What does the cryptic crossword clue "Mineral primarily a compound of 13, reduced by measure unknown (6)" mean?

The definition is "Mineral". The wordplay uses a charade (building blocks). QUART (synonym="measure") + Z (abbreviation="unknown") = QUARTZ; definition: "Mineral" The answer is QUARTZ.

Mineral primarily a compound of 13, reduced by measure unknown (6)

Guardian Cryptic #27904 | 19 Down | 2019-08-20 flower

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