Rapidcreekite is a rare mineral with formula Ca2(SO4)(CO3)·4H2O. The mineral is white to colorless and occurs as groupings of acicular (needle-shaped) crystals. It was discovered in 1983 in northern Yukon, Canada, and described in 1986. Rapidcreekite is structurally and compositionally similar to gypsum.
Description
Rapidcreekite is transparent and white to colorless.[4] The mineral occurs as isolated clusters or pervasive crusts of radiating sprays of acicular crystals up to 3 mm (0.12 in) in length.[2][4]
Structure
In the structure of rapidcreekite, there are two distinct calcium sites coordinated by six oxygen anions and two H2O groups arranged as a square antiprism. The sulfur site is tetrahedrally coordinated by oxygen anions and the carbon site is coordinated by a triangle of oxygen anions. The structural unit of rapidcreekite is a sheet that consists of edge-sharing CaΦ8 polyhedra (Φ, unspecified species: O or OH) cross-linked by carbonate and sulfate groups. Sheets are held together by weakening hydrogen bonds, accounting for the perfect cleavage along {100}.[5]
The structure and composition of rapidcreekite is similar to that of gypsum. If half of the sulfate groups in gypsum were replaced by carbonate, the formula of rapidcreekite is obtained.[6] If gypsum were transformed by twinning along alternate rows of sulfate groups and the resultant triangles of oxygen along the boundary occupied by carbon, the structure of rapidcreekite would result.[7]
History
Rapidcreekite was first encountered in 1983 in a tributary of Rapid Creek unofficially known as Crosscut Creek at 68°33′45″N 136°47′30″W in northern Yukon.[8] The mineral was named rapidcreekite for the general area in which it was found.[2] The mineral and name were approved by the IMA Commission on New Minerals and Mineral Names (IMA1984-035).[1][2] Rapidcreekite was described in 1986 in the journal Canadian Mineralogist.[8]
The holotype specimen, which consists of a few grams of rapidcreekite on a matrix, is held in the National Mineral Collection of the Geological Survey of Canada in Ottawa. Other specimens are held there and in the National Museum of Natural Sciences in Ottawa.[2]
Occurrence
Rapidcreekite occurs in association with aragonite, gypsum, and kulanite.[4] It has been found in Canada, Germany, Norway, and Romania.[1] The rare mineral occurs as a secondary phase that formed along the surfaces of joints and bedding planes in a quartz-rich sideritic formation.[4][8] In the Rapid Creek area, the formation occurred in Albian ironstones and shales.
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