What is a black or greenish black mineral occurring in foliated flates, also in velvety bronze-colored incrustations. It is a hydrous silicate of iron and alumina called?

stilpnomelane — A black or greenish black mineral occurring in foliated flates, also in velvety bronze-colored incrustations. It is a hydrous silicate of iron and alumina. (Webster's 1913.)

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