Shibkovite

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Shibkovite
General and classification
chemical formula K (□ K) Ca 2 Zn 3 Si 12 O 30
Mineral class
(and possibly department) 		Silicates and germanates - ring silicates
System no. to Strunz
and to Dana 		9.CM.05 ( 8th edition : VIII / E.22)
63.02.01a.16
Similar minerals quartz
Crystallographic Data
Crystal system hexagonal
Crystal class ; symbol dihexagonal-dipyramidal; 6 / m  2 / m  2 / m
Space group P 6 / mcc (No. 192)Template: room group / 192
Lattice parameters a  = 10.502  Å ; c  = 14.184 Å
Formula units Z  = 2
Physical Properties
Mohs hardness 5.5 to 6
Density (g / cm 3 ) measured: 2.89 calculated: 2.90
Cleavage -
colour colorless
Line color White
transparency transparent
shine Glass gloss
Crystal optics
Refractive indices n ω  = 1.561
n ε  = 1.563
Birefringence δ = 0.002
Optical character uniaxial positive, sometimes weakly biaxial
Other properties
Special features bright red fluorescence under short-wave UV light

The mineral shibkovite is a very rare ring silicate from the milarite group and has the idealized chemical composition K (□ K) Ca 2 Zn 3 Si 12 O 30 . It crystallizes in the hexagonal crystal system and develops colorless, isometric grains of less than a millimeter in size. Outwardly, it can easily be confused with quartz, with which it is sometimes closely fused.

Shibkovite forms in alkali-rich , water-poor pegmatites at low pressure (<2 kbar ) and average temperatures around 450–500 ° C. He has only been at its type locality , the moraines of the Darai-Pioz- glacier in Alay Mountains in Tajikistan have been found.

Etymology and history

Shibkovit was discovered in the moraines of the Darai-Pioz- glacier in Alay Mountains in Tajikistan described and 1998 by LA Pautov, AA Agakhanov, E. V Sokolova as a new mineral of Milaritgruppe. They named the mineral after the well-known Russian geologists Viktor Sergejewitsch Schibkow (1926-1992) and Nikolai Viktorowitsch Schibkow (1951-1991).

classification

In the outdated, but still partially common 8th edition of the mineral classification by Strunz Shibkovit belongs Almarudit , Armenit , Berezanskit , Brannockit , Chayesit , Darapiosit , Dusmatovit , Emeleusit , Faizievit , Merrihueit , Oftedalit , Osumilith , Osumilith- (Mg) , Poudretteit , Roedderite , Sogdianite , Sugilite , Trattnerite and Yagiite to the general division of " Ring Silicates (Cyclosilicates)" into the " Milarite Osumilith Group " with the system no. VIII / E.22 .

The 9th edition of Strunz's mineral systematics , which has been in effect since 2001 and is used by the International Mineralogical Association (IMA), also classifies Shibkovite in the "ring silicates" section. This is, however, further subdivided according to the structure of the rings, so that the mineral can be found in the sub-section "[Si 6 O 18 ] 12− -six double rings" according to its structure . Therein it is part of Agakhanovit- (Y) , Almarudit, Armenit, Berezanskit, Brannockit, Chayesit, Eifelit , Darapiosit, Dusmatovit, Friedrichbeckeit , Klöchit , Merrihueit, Milarit, Oftedalit, Osumilith, Osumilith- (Mg), Poudretteit, Roedderit, Sogdianit, Sugilite, trattnerite and yagiite to the " milarite group " with the system no. 9.CM.05 .

The systematics of minerals according to Dana , which is mainly used in the English-speaking world , assigns Shibkovite to the class of "silicates and Germanates", but there in the more finely divided division of "ring silicates: condensed rings". Here it is in the " Milarite Osumilith Group (Milarite Osumilith Subgroup) " with the system no. 63.02.01a can be found in the subsection “ Ring Silicates : Condensed, 6-membered Rings”.

Chemism

Shibkovit is the K-Ca analogue of Klöchit or Dusmatovit or the K-Zn analogue of Milarit and the measured composition from the type locality is [12] K [9] (K 1.20 □ 0.80 ) [6] (Ca 1.26 Mn 0.40 Na 0.39 Fe 0.01 ) [4] Zn 3.01 [4] (Si 12.01 Al 0.01 ) O 30 , with the coordination number of the respective in square brackets Position in the crystal structure is indicated.

