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(1) The writer studied the mineralogical properties of opal and opal-like mineral found in some clays and altered volcanic rocks in Japan and found that some of these minerals show the mineralogical properties of cristobalite., (2) From this, it is reasonably infered that in some cases there is an intimate relation between opal and cristobalite and that cristobalite is formed as a secondary mineral
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Toshio Sudô (1950) studied this question.
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