Alstonite

• Bariumaragonit

Alstonite is a rarely occurring mineral from the mineral class of " carbonates and nitrates ." It crystallizes in the triclinic crystal system with the chemical composition Baca [ CO3 ] 2, is therefore a barium-calcium - carbonate.

Alstonite occurs mainly in the form of pseudo- orthorhombic and pseudohexagonal, dipyramidaler crystal twins. These are usually recognizable by striped crystal faces perpendicular to the pseudo-hexagonal c- axis due to the formation of medial repetition twins in this direction. In its pure form Alstonite is colorless and transparent glass with glass-like shine on the crystal faces. But it may also appear white and accept by foreign admixtures a gray, beige (cream ) or light pink color by multiple refraction due to lattice defects or polycrystalline training, the transparency decreases accordingly.

With a Mohs hardness of 4 to 4.5 corresponds approximately to the reference Alstonite mineral fluorite, can not, however, quite so easily scratched with a pocket knife.

Special Features

Under UV light, show some Alstonite yellow fluorescence, similar to that of neon-colored highlighters.

Etymology and history

Alstonite was first discovered in the " Brownley Hill Mine " at Nenthead ( Alston Moor district ) in the county of Cumbria and in the " Fallowfield Mine" at Acomb in the county of Northumberland in England part of the country of the United Kingdom (UK). The mineral in 1841 was described by August Breithaupt, who named it after his first type locality Alston Moor.

Alstonite was mostly recognized as a separate mineral even before the establishment of the International Mineralogical Association (IMA ), which was adopted without examination by the Commission CNMNC (G = Grandfathered ). Due to its chemical composition is Alstonite depending on the source but also occasionally as a solid solution between aragonite (Ca [ CO3 ] ) and Whiterit (Ba [ CO3 ] ) or as AragonitVarietät ( Bariumaragonit with up to 50 % BaCO3 )

Type material of the mineral is among others in the Freiberg in Saxony, Germany kept ( Register-Nr. 15818 ).

Classification

In the outdated, but partly still in use 8th edition of the mineral classification by Strunz the Alstonite belonged to common mineral class of " carbonates, nitrates and borates " and then to the Department of " water clear carbonate [ CO3 ] 2 - without foreign anions ", where he with aragonite, Barytocalcit, Cerussite, Olekminskit, Paralstonit, strontianite and witherite the " aragonite group " with the system no. V/B.04 formed.

The 9th edition used since 2001 and valid by the International Mineralogical Association (IMA ) of the Strunz'schen Mineral classification assigns the Alstonite in the newly defined class of " carbonates and nitrates " and there is also no in the department of " Carbonates without additional anions, H2O " one. However, this is further subdivided according to the affiliation of the cations 5.AB. to specific element families, so that the mineral is found according to its composition in the subsection " alkaline-earth (and other M2 ) carbonates ", where it is the only member of the unnamed group 35 forms.

The mostly commonly used in English-speaking classification of minerals according to Dana assigns the Alstonite as the obsolete, Strunz'sche systematics in the common class of " carbonates, nitrates and borates " and there in the department of " Anhydrous carbonates " one. Here he can be found as the only member of the unnamed group 14:02:05 within the subdivision " Anhydrous carbonates having the formula A B2 ( CO3) 2 ".

Modifications and varieties

The connection Baca [ CO3 ] 2 is trimorph and occurs in nature in addition to the triclinic crystallizing Alstonite still as monoclinic Barytocalcit crystallizing and crystallizing as trigonal Paralstonit ago.

Education and Locations

Alstonite be hydrothermally formed at rather low temperatures in lead - zinc deposits, rarely in carbonatites. Accompanying minerals among other ankerite, barite, Benstonit, calcite, galena, pyrite, quartz, siderite and sphalerite may occur.

A rare mineral formation Alstonite could be detected only in a few localities, where so far (as of 2013) are about 20 localities known as. In addition to its type localities " Brownley Hill Mine " in the circle Alston Moor ( Cumbria ) and " Fallowfield Mine" at Acomb (Durham ) entered the mineral in the UK still in the coal mine " New Brancepeth " in the Deerness Valley in the English county of County Durham and in the quarry " Dolyhir " at Wethel in Powys and Rhondda Cynon Taf area in Llantrisant in Wales.

In Germany you know Alstonite so far only from the mining area Neubulach (Black Forest) in Baden -Württemberg and the only known locality in Austria is Arzberg in Styria.

Other known localities include

• The ore deposit Rosebery on the Australian island of Tasmania
• The phosphate quarry near Anitápolis in the Brazilian state of Santa Catarina
• The mining area Kuang Bayan ( Bayan Obo ) in Inner Mongolia Autonomous Region of the People's Republic of China
• The carbonatite complex " Qaqqaarsuk " ( Qaqarssuk ) near the village of Maniitsoq in West Greenland
• The zinc-lead - silver deposit near the mining town of Rosh Pinah in Namibia
• The Chibinen and Vuorijärvi Massif ( Vuoriyarvi ) on the Russian Kola Peninsula
• The municipality Cave -in- Rock in the Hardin County ( Illinois) and the Green River Formation in Sweetwater County ( Wyoming) in the United States (USA)

Crystal structure

Alstonite crystallized in the triclinic space group C1 or C1 ( Raumgruppen-Nr. 1 or 2) with the lattice parameters a = 30.14 Å; b = 17.40 Å; c = 6.12 Å; α = 90 °; β = 90 ° and γ = 90 ° and 24 formula units per unit cell.