A New Occurrence of Well-Crystallized Sepiolite from the Kuzu District, Tochigi Prefecture, Central Japan

Abstract

A new occurrence of white clay-vein consisting almost entirely of well-crystallized sepiolite along the fault running through the Upper Limestone Member of the Nabeyama Formation in the Kuzu district is shortly reported. Field observation and its mineralogical studies led to the following results;
1) The vein runs N80°E, with dip 40°NW, and the width usually ranges from 6cm to 10cm, but the maximum attains 25cm. The contact between the vein and the wallrock is sharply defined. And it often contains lenticular masses of microcrystalline quartz.
2) The result of the chemical analysis of minus 2 microns fraction of the present material is generally in good agreement with the pablished data of common a-sepiolite as listed in Table 1. The structural formula of “dehydrated sepiolite” calculated from the result on abasis of Brauner-Preisinger model was as follows:
(Al_0.06Fe³⁺_0.01Mg_7.89) _7.96 (Si_11.79Al_0.21) ₁₂O₃₂·Ca_0.21.
3) X-ray diffraction patterns of the present material unheated and heated at various temperatures as shown in Fig. 5 clearly indicated that the material is a well-crystallized sepiolite.
4) Thermograph of the present sepiolite clearly showed four steps of dehydration, in the temperature ranges of <250°C, 250°-450°C, 450°C-610°C and 730°-860°C. The total weight loss of the second and third steps is nearly twice as much as the final weight loss.
5) Chemical and thermal data of the present sepiolite as noted above seem to support the Brauner-Preisinger model rather than the Nagy-Bradley one as respects the structure of sepiolite.
6) The electron micrograph of the present sepiolite showed that it occurs in single lath and bundles of laths. The laths attain a maximum length of about 5 microns.
7) The mode of occurrence of the vein as well as the mineralogical properties of the sepiolite suggests two possible geneses. Namely, the one is supergene origin, the other is hypogene.