Paracelsian

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Paracelsian
Paracelsian-27532.jpg
Paracelsian in free-standing, sharp-edged crystals up to 1.5 cm in length from the Benallt Mine near Rhiw, Gwynedd, Wales (step size: 6.2 × 4.1 × 3.9 cm)
General and classification
chemical formula Ba [Al 2 Si 2 O 8 ]
Mineral class
(and possibly department) 		Silicates and Germanates - framework silicates
System no. to Strunz
and to Dana 		9.FA.40 ( 8th edition : VIII / J.06)
76.01.05.01
Crystallographic Data
Crystal system monoclinic
Crystal class ; symbol monoclinic prismatic; 2 / m
Space group P 2 1 / a (No. 14, position 3)Template: room group / 14.3
Lattice parameters a  = 9.07  Å ; b  = 9.59 Å; c  = 8.58 Å
β  = 90.2 °
Formula units Z  = 4
Frequent crystal faces {110}, {120}, {201}, { 2 01}, {001}
Twinning simple according to (100), lamellar according to (201)
Physical Properties
Mohs hardness ≈ 6
Density (g / cm 3 ) 3.29 to 3.32 (measured), 3.342 (calculated)
Cleavage imperfect according to (110); Angle between (110) and (1 1 0) = 87 °
Break ; Tenacity half-mussel; -
colour colorless to white, pale yellow
Line color White
transparency translucent to translucent
shine Glass gloss
Crystal optics
Refractive indices n α  = 1.5702
n β  = 1.5824
n γ  = 1.5869
Birefringence δ = 0.0167
Optical character biaxial negative
Axis angle 2V = 50 ° 35 ′ - 52 ° 42 ′
Other properties
Special features red fluorescence in short-wave UV light

Paracelsian is a very rarely occurring minerals from the mineral class of " silicates and germanates ," specifically a framework silicate (tectosilicate) having the chemical formula Ba [Al 2 Si 2 O 8 ]. It crystallizes in the monoclinic crystal system and develops pseudorhombic crystals up to 5 cm in length and 1 cm in width, which are often characterized by chisel-shaped end faces. Furthermore, there are aggregates of jumbled crystals and massive formations.

Etymology and history

In 1905 the Italian mineralogist Emilio Tacconi from Pavia described a new mineral from the marble quarries of Candoglia near Mergozzo, Valle d'Ossola, Piedmont, which appeared in the form of pale yellow grains and rarely tiny crystals on corridors in the crystalline slates there. He named it because of its composition very similar to the feldspar Celsian and after the Greek word παρά [para] for "related to" - put together "related to Celsian" - as Paracelsian. Celsian himself was named after the Swedish astronomer and naturalist Anders Celsius (1701–1744). The exact type locality for the Paracelsian is very likely the "Cava Madre" quarry near Candoglia.

Only a few years later (1911), significantly better material was found in the Benallt manganese mine in North Wales, which - to this day - should provide the world's best crystals for Paracelsian. A Mr. GJ Williams, HM Inspector of Mines at the Crown Estate's manganese mine, had reported to the famous Arthur Russell that crystals of the Celsian barium feldspar had been found. Sir Arthur Edward Ian Montagu Russell, British mineralogist and well-known collector of minerals and mineral collections, wrote a short article about this in Nature magazine . The description of the two barium feldspars - Celsian and Paracelsian - from this find was only carried out many years later by the British mineralogist and editor of the Mineralogical Magazine Leonard James Spencer on the material collected by Williams and Russel.

Type material for the mineral is not defined.

classification

In the outdated but still partially in use 8th edition of the mineral classification by Strunz of Paracelsian belonged to the mineral class of "silicates and Germanates" and then to the Department of " framework silicates (tectosilicates)" where he collaborated with Buddingtonit , celsian , Hyalophane , Kokchetavit , Mikroklin , Orthoclase , Rubiklin , Sanidin and Slawsonit the group of "alkali feldspars" with the system no. VIII / J.06 formed.

The 9th edition of Strunz's mineral systematics , which has been valid since 2001 and is used by the IMA, also assigns the Paracelsian to the class of “silicates and germanates” and there in the “tectosilicates” section. This section is, however, further subdivided according to the possible presence of further anions , so that the mineral can be found according to its composition in the subdivision of "tectosilicates (tectosilicates) without additional anions", where it is the only member of the unnamed group 9.FA.40 forms.

