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Rapidcreekite

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Rapidcreekite
Rapidcreekite from the Rapid Creek area, Yukon, Canada
General
CategorySulfate minerals
Carbonate minerals
Formula
(repeating unit)
Ca2(SO4)(CO3)·4H2O
IMA symbolRck[1]
Strunz classification7.DG.20[2]
Dana classification32.2.1.1[2]
Crystal systemOrthorhombic
Crystal classDipyramidal (mmm)
H-M symbol: (2/m 2/m 2/m)
Space groupPcnb
Unit cella = 15.517(2) Å
b = 19.226(3) Å
c = 6.1646(8) Å; Z = 8
Identification
ColorWhite to colorless
Crystal habitElongated, flattened to acicular crystals in radiating sprays or crust forming
CleavagePerfect on {010}
Good on {100}
FractureSplintery
TenacityBrittle
Mohs scale hardness2
LusterVitreous
StreakWhite
DiaphaneityTransparent
Specific gravity2.239 (calculated)
Optical propertiesBiaxial (+)
Refractive indexnα = 1.516
nβ = 1.518
nγ = 1.531[2]
Birefringenceδ = 0.015[2]
2V angle45° (measured)
DispersionNone[2]
Ultraviolet fluorescenceNon-fluorescent[3]
SolubilityDissolves slowly in 10% HCl[4]
References[5]

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

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White crystals of rapidcreekite with quartz from the Rapid Creek area

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]

Structure

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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]

History

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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 / 68.56250°N 136.79167°W / 68.56250; -136.79167 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]

Occurrence

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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]

References

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  1. ^ Warr, L.N. (2021). "IMA–CNMNC approved mineral symbols". Mineralogical Magazine. 85 (3): 291–320. Bibcode:2021MinM...85..291W. doi:10.1180/mgm.2021.43. S2CID 235729616.
  2. ^ a b c d e f g "Rapidcreekite". Mindat. Retrieved August 6, 2012.
  3. ^ a b c d e Roberts et al. 1986, p. 52
  4. ^ Roberts et al. 1986, p. 53
  5. ^ a b c d e Anthony, John W.; Bideaux, Richard A.; Bladh, Kenneth W.; Nichols, Monte C. (eds.). "Rapidcreekite" (PDF). Handbook of Mineralogy. Chantilly, VA: Mineralogical Society of America.
  6. ^ a b Cooper & Hawthorne 1996, p. 99
  7. ^ Cooper & Hawthorne 1996, p. 103
  8. ^ Cooper & Hawthorne 1996, p. 104
  9. ^ a b c d Roberts et al. 1986, p. 51
Bibliography
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Media related to rapidcreekite at Wikimedia Commons