• journal_title：The Canadian Mineralogist
• Contributor：Bruce A. Kjarsgaard ; Roger H. Mitchell
• Publisher：Mineralogical Association of Canada
• Date：2008-
• Format：text/html
• Language：en
• Identifier：10.3749/canmin.46.4.981
• journal_abbrev：Can Mineral
• issn：0008-4476
• volume：46
• issue：4
• firstpage：981
• section：Articles

Experimental data are presented for an evaluation of the solubility of tantalum in the ternary system CaCO₃ – Ca(OH)₂ – NaTaO₃ (or calcite – portlandite – sodium tantalate) over the temperature range 550 to 900°C at 0.1 GPa pressure. Near-liquidus phase relationships are given for the pseudobinary join ([CaCO₃]₄₅[Ca(OH)₂]₅₅)_100–x – (NaTaO₃)_x, 30 < x < 70 wt.%. These data show the presence of a large field of calcite plus liquid and the presence of a primary phase field for microlite plus liquid in melts that contain greater than ~55 wt.% NaTaO₃. The primary crystallization fields of calcite and microlite are separated by an exceptionally steep thermal valley located at about 55 wt.% NaTaO₃. Quenched liquids contain calcite, portlandite, Na–Ca carbonates and calcium–tantalum oxide. The maximum solubility of tantalum in this system is estimated to be on the order of ~48 wt.% Ta₂O₅. At higher NaTaO₃ contents, microlite (isostructural with pyrochlore) crystallizes. In contrast, our previous work has demonstrated that lueshite, a perovskite-structured compound, crystallizes in the system CaCO₃ – Ca(OH)₂ – NaNbO₃ at high NaNbO₃ contents. The addition of 1.75 wt.% F to theTa system studied depresses the liquidus by >150°C, and tantalum solubility exceeds 61 wt.% Ta₂O₅. In experiments in the system CaCO₃ – Ca(OH)₂ – NaTaO₃ – NaNbO₃, we found primary niobian microlite to crystallize at high temperature, and tantalian pyrochlore at lower temperature.