Meteoritos versus rocas terrestres; el seudometeorito de Getafe

Abstract

Larnite ( b-Ca2SiO4) is a very rare and little known compound in nature. It forms part --along with forsterite (Mg2SiO4), fayalite (Fe2SiO4), and tephroite (Mn2SiO4)--, of the monticellite and knebelite series (general group of olivine). However, despite its scarcity, larnite has been found in different natural settings, almost always under thermodynamic conditions of around 0.2-1 kbar and 1,000 to 1,100C. Larnite can also be artificially formed, especially during the synthesis of high technology refractory and ceramics materials, and as a mineral component of some industrial slags. Typical portland cements are mixtures of tricalcium silicate (3CaO SiO2), tricalcium aluminate (3CaO Al2O3), and dicalcium silicate (2CaO SiO2), in varying proportions, together with small amounts of magnesium and iron compounds. This work displays the analysis and compositional properties of larnite-rich ultrarefractory materials, cataloged as possible meteorite specimens (Getafe rock, Fig. 1), 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 (Fig. 2). Bulk chemical composition was determined by XRF (specific standards and analytical routines were designed, Table 1 and Fig. 3). Minor and trace elements (including rare earths) were analyzed by the combination of NAA, ICP-MS and ICP-AES. These ultrarefractory materials are rich in iron and calcium (Table 2). Larnite occurs as imperfectly developed tabular crystals, mainly displaying rhombic shapes of around 25 x 35 mm. SEM and microprobe analyses (Fig. 4) indicate that its chemical composition closely matches the theoretical formula (x = Ca1.96 Si0.98 O4), 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 Wt % to 8.4 Wt %, and from 1058 ppm to 1530 ppm, respectively.