Rapidcreekite

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Rapidcreekite
Rapidcreekite - Rapid Creek, Yukon.jpg
Rapidcreekite from the Rapid Creek area, Yukon, Canada
General
Category Sulfate minerals
Carbonate minerals
Formula
(repeating unit)
Ca2(SO4)(CO3)·4H2O
Strunz classification 7.DG.20[1]
Dana classification 32.2.1.1[1]
Crystal symmetry Space group: Pcnb
Point group: 2/m 2/m 2/m
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
Crystal system Orthorhombic
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[1]
Birefringence δ = 0.015[1]
2V angle 45° (measured)
Dispersion None[1]
Ultraviolet fluorescence Non-fluorescent[2]
Solubility Dissolves slowly in 10% HCl[3]
References [4]

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[edit]

White crystals of rapidcreekite with quartz from the Rapid Creek area

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 millimetres (0.12 in) in length.[2][4]

Structure[edit]

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 weaking 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[edit]

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.[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[edit]

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.[5][8]

References[edit]

  1. ^ a b c d e f g "Rapidcreekite". Mindat. Retrieved August 6, 2012. 
  2. ^ a b c d e Roberts et al. 1986, p. 52
  3. ^ Roberts et al. 1986, p. 53
  4. ^ 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. 
  5. ^ a b Cooper & Hawthorne 1996, p. 99
  6. ^ Cooper & Hawthorne 1996, p. 103
  7. ^ Cooper & Hawthorne 1996, p. 104
  8. ^ a b c d Roberts et al. 1986, p. 51
Bibliography

External links[edit]

Media related to rapidcreekite at Wikimedia Commons