One of the most illustrative cases of isomorphism in systematic mineralogy is furnished by the jarosite division of the alunite-beudantite group. The original jarosite, known since 1838, is a hydrous sulphate of ferric iron and potassium. Sixty-four years later Hillebrand and Penfield described two new jarosites in one of which the potassium was replaced by sodium while the other contained, in place of the alkali metal, lead. The latter mineral has been found in a number of places since and in some localities is an important constituent of ores. Still more recently Schaller has called attention to a very interesting member of the group in which silver enters as an essential constituent. The purpose of the present short paper is to describe briefly a fifth jarosite in which the essential alkali is ammonium oxide. The present members of the jarosite group are as follows.

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