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Summary The principal clay mineral in the Stafford tonstein is a mixed-layer mica-montmorillonite, which is comparable on X-ray evidence with potassium bentonites. The restricted heavy mineral suite in the tonstein is also diagnostic of bentonites, and from this it is suggested that heavy minerals may well offer one of the most reliable means of establishing whether or not a tonstein is volcanic in origin. The position of tonsteins in the coal-bearing sequence is discussed, and also the influence position has upon mineralogy and chemistry.
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D. A. Spears (1971) studied this question.
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