31.
Feldspar is described as an incongruently melting mineral, which means it
Answer: C
Correct answer: melts to a liquid plus a solid phase together
Heated past about 1150 degrees, potash feldspar does not melt cleanly: it decomposes to a potassium aluminosilicate liquid together with solid leucite. That behaviour is what makes it a useful body flux rather than a glass - the melt appears gradually over a range of temperature instead of all at once, which is precisely what gives a feldspathic body any firing range at all.
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