Synthetic Mica Investigations: II, Role of Fluorides in Mica Batch Reactions

Various fluorides were studied to determine which might be suitable for the synthesis of fluor-phlogopite mica (K2Mg6Al2Si6O20F4). K2SiF6, MgF2, K2AlF5, and K3AlF6 are preferred in that order. KF, KMgF3, AlF3, KAlF4, and MgSiF6· 6H2O are less suitable for the purpose. Solid-state reaction studies were made on the following binary partial systems of the mica batch: MgF2+ MgO, 3MgF2+ Al2O3, 2MgF2+ SiO2, 3MgF2+ KAlSi3O8, K2SiF6+ 3MgO, K2SiF6+ Al2O3, K3AlF6+ 3MgO, and K3AlF6+ 3SiO2. Solid-state reaction studies on various types of fluor-phlogopite batches showed that this mica compound can be synthesized at temperatures as low as 750°C. For best results, however, it should be done at 1000° to 1300°C. in closed containers, using anhydrous batch materials to minimize hydrolysis of the fluorides and loss of HF. Many of the mica batches expand as they react in the solid state, and there is danger in large-scale experiments that the container may break unless heating is rapid between 750° and 1200°C.