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July-August 2016

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Who's Who in Mineral Names: Alpheus Fuller Williams (1874–1953)

Afwillite, Ca3(SiO3OH)2•2H2O, is a monoclinic hydrous calcium silicate that was named in honor of Alpheus Fuller Williams, who was general manager of De Beers Consolidated Mines, Ltd. in Kimberley, South Africa, from 1905 to 1931. He discovered the mineral at the Dutoitspan diamond mine in Kimberley in 1923 and, recognizing that it appeared to be something unusual, provided samples for analyses. The mineral was found in a dolerite xenolith in the kimberlite, associated with calcite, apophyllite, and natrolite, between the 150-meter and 250-meter levels at the mine. The mineral is brittle, with a hardness of 3–4, is colorless to white with a vitreous luster, and has perfect {101} cleavage. It displays a conchoidal fracture and has a white streak. The original discovery consisted of less than a quarter-kilogram (half-pound) of material composed of easily cleavable crystals, the largest specimen of which is reported to have measured 11 cm (Parry and Wright 1925). However, the British Museum of Natural History in London has a specimen (BM 1925.463) that is almost 16 cm in length, and Williams had a specimen of 24 cm (9.5 inches) illustrated in his 1932 book The Genesis of the Diamond.

Dr. Bruce Cairncross, a consulting editor of Rocks & Minerals, is a professor of geology at the University of Johannesburg.

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