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 Yukon (; ; formerly called Yukon Territory and also referred to as the Yukon) is the smallest and westernmost of Canada's three territories. It also is the second-least populated province or territory in Canada, with a population of 43,964 as ...

, Canada, and described in 1986. Rapidcreekite is structurally and compositionally similar to

gypsum Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula . It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, blackboard or sidewalk chalk, and drywal ...

Description

Rapidcreekite is transparent and white to colorless. The mineral occurs as isolated clusters or pervasive crusts of radiating sprays of acicular crystals up to in length.

Structure

In the structure of rapidcreekite, there are two distinct

calcium Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar t ...

sites coordinated by six oxygen

anion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by conve ...

s and two H₂O groups arranged as a

square antiprism In geometry, the square antiprism is the second in an infinite family of antiprisms formed by an even-numbered sequence of triangle sides closed by two polygon caps. It is also known as an ''anticube''. If all its faces are regular, it is a sem ...

. The sulfur site is tetrahedrally coordinated by oxygen anions and the

carbon Carbon () is a chemical element with the symbol C and atomic number 6. It is nonmetallic and tetravalent—its atom making four electrons available to form covalent chemical bonds. It belongs to group 14 of the periodic table. Carbon mak ...

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 . 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. 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.

History

Rapidcreekite was first encountered in 1983 in a tributary of Rapid Creek unofficially known as Crosscut Creek at in northern Yukon. The mineral was named ''rapidcreekite'' for the general area in which it was found. The mineral and name were approved by the IMA Commission on New Minerals and Mineral Names (IMA1984-035). Rapidcreekite was described in 1986 in the journal ''Canadian Mineralogist''. The

holotype specimen A holotype is a single physical example (or illustration) of an organism, known to have been used when the species (or lower-ranked taxon) was formally described. It is either the single such physical example (or illustration) or one of several ...

, which consists of a few grams of rapidcreekite on a

matrix Matrix most commonly refers to: * ''The Matrix'' (franchise), an American media franchise ** ''The Matrix'', a 1999 science-fiction action film ** "The Matrix", a fictional setting, a virtual reality environment, within ''The Matrix'' (franchis ...

, is held in the National Mineral Collection of the

Geological Survey of Canada The Geological Survey of Canada (GSC; french: Commission géologique du Canada (CGC)) is a Canadian federal government agency responsible for performing geological surveys of the country, developing Canada's natural resources and protecting the e ...

in Ottawa. Other specimens are held there and in the National Museum of Natural Sciences in Ottawa.

Occurrence

Rapidcreekite occurs in association with

aragonite Aragonite is a carbonate mineral, one of the three most common naturally occurring crystal forms of calcium carbonate, (the other forms being the minerals calcite and vaterite). It is formed by biological and physical processes, including pre ...

, gypsum, and kulanite. It has been found in Canada, Germany, Norway, and Romania. The rare mineral occurs as a secondary phase that formed along the surfaces of

joints A joint or articulation (or articular surface) is the connection made between bones, ossicles, or other hard structures in the body which link an animal's skeletal system into a functional whole.Saladin, Ken. Anatomy & Physiology. 7th ed. McGraw- ...

and bedding planes in a quartz-rich sideritic formation. In the Rapid Creek area, the formation occurred in

Albian The Albian is both an age of the geologic timescale and a stage in the stratigraphic column. It is the youngest or uppermost subdivision of the Early/Lower Cretaceous Epoch/ Series. Its approximate time range is 113.0 ± 1.0 Ma to 100.5 ± 0 ...

ironstone Ironstone is a sedimentary rock, either deposited directly as a ferruginous sediment or created by chemical replacement, that contains a substantial proportion of an iron ore compound from which iron (Fe) can be smelted commercially. Not to be con ...

s and shales.

References

;Bibliography * *

External links

{{commonscat-inline, Rapidcreekite, lcfirst=yes Orthorhombic minerals Minerals in space group 60 Calcium minerals Sulfate minerals Carbonate minerals Hydrates