The investigation of hydrothermally synthetically produced Shibkovite with the composition K (K 1.67 0.33H 2 O) (Ca 1.3 Na 0.7 ) Zn 3 Si 12 O 30 showed that the crystallization conditions such as pressure, temperature and water content of the fluid phase determine whether and how much crystal water is incorporated into minerals with a milarite structure.

Crystal structure

Shibkovit crystallizes in the hexagonal crystal system in the space group P 6 / mcc (space group no. 192) with the lattice parameters a  = 9.970  Å and c  = 14.130 Å as well as two formula units per unit cell .  Template: room group / 192

The weak, optically biaxial character of Shibkovit indicates a lowered symmetry.

Shibkovite is isotypic to milarite , ie it crystallizes with the same structure as milarite. The 12-fold coordinated C position is fully occupied with potassium (K + ). The 9-fold coordinated B position is also more than half occupied by potassium (K + ). Calcium (Ca 2+ ), manganese (Mn 2+ ), sodium (Na + ) and iron (Fe 2+ ) fill the 6-fold coordinated A position. In addition to the Eifelite, which also belongs to the milarites, it is one of the few minerals that incorporates significant amounts of sodium in octahedral coordinated positions. The tetrahedrally coordinated T2 position contains only zinc (Zn 2+ ). The T1 position, which builds up the 6 double rings, only contains silicon (Si 4+ ).

Education and Locations

Shibkovite forms in alkali-rich , H 2 O-undersaturated pegmatites at low pressure (<2 kbar ) and temperatures around 450–500 ° C.

So far the only documented occurrence of Shibkovit is its type locality , the Darai-Pioz- glacier in Alay Mountains in Tajikistan , where in pegmatitic rocks in the moraine is found. Shibkovit occurs here together with quartz , the feldspars microcline and albite , the sodium pyroxene Aegirin , the lithium Glimmer Polylithionit , Reedmergnerit (a framework silicate of the feldspar group ), the Milaritgruppenmineral Sogdianit , pyrochlore and Pektolith , Turkestanit , a thorium -4er double-ring silicate and sphalerite .

This very mineral-rich site is the type locality of 35 minerals (as of 2016), 5 of them from the milarite group alone: Berezanskite , Darapiosit , Dusmatovite , Shibkovite and Sogdianite . Furthermore, the milarite group minerals milarite, osumilite and sugilite were detected here.

See also

literature

Web links

Individual evidence

  1. ↑ a b FC Hawthorne (2002): The Use Of End-Member Charge-Arrangements In Defining New Mineral Species And Heterovalent Substitutions In Complex Minerals . In: The Canadian Mineralogist , 40, pp. 699–710 ( PDF (309 kB) )
  2. ↑ a b c d e f g h i j k l m n o LA Pautov, AA Agakhanov, E. V Sokolova (1998): Shibkovite K (Ca, Mn, Na) 2 (K 2-x □ x) Zn3 Si12O30 - a new mineral from the milarite group. In: Zapiski Vseross. Mineral. Obshch. , 127, pp. 89–94 ( Russian with English summary )
  3. ↑ a b c d e f g h i j k l m JL Jambor, J. Puziewicz, AC Roberts (2000): New Mineral Names - Shibkovite , In: American Mineralogist , 85, pp. 627–630 ( PDF (34, 3 kB) )
  4. ↑ a b c d e f g h EV Sokolova, VB Rybakov, LA Pautov (1999): Crystal structure of shibkovite . In: Doklady Earth Sciences , 369A, pp. 1288–1290 ( PDF (202 kB) )
  5. ↑ Webmineral - Shibkovit (English)
  6. ↑ GV Kiriukhina, OV Yakubovich, OV Dimitrova (2015): Synthetic shibkovite K (K1.67H2O0.33) (Ca1.3Na0.7) [Zn3Si12O30]: the crystal structure and comparative crystal chemistry . In: Reports Crystallography , 60, pp. 37–45 ( doi: 10.1134 / S1063774515010101 )
  7. ↑ a b Darai-Pioz Glacier (Dara-i-Pioz; Dara-Pioz), Alai Range (Alayskiy), Tien Shan Mtn, Region of Republican Subordination, Tajikistan