Paracelsian also assigns the systematics of minerals according to Dana to the class of "silicates and germanates" and there in the department of "framework silicates: Al-Si lattice". Here, together with Slawsonite, it forms the "Paracelsian group" with system no. 76.01.05 within the subdivision " With (simple) Al-Si grid ".

Crystal structure

Crystal structure of Paracelsian projected onto the (a, c) plane

Paracelsian crystallizes in the monoclinic crystal system in the space group P 2 1 / a (space group no. 14, position 3) , with the lattice parameters a  = 9.07  Å ; b  = 9.59 Å; c  = 8.58 Å and β = 90.2 ° as well as four formula units per unit cell . Template: room group / 14.3

The crystal structure of Paracelsian consists of a backbone of ordered feldspatartigen SiO 4 - and AlO 4 - tetrahedra with Ba [9] resulting gaps in between. The coordination number of the position in the crystal structure is indicated in square brackets, which in this case means that barium (Ba 2+ ) is in a 9-coordinate position.

properties

morphology

Drawing of a pseudorhombic Paracelsian crystal

The crystals of Paracelsians similar in costume and habit face poor Topaz - and Danburit crystals, which is very unusual for a monoclinic feldspar actually. However, the angle β is 90.2 °, which results in a pseudo-orthorhombic appearance, which explains the similarity with topaz and danburite crystals.

The habit of the Paracelsian crystals is prismatic, their crystal costume consists of only a few shapes. The prisms {110} and {120} determine the costume . The surfaces in the prism zone are striped vertically. Between the areas of {110} and {120} there are numerous vicinal areas with indices ranging from {780} to {7.13.0}. With regard to the end faces, characteristic chisel-shaped crystals with the pinacoids {201} and { 2 01} are to be distinguished from those in which the base pinacoid {001} forms the dominant end faces. The Paracelsian crystals are mostly symmetrical, but occasionally show an uneven formation of the end faces with the chisel-shaped faces of the pinacoid {201} and { 2 01} at one end and the base pinacoid {001} at the other end of the crystal.

physical and chemical properties

The color of the crystals and aggregates of Paracelsians is colorless to white, occasionally also pale yellow. The line color of the translucent to transparent mineral, on the other hand, is always white. The Paracelsian crystals have a glass-like sheen .

According to (110), the mineral shows a very unusual, only imperfect cleavage ability for a feldspar , but due to its brittleness it breaks like glass or quartz , whereby the break edges are half-shelled. With a Mohs hardness of ≈ 6, Paracelsian is one of the medium-hard minerals that, like the reference mineral orthoclase, can be scratched with a steel file. The calculated density of the mineral is a maximum of 3.342 g / cm³.

Paracelsian shows red fluorescence in short-wave UV light.

Modifications and varieties

The compound Ba [Al 2 Si 2 O 8 ] is dimorphic and occurs in nature in addition to the monoclinic-pseudorhombic crystallizing Paracelsian as a monoclinic Celsian that crystallizes in a different space group . In addition, two other artificial polymorphs of this compound are known. Paracelsian is very likely metastable and converts to Celsian via "Hexacelsian" at temperatures of at least 500 ° C (possibly also at even lower temperatures).

Paracelsian is the natural barium-dominant analogue of the strontium-dominated slavsonite, Sr [Al 2 Si 2 O 8 ], with which it is also isostructural. It also forms the natural barium-dominant analogue of the calcium-dominated anorthite, Ca [Al 2 Si 2 O 8 ].

Education and Locations

Paracelsian is a very rare feldspar and was found by its type locality in grains and granular masses in a calciphyr. At its second location in Benallt, Wales, it appeared in a volume in Ordovician slates and sandstones, which were associated with lava flows and volcanic tuffs and layers of black manganese ores. The non-oxidized manganese ore consisted mainly of carbonates and, to a lesser extent, of silicates . Paracelsian is accompanied by Celsian at both locations; in Benallt, secondary iron and manganese oxides are still found in paragenesis . Finally, Paracelsian was found in the form of microscopic grains of manganogrunerite-rich ribbons in manganese ores from the Assitza deposit in Romania.

Paracelsian has only been described as a very rare mineral formation from a few localities and was only a little more common in Benallt. So far (as of 2016) only four sites are known.

