著者

砂川 一郎 堀田 修三 沢田 秀穂

出版者

宝石学会(日本)

雑誌

宝石学会誌 (ISSN:03855090)

巻号頁・発行日

vol.10, no.1, pp.3-35, 1983

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Diamonds have been found occassionally in the tails of dressing of stream tin deposits in Phuke, Phangnga and Takua Pa Areas, South Thailand. From a geological point of view, the occurrence of diamond in Thailand implies unique and important meanings, since in Thailand kimberlites have not been found so far and there is no continental shield (craton) area in which most diamonds and kimberlites of the world have been found. In spite of this importance, no detailed studies on diamond crystals from Thailand have been made so far, although there have been four reports on their occurrences. In the present study, detailed surface microtopographic observations have been made of 15 diamond crystals, which were collected during the past 20 years by a gem dealer in Phangnga at a tin deposit in a small island, Dog Mai, off Phangnga and Phuket, South Thailand. Crystals are colourless, pale yellow, pale green and pale brown in colour, from 0.1 to over 0.7 ct in weight, and are very much rounded, showing rounded octahedral, tetrahedral, hexa-octahedral, dodecahedroidal and elongated dodecahedroidal morphologies (Fig. 3, 4, Table 1). The rounded morphology indicates that the crystals have suffered severe dissolution, most probably during the ascension of crystals in kimberlite magma, not during the transportation on the Earth's surface. Surface microtopographs representing various degrees of dissolution have been observed, which include rounded hexa-octahedral and dod-ecahedroidal faces (Fig. 12-16), circular disk depressions, often superimposed, and similar necklace-like patterns (Fig. 16-19), net work patterns (Fig. 20), all of which appear on rounded surfaces, and trigons appearing on flat {111} faces. Among these, trigons attract a particular attention. On the (111) faces of one of these crystals, all varieties of trigon morphologies are observed (Fig. 23, 25), which include trigons having opposite orientation to the triangle of the octahedral face (negative trigon, NT type), truncated NT (NT' type), hexagon (H type), and those having the same orientation as the triangle of the octahedral face (positive type, PT type), and truncated PT (PT' type). Both point-bottomed (P type) and flat-bottomed (F type) trigons are seen in a ratio of ca. 24 : 1. Trigons so far observed on {111} faces of natural diamond crystals are exclusively NT type and its deviation, and neither PT type trigons nor co-existence of both PT and NT types have been reported. Recent experimental studies have shown that trigons are etch pits and their orientations and morphologies change depending on temperature and oxygen fugacity (Fig. 21). Therefore, the co-existence of both PT and NT types on one octahedral face observed in the present study is very unique and suggests a rather peculiar condition under which diamonds from Thailand have experienced dissolution. The distributions of different types (Table 2), sizes (Table 3, Fig.26), and inclination angles of side faces (Table 4, Fig. 28) of ca. 8000 trigons occurring on flat table area and narrow terrace area on one (111) face are measured. The results are analysed, using a concept of surface reconstruction model proposed for Si etch pits by van Enckevort and Giling (1978) (Fig. 31, 33,34) and phase diagram of orientations of trigons (Fg. 21) by Yamaoka et al. (1980). Based on this analysis, the dissolution history of the crystal is suggested, in which the effect of change of oxygen fugacity is stressed (Fig.35). It is suggested that diamonds in Thailand came from kimberlite magma, whose chemistry was slightly different from that of kimberlites in other localities. This is in harmony with geological peculiarity of diamond occurrence in Thailand.