Jimthompsonite is a

magnesium Magnesium is a chemical element with the symbol Mg and atomic number 12. It is a shiny gray metal having a low density, low melting point and high chemical reactivity. Like the other alkaline earth metals (group 2 of the periodic ta ...

iron Iron () is a chemical element with symbol Fe (from la, ferrum) and atomic number 26. It is a metal that belongs to the first transition series and group 8 of the periodic table. It is, by mass, the most common element on Earth, right in f ...

silicate mineral with chemical formula . It is a triple chain silicate (or inosilicate) along with clinojimthompsonite and

chesterite Chesterite is a rare silicate mineral that can be compared to amphiboles, micas, and jimthompsonite. Its chemical formula is . Chesterite is named after Chester, Vermont, where it was first described in 1977.Konishi, Hiromi, Reijo, Alviola, and ...

. They were described in 1977 by Burham and Veblen. They attracted great mineralogical attention because they were the first examples of new chain silicate structures among a large group known as biopyriboles whose name is derived from the words

biotite Biotite is a common group of phyllosilicate minerals within the mica group, with the approximate chemical formula . It is primarily a solid-solution series between the iron-endmember annite, and the magnesium-endmember phlogopite; more alumino ...

pyroxene The pyroxenes (commonly abbreviated to ''Px'') are a group of important rock-forming inosilicate minerals found in many igneous and metamorphic rocks. Pyroxenes have the general formula , where X represents calcium (Ca), sodium (Na), iron (Fe II) ...

, and

amphibole Amphibole () is a group of inosilicate minerals, forming prism or needlelike crystals, composed of double chain tetrahedra, linked at the vertices and generally containing ions of iron and/or magnesium in their structures. Its IMA symbol is A ...

s. James B. Thompson, Jr. postulated the existence of the new biopyroles in 1970. The theory proved correct when jimthompsonite, clinojimthompsonite and chesterite were discovered in the Carlton Quarry in

Windsor County Windsor County is a county located in the U.S. state of Vermont. As of the 2020 census, the population was 57,753. The shire town (county seat) is the town of Woodstock. The county's largest municipality is the town of Hartford. History Wind ...

Vermont Vermont () is a state in the northeast New England region of the United States. Vermont is bordered by the states of Massachusetts to the south, New Hampshire to the east, and New York to the west, and the Canadian province of Quebec to ...

in 1977. The new minerals are found intergrown with the amphiboles

anthophyllite Anthophyllite is an orthorhombic amphibole mineral: ☐Mg2Mg5Si8O22(OH)2 (☐ is for a vacancy, a point defect in the crystal structure), magnesium iron inosilicate hydroxide. Anthophyllite is polymorphic with cummingtonite. Some forms of anthoph ...

and

cummingtonite Cummingtonite ( ) is a metamorphic amphibole with the chemical composition , magnesium iron silicate hydroxide. Monoclinic cummingtonite is compositionally similar and polymorphic with orthorhombic anthophyllite, which is a much more common fo ...

in sprays up to 5 cm long. They are all colorless to pale pinkish-brown and transparent. As for pyroxene and amphiboles, the type of chain structure dictates the angle between the two distinctive cleavages. The cleavages of jimthompsonite are at 142 degrees and 38 degrees, and 135 degrees and 45 for chesterite; compared to the cleavage angles of pyroxene at about 94 degrees and 86 degrees and amphibole about 124 and 56 degrees.

Composition

The chemical formula of jimthompsonite is . Veblen and Durham determined the percentages of jimthompsonite as follows: * Magnesium 14.71% Mg 24.40% MgO * Iron 11.27% Fe 14.50% FeO * Silicon 27.20% Si 58.20% SiO₂ * Hydrogen 0.33% H 2.91% H₂O * Oxygen 46.49% O Some of the other characteristics of jimthompsonite are that it has an

orthorhombic In crystallography, the orthorhombic crystal system is one of the 7 crystal systems. Orthorhombic lattices result from stretching a cubic lattice along two of its orthogonal pairs by two different factors, resulting in a rectangular prism with a r ...

crystal system dipyramidal with H-M symbol (2/m 2/m 2/m) and space group: Pbca. It has a perfect cleavage, its colorless to pink brown with a density of 3.03 g/cm³ and hardness of 2-2.5

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 drywall. ...

. It has a white streak and silky-pearly luster.

Origin

It is found in the Carleton talc quarry, near Chester, Windsor County, Vermont, US. The name triple chain

silicate In chemistry, a silicate is any member of a family of polyatomic anions consisting of silicon and oxygen, usually with the general formula , where . The family includes orthosilicate (), metasilicate (), and pyrosilicate (, ). The name is al ...

comes from a wide complex chain structure as

silicon Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic tab ...

being the most dominant element in its composition. It was named for James Burleigh Thompson, Jr. (1921–2011), petrologist of

Harvard University Harvard University is a private Ivy League research university in Cambridge, Massachusetts. Founded in 1636 as Harvard College and named for its first benefactor, the Puritan clergyman John Harvard, it is the oldest institution of higher le ...

Cambridge, Massachusetts Cambridge ( ) is a city in Middlesex County, Massachusetts, United States. As part of the Boston metropolitan area, the cities population of the 2020 U.S. census was 118,403, making it the fourth most populous city in the state, behind Boston, ...

, US.

References

External links

J is for Jimthompsonite
in Geology Word of the Week Inosilicates Orthorhombic minerals Minerals in space group 61

Composition

Origin

References

Further reading

External links