Polyhalite

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Polyhalite
History museum of Truskavets 081.jpg
Anhydrite and polyhalite
General and classification
chemical formula K 2 Ca 2 Mg [SO 4 ] 4 • 2H 2 O
Mineral class
(and possibly department) 		Sulphates, Selenates, Tellurates, Chromates, Molybdates and Wolframates
System no. to Strunz
and to Dana 		7.CC.65 ( 8th edition : VI / C.20)
04/29/05/01
Crystallographic Data
Crystal system triclinic
Crystal class ; symbol triclinic pinacoidal; 1
Room group (no.) P 1 (No. 2)
Lattice parameters a  = 6.975  Å ; b  = 6.984 Å; c  = 8.899 Å,
α  = 104.01 °; β  = 101.19 °; γ  = 114.10 °
Formula units Z  = 4
Twinning predominantly after {010} and {100} polysynthetic twins
Physical Properties
Mohs hardness 3.5
Density (g / cm 3 ) 2.76 to 2.78
Cleavage completely according to {10 1 }
Break ; Tenacity shell-like, brittle
colour colorless, gray-white, brown, rose-red, red-brown
Line color White
transparency transparent to translucent
shine Glass gloss to fat gloss
Crystal optics
Refractive indices n α  = 1.546 to 1.548
n β  = 1.558 to 1.562
n γ  = 1.567
Birefringence δ = 0.021
Optical character biaxial negative
Axis angle 2V = measured: 60 to 62 °; calculated: 60 to 80 °

Polyhalite is a rather seldom occurring mineral from the mineral class of " sulfates ". It crystallizes in the triclinic crystal system with the chemical composition K 2 Ca 2 Mg [SO 4 ] 4 · 2H 2 O and develops mostly needle-like, tabular or prismatic-columnar crystals , but also flaky or fibrous mineral aggregates . By forming twins, polyhalite often simulates an orthorhombic symmetry.

Special properties

Pure polyhalite is colorless. However, it can also be gray-white, brown, pink-red or red-brown in color due to foreign admixtures.

The mineral is slowly decomposed by water with the separation of gypsum and possibly syngenite and partial dissolution. A taste is barely noticeable, sometimes slightly salty.

Etymology and history

Even if the name suggests a similarity, the sulfatic polyhalite has nothing to do with the chloride mineral halite . The ancient Greek words πολύς (polýs) for "a lot" and ἅλς (neck) for "salt" are an allusion to its complex composition with several salt-forming metals.

Polyhalite was first found in 1818 in the Bad Ischler Salzberg in Austria and described by Friedrich Stromeyer .

classification

In the old (8th edition) and new systematics of minerals according to Strunz (9th edition) , polyhalite belongs to the water-containing sulfates without foreign anions . However, the new Strunz'sche mineral classification now subdivides more precisely according to the size of the cations , and the mineral is accordingly in the subdivision “C. Found with medium-sized and large cations ”, where he and Wattevilleit form the unnamed group“ 7.CC.65 ”.

The systematics of minerals according to Dana , which is common in the English-speaking world, also classifies polyhalite in the class of sulfates, but there in the division of " hydrated acids and sulfates with (A + ) 2 B n (XO 4 ) p  • x (H 2 O) ", where he forms the unnamed group" 29.4.5 " together with Leightonit .

Education and Locations

Polyhalite is mainly formed by sedimentary deposits in marine salt deposits , but in rare cases also occurs as a sublimate on fumaroles .

In Austria, in addition to its type locality Bad Ischler Salzberg , polyhalite was also found near Hallstatt in Upper Austria, Abtenau and Hallein in Salzburg, Altaussee in Styria and Hall in Tyrol .

In Germany, polyhalite was found in the Bavarian salt mine Berchtesgaden , in Neuhof (near Fulda) and in the Werra Valley in Hesse, near Celle , Lüneburg and Nordhorn in Lower Saxony, Staßfurt in Saxony-Anhalt and Gera and Bad Salzungen in Thuringia.

So far (as of 2010), polyhalite has been detected at over 60 sites worldwide, including in Chile , China , France , Greece , Iran , Italy , Kazakhstan , Mexico , the Netherlands , Poland , Russia , Spain , Turkey , Ukraine , Uzbekistan , United Kingdom (Great Britain) and the United States of America .

Crystal structure

Polyhalite crystallizes triclinically in the space group P 1 (space group no. 2) with the lattice parameters a  = 6.975  Å ; b  = 6.984 Å; c  = 8.899 Å; α = 104.01 °; β = 101.19 ° and γ = 114.10 ° as well as four formula units per unit cell .

See also

Individual evidence

  1. ↑ a b c d e Handbook of Mineralogy - Polyhalite (English, PDF 66.5 kB)
  2. ↑ a b American Mineralogist Crystal Structure Database - Polyhalite (English, 2005)
  3. ↑ Webmineral - Polyhalite (English)
  4. ↑ a b c Polyhalite at mindat.org (engl.)
  5. ^ 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. 391 .
  6. ↑ Carl Hintze : Handbook of Mineralogy. First volume. Third division. Second half: sulfates, chromates, molybdates, tungstates, uranates . 1st edition. Walter de Gruyter & Co., Berlin and Leipzig 1930, p. 4477-4485 .
  7. ↑ American Mineralogist Crystal Structure Database - Polyhalite (English, 2005)

literature

Web links