The type locality is the quarry "Cava Madre", Candoglia, Mergozzo , Valle d'Ossola , Piedmont , Italy . The second location was the Benallt manganese mine near Rhiw, Lleyn Peninsula , County Gwynedd (formerly Caernarfonshire ), Wales , United Kingdom . In the form of microscopic grains of manganogrunerite-rich ribbons in manganese ores from the Assitza deposit, Iacobeni (German Jakobeny), Suceava district , Bistrița Mountains, Romania . Allegedly also from the type locality of the Celsian, the “Jakobsberg” pit (“Jakobsbergsgruvan”), Jakobsberg-Erzfeld, Nordmark District, Filipstad , Värmland Province , Sweden .

use

Due to its rarity, Paracelsian is completely meaningless in every respect and is only a coveted mineral for the collector.

See also

literature

  • Leonard James Spencer : Barium felspars (celsian and paracelsian) from Wales . In: Mineralogical Magazine . tape 26 , no. 178 , 1942, pp. 231–245 (English, v [PDF; 1,3 MB ; accessed on May 14, 2019]).
  • Paracelsian . In: John W. Anthony, Richard A. Bideaux, Kenneth W. Bladh, Monte C. Nichols (Eds.): Handbook of Mineralogy, Mineralogical Society of America . 2001 (English, handbookofmineralogy.org [PDF; 68 kB ; accessed on May 14, 2019]).

Web links

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

Individual evidence

  1. ↑ a b c d e f g Hugo Strunz , Ernest H. Nickel : Strunz Mineralogical Tables. Chemical-structural Mineral Classification System . 9th edition. E. Schweizerbart'sche Verlagbuchhandlung (Nägele and Obermiller), Stuttgart 2001, ISBN 3-510-65188-X , p.  696 .
  2. ↑ a b c d e f g h i Paracelsian . In: John W. Anthony, Richard A. Bideaux, Kenneth W. Bladh, Monte C. Nichols (Eds.): Handbook of Mineralogy, Mineralogical Society of America . 2001 (English, handbookofmineralogy.org [PDF; 68  kB ; accessed on May 14, 2019]).
  3. ↑ a b Webmineral - Paracelsian
  4. ^ A b Jan H. Bernard, Jaroslav Hyršl: Minerals and their localities . 1st edition. Granit, Praha 2004, ISBN 80-7296-039-3 , p. 450 .
  5. ^ Enrico Tacconi: Di un silicato di alluminio e bario [Paracelsian] dei calcefiri di Candoglia in valle del Toce . In: Rendiconti R. Istituto, Lombardo, Science. Letters . tape 38 , 1905, pp. 636-643 (Italian).
  6. ↑ Marco E. Ciriotti, Lorenza Fascio, Marco Pasero: Italian Type Minerals . 1st edition. Edizioni Plus - Università di Pisa, Pisa 2009, ISBN 978-88-8492-592-3 , p. 209 .
  7. ^ Arthur Russell : An occurrence of the Barium-felspar Celsian in North Wales . In: Nature . tape  86 , no. 2162 , 1911, pp. 180 , doi : 10.1038 / 086180c0 ( zenodo.org [PDF; 132 kB ; accessed on May 14, 2019]).
  8. ↑ a b c d Leonard James Spencer : Barium felspars (celsian and paracelsian) from Wales . In: Mineralogical Magazine . tape  26 , no. 178 , 1942, pp. 231–245 (English, v [PDF; 1,3 MB ; accessed on May 14, 2019]).
  9. ^ HC Lin, WR Foster: Studies in the system BaO-Al 2 O 3 -SiO 2 I. The polymorphism of celsian . In: American Mineralogist . tape 53 , 1968, pp. 134–144 (English, minsocam.org [PDF; 713 kB ; accessed on May 14, 2019]).
  10. ↑ M. Bălan: Mineralogia zăcămintelor manganifere de la Iacobeni [minerals of manganese deposits in Iacobeni] . 1st edition. Edit. Academiei, Bucureşti 1976, OCLC 3479779 (Romanian).
  11. ↑ Localities for Paracelsian. In: mindat.org. Hudson Institute of Mineralogy, accessed May 14, 2019 .
  12. ↑ a b List of locations for Paracelsian in the Mineralienatlas and in Mindat