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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.
| Rapidcreekite | |
|---|---|
 Rapidcreekite from the Rapid Creek area, Yukon, Canada | |
| General | |
| Category | Sulfate minerals Carbonate minerals |
| Formula (repeating unit) | Ca2(SO4)(CO3)·4H2O |
| IMA symbol | Rck[1] |
| Strunz classification | 7.DG.20[2] |
| Dana classification | 32.2.1.1[2] |
| Crystal system | Orthorhombic |
| Crystal class | Dipyramidal (mmm) H-M symbol: (2/m 2/m 2/m) |
| Space group | Pcnb |
| Unit cell | a = 15.517(2) Å b = 19.226(3) Å c = 6.1646(8) Å; Z = 8 |
| Identification | |
| Color | White to colorless |
| Crystal habit | Elongated, flattened to acicular crystals in radiating sprays or crust forming |
| Cleavage | Perfect on {010} Good on {100} |
| Fracture | Splintery |
| Tenacity | Brittle |
| Mohs scale hardness | 2 |
| Luster | Vitreous |
| Streak | White |
| Diaphaneity | Transparent |
| Specific gravity | 2.239 (calculated) |
| Optical properties | Biaxial (+) |
| Refractive index | nα = 1.516 nβ = 1.518 nγ = 1.531[2] |
| Birefringence | δ = 0.015[2] |
| 2V angle | 45° (measured) |
| Dispersion | None[2] |
| Ultraviolet fluorescence | Non-fluorescent[3] |
| Solubility | Dissolves slowly in 10% HCl[4] |
| References | [5] |
Rapidcreekite is transparent and white to colorless.[5] The mineral occurs as isolated clusters or pervasive crusts of radiating sprays of acicular crystals up to 3 mm (0.12 in) in length.[3][5]
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}.[6]
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.[7] 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.[8]
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.[9] The mineral was named rapidcreekite for the general area in which it was found.[3] The mineral and name were approved by the IMA Commission on New Minerals and Mineral Names (IMA1984-035).[2][3] Rapidcreekite was described in 1986 in the journal Canadian Mineralogist.[9]
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.[3]
Rapidcreekite occurs in association with aragonite, gypsum, and kulanite.[5] It has been found in Canada, Germany, Norway, and Romania.[2] The rare mineral occurs as a secondary phase that formed along the surfaces of joints and bedding planes in a quartz-rich sideritic formation.[5][9] In the Rapid Creek area, the formation occurred in Albian ironstones and shales.[6][9]
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