O3-MnO2 and CuO-Cu2O buffers. Ca-piemontite crystallized as almost a single phase from a starting material of Ca2Al2Mn3+Si3O12.5-oxide mixture with excess H2O. Sr-bearing piemontites crystallized from starting materials containing both Ca2Al2Mn3+Si3O12(OH)- and Sr2Al2Mn3+Si3O12(OH)-components. They were accompanied by slawsonite (SrAl2Si2O8), bixbyite (Mn2O3) and other minor phases. Piemontite did not crystallize from Sr2Al2Mn3+Si3O12.5-oxide mixture with excess H2O at 300 MPa and 550°C; slawsonite+SrSiO3+SrSi2O5·H2O+Al2O3+bixbyite assemblage was formed instead.
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 Sr2+ atom per formula unit (O12(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 a- and c-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.
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 Sr2+-ions occupy only the ten-coordinated A(2) site in natural Sr-bearing piemontites.
