抄録
Origin of tosudite is controversy and two hypotheses have been postulated including the diagenesis of precursor materials or the direct precipitation from hydrothermal solutions. The newly-discovered dioctahedral Li-tosudite from the kaolin deposits of north-west of Gonabad, Mashhad, Iran was studied in the current paper to examine its origin on the light of these hypotheses. The studied tosudite occurs as soft, white to grayish white and fine-grained lenses ranged in diameter from 30 cm up to 2 m in diameter. The lenses are composed of thin uniform flakes (up to 5 μm in size) with very fine and rounded anatase crystals between flakes. In the XRD pattern, the characteristic basal spacing of 27.6Å for the air-dried mineral was changed to 30.1Å when glycolated and to 22.5Å upon heating. Infrared and differential thermal analyses confirmed the occurrence of tosudite and suggested that it is Li-tosudite. Chemical analysis indicated that the studied tosudite is composed mainly of Al_2O_3 and SiO_2 with relatively high TiO_2 content (0.9%) due to the presence of anatase. The high Li_2O content (1.4%) confirmed the Li nature of the tosudite. Ba, Pb, V, and Zr were detected in relatively high concentrations compared to other trace elements. Chondrite-normalized REEs patterns exhibited LREE enrichment relative to HREE as shown by (La/Yb)_N ratio of 35 and negative Eu anomaly (Eu/Eu* = 0.57). The occurrence of tosudite as uniform and homogeneous lenses as well as the absence of any preexisting minerals suggests the authigenic origin of the studied tosudite as direct precipitation from hydrothermal solutions that was previously reported in association with the magmatic activities of the studied locality. The relative enrichment of LREE relative to HREE and the Eu anomaly suggests an acidic igneous rock as a possible source of the studied tosudite.
