The partial dehydration of gypsum to the hemihydrate with the loss of one and a half of the two water molecules of gypsum has been studied by many investigators, a recent and definitive paper being that by E. Posnjak. As the transformation of the gypsum to the hemihydrate, and the hydration of the latter back to gypsum, is what occurs in the manufacture and use of plaster-of-Paris, the subject is of great practical importance, and the hemihydrate itself, plaster-of-Paris, is a commodity produced and used in enormous quantities. Also as gypsum and anhydrite are among the commonest of minerals, and since the artificial intermediate hemihydrate is an easily prepared every day article of commerce, it would seem reasonable to suppose that naturally-occurring hemihydrate should be listed as a known mineral. This would seem even more likely, when the well-known partial dehydration of gypsum on simple grinding, or on gentle warming, is recalled. Yet, the hemihydrate is not mentioned in the standard reference works of Dana-Ford, Winchell, or Larsen-Berman.

This content is PDF only. Please click on the PDF icon to access.

First Page Preview

First page PDF preview
You do not have access to this content, please speak to your institutional administrator if you feel you should have access.