Alstonite

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Alstonite
Alstonite-Witherite-oldeuro-107a.jpg
Alstonite (colorless, pyramidal crystals) and Witherite (gray, spherical aggregate) from the "Brownley Hill Mine", Alston Moor District, Northern Pennines , England (size: 9.7 × 4.1 × 3.7 cm)
General and classification
other names

Barium aragonite

chemical formula BaCa [CO 3 ] 2
Mineral class
(and possibly department) 		Carbonates and nitrates (formerly carbonates, nitrates and borates)
System no. to Strunz
and to Dana 		5.AB.35 ( 8th edition : V / B.04)
02/14/05/01
Similar minerals Aragonite , witherite
Crystallographic Data
Crystal system triclinic
Crystal class ; symbol triclinic pedial 1 or -pinacoidal 1
Room group (no.) C 1 or C 1 (No. 1 or 2)
Lattice parameters a  = 30.14  Å ; b  = 17.40 Å; c  = 6.12 Å
α  = 90 °; β  = 90 °; γ  = 90 °
Formula units Z  = 24
Twinning according to {1 1 0} and {310}
Physical Properties
Mohs hardness 4 to 4.5
Density (g / cm 3 ) measured: 3.707; calculated: 3.69
Cleavage imperfect after {110}
Break ; Tenacity shell-like to uneven
colour colorless, white, gray, cream, light pink
Line color White
transparency transparent to translucent
shine Glass gloss
Crystal optics
Refractive indices n α  = 1.526
n β  = 1.671
n γ  = 1.672
Birefringence δ = 0.146
Optical character biaxial negative
Axis angle 2V = measured: 6 °; calculated: 8 °
Other properties
Special features yellow fluorescence

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 [CO 3 ] 2 , so it is a barium - calcium carbonate.

Alstonite occurs predominantly in the form of pseudo-orthorhombic and pseudohexagonal, dipyramidal crystal twins . These are usually recognizable by striped crystal faces at right angles to the pseudo-hexagonal c-axis due to the formation of medial repeat twins in this direction. In its pure form, Alstonite is colorless and transparent with a glass-like gloss on the crystal surfaces . However, due to multiple refraction due to lattice construction defects or polycrystalline formation, it can also appear white and, due to foreign admixtures, take on a gray, beige (cream-colored) or light pink color, with the transparency decreasing accordingly.

With a Mohs hardness of 4 to 4.5, alstonite roughly corresponds to the reference mineral fluorite , but it is not that easy to scratch with a pocket knife .

Etymology and history

Pseudohexagonal-dipyramidal alstonite on barite from the type locality "Brownley Hill Mine", Alston Moor, England ( overall size : 4.3 × 3.4 × 3.0 cm)

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

Alstonite was already predominantly recognized as an independent mineral before the founding of the International Mineralogical Association (IMA), which was adopted by its CNMNC committee without examination (G =  grandfathered ). Due to its chemical composition, depending on the source, alstonite is sometimes also used as a mixed crystal between aragonite (Ca [CO 3 ]) and witherite (Ba [CO 3 ]) or as an aragonite variety ( barium aragonite with up to 50% BaCO 3 )

Type material of the mineral is stored in the Freiberg Mining Academy in Saxony, Germany (register no. 15818).

classification

In the outdated, but partly still in use 8th edition of the mineral systematics according to Strunz , the alstonite belonged to the common mineral class of "carbonates, nitrates and borates" and there to the department of "anhydrous carbonates [CO 3 ] 2− without foreign anions ", where it together with aragonite , barytocalcite , cerussite , olekminsite , paralstonite , strontianite and withitherite the "aragonite group" with the system no. V / B.04 formed.

The 9th edition of Strunz's mineral systematics , which has been in effect since 2001 and is used by the International Mineralogical Association (IMA), assigns alstonite to the newly defined class of “carbonates and nitrates” and there also to the section of “carbonates without additional anions; without H 2 O “. However, this is further subdivided according to the membership of the cations in certain element families, so that the mineral can be found according to its composition in the sub-section "alkaline earth (and other M 2+ ) carbonates", where it is the only member of the unnamed group 5. AB.35 forms.

The systematics of minerals according to Dana , which is mainly used in the English-speaking world , assigns the alstonite to the common class of "carbonates, nitrates and borates" and there in the department of "anhydrous carbonates" like the outdated Strunzian system. Here he is to be found as the only member in the unnamed group 02/14/05 within the sub-section "Anhydrous carbonates with the formula A + B 2+ (CO 3 ) 2 ".

Crystal structure

Alstonite crystallizes triclinically in the space group C 1 or C 1 (space group no. 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 ° as well as 24 formula units per unit cell .

properties

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

Modifications and varieties

The compound BaCa [CO 3 ] 2 is trimorphic and occurs in nature in addition to the triclinic crystallizing alstonite as monoclinic crystallizing barytocalcite and as trigonal crystallizing paralstonite .

Education and Locations

Alstonite from Cumberland, Cumbria, England (size: 5.1 × 4.1 × 2.7 cm)

Alstonit formed hydrothermally at rather low temperatures in lead - zinc - deposits rare in carbonatites . As accompanying minerals may include Ankerit , barite , Benstonit , calcite , galena , pyrite , quartz , siderite and sphalerite occur.

As a rare mineral formation, Alstonite could only be proven at a few sites, whereby so far (as of 2013) around 20 sites are known. In addition to its type localities "Brownley Hill Mine" in the district of Alston Moor (Cumbria) and "Fallowfield Mine" near Acomb (Durham), the mineral occurred in the United Kingdom in the colliery "New Brancepeth" in the Deerness Valley in the English county of County Durham as well in the "Dolyhir" quarry at Wethel in the Powys area and at Llantrisant in the Rhondda Cynon Taf area in Wales.

In Germany, alstonite is only known from the mining area of Neubulach (Black Forest) in Baden-Württemberg and the only known site in Austria so far is Arzberg in Styria.

Other well-known sites are among others

See also

literature

  • JFA Breithaupt : Holoëdrites syntheticus or Alstonit. In: Complete characteristics of the mineral system. 2nd edition, Arnoldische Buchhandlung, Dresden and Leipzig 1841, pp. 255–256 ( PDF 145.2 kB )

Web links

Commons : Alstonite  - collection of images, videos and audio files

Individual evidence

  1. ↑ a b Hans Jürgen Rösler : Textbook of Mineralogy . 4th revised and expanded edition. German publishing house for basic industry (VEB), Leipzig 1987, ISBN 3-342-00288-3 , p.  706 .
  2. ↑ a b c d e Hugo Strunz , Ernest H. Nickel: Strunz Mineralogical Tables . 9th edition. E. Schweizerbart'sche Verlagbuchhandlung (Nägele and Obermiller), Stuttgart 2001, ISBN 3-510-65188-X , p.  289 .
  3. ↑ a b c d e Alstonite , In: John W. Anthony, Richard A. Bideaux, Kenneth W. Bladh, Monte C. Nichols (Eds.): Handbook of Mineralogy, Mineralogical Society of America , 2001 ( PDF 65.3 kB )
  4. ↑ a b c Mindat - Alstonite
  5. ^ IMA / CNMNC List of Mineral Names; August 2013 (PDF 1.3 MB)
  6. ^ Friedrich Klockmann : Klockmanns textbook of mineralogy . Ed .: Paul Ramdohr , Hugo Strunz . 16th edition. Enke , Stuttgart 1978, ISBN 3-432-82986-8 , pp.  575 (first edition: 1891).
  7. ↑ Mindat - Number of localities for Alstonite
  8. ↑ Find location list for Alstonite in the Mineralienatlas and Mindat