Analysis and chemical composition of larnite-rich ultrarefractory materials

Larnite (b-Ca2SiO4) is a rare, little known compound. However, despite its scarcity, larnite is found in different natural settings, almost always under thermodynamic conditions of around 0.2–1 kbar and 1000–1100 °C. Larnite can also be formed artificially, especially during the synthesis of high technology refractory and ceramic materials, and as a mineral component of some industrial slags and portland cements. This work describes the analysis and compositional properties of larnite-rich ultrarefractory materials, cataloged as possible meteorite specimens, from the collection of the “Museo Nacional de Ciencias Naturales” (Madrid). Larnite is associated with metal oxides and sulfides, native iron and copper, and calcium-aluminium silicates. Bulk chemical composition was determined by XRF (specific standards and analytical routines were designed). Minor and trace elements (including rare earths) were analyzed by the combination of INAA, ICP-MS and ICP-AES. Larnite occurs as imperfectly developed tabular crystals, mainly displaying rhombic shapes of around 25 mm × 35 mm. SEM and microprobe analyses indicate its chemical composition closely matches the theoretical formula (x=Ca1.96Si0.98O4), although significant amounts of Al (Al0.19–Al0.54), Fe (Fe0.01–Fe0.14), Mn (Mn0.01–Mn0.03) and Mg (Mg0.01–Mg0.02) have also been detected in some crystals. PIXE analyses display high Fe and Ba values, ranging from 3.9 to 8.4 wt.%, and from 1058 to 1530 ppm, respectively.