Gehlenite

Gehlenite
Gehlenite from the Fassa Valley , South Tyrol , Italy
General and classification
chemical formula	Ca 2 Al [4] [AlSiO 7 ]
Mineral class (and possibly department)	Silicates and Germanates
System no. to Strunz and to Dana	9.BB.10 ( 8th edition : VIII / C.02) 04.55.01.02
Crystallographic Data
Crystal system	tetragonal
Crystal class ; symbol	tetragonal-scalenohedral; 4 2 m
Space group	P 4 2 1 m (No. 113)
Lattice parameters	a  = 7.69  Å ; c  = 5.07 Å
Formula units	Z  = 2
Twinning	after {100}, lamellar after {001}
Physical Properties
Mohs hardness	5 to 6
Density (g / cm 3 )	measured: 3.038; calculated: 3.03
Cleavage	clearly after {001}; indistinct after {110}
Break ; Tenacity	uneven, splintery to mussel-like
colour	colorless, brown, yellowish, greenish-gray
Line color	white to off-white
transparency	transparent to opaque
shine	Glass gloss to fat gloss
Crystal optics
Refractive indices	n ω  = 1.670 n ε  = 1.660
Birefringence	δ = 0.010
Optical character	uniaxial negative

Gehlenite is a rather seldom occurring mineral from the mineral class of " silicates and germanates ". It crystallizes in the tetragonal crystal system with the chemical composition Ca ₂ Al ^[4] [AlSiO ₇ ] and develops mostly transparent to translucent crystals with a thick tabular or short prismatic habit and white, gray or yellowish color, but also granular to massive mineral aggregates .

Gehlenite and Åkermanit form a perfect mixed series .

Etymology and history

Gehlenite was first found in 1815 on Monte Monzoni in the Fassa Valley in Italy and described by Johann Nepomuk von Fuchs , who named the mineral after the German chemist Adolf Ferdinand Gehlen , the type locality is in the Fassa Valley in the province of Trento , Italy .

classification

In the meanwhile outdated, but still in use 8th edition of the mineral classification according to Strunz , the gehlenite belonged to the mineral class of "silicates and germanates" and there to the department of " group silicates (sorosilicates)", where together with Åkermanit , Andrémeyerit , Barylith , Gugiait , Hardystonit , Jeffreyit , Kaliobarylith , melilite , Meliphan and Okayamalith the "melilite group" with the system number. VIII / C.02 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), also assigns gehlenite to the class of "silicates and germanates" and there in the department of "group silicates (sorosilicates)". However, this section is further subdivided according to the type of silicate group formation, the possible presence of further anions and the coordination of the cations , so that the mineral according to its composition and structure in the subdivision “Si ₂ O ₇ groups, without non-tetrahedral anions; Cations in tetrahedral [4] and greater coordination ”can be found, where together with Åkermanite, Barylite, Cebollite , Gugiaite, Hardystonite, Jeffreyite, Melilith and Okayamalith the“ Melilite group ”with the system no. 9.BB.10 forms.

The systematics of minerals according to Dana , which is mainly used in the English-speaking world , assigns gehlenite to the class of “silicates and germanates”, although there it is in the more finely subdivided division of “group silicates: Si ₂ O ₇ groups, generally without additional anions” . Here he is together with Åkermanit, Melilith and Okayamalith in the "Melilith group" with the system no. 55.04.01 to be found within the subsection of the " Group silicates: Si ₂ O ₇ groups, generally without additional anions and with cations in [8] and lower coordination ".

Crystal structure

Gehlenite crystallizes tetragonally in the space group P 4 2 ₁ m (space group no. 113) with the lattice parameters a  = 7.69  Å and c  = 5.07 Å as well as two formula units per unit cell .

The crystal structure is built up by groups of [AlSiO ₇ ] and [AlO ₄ ] tetrahedra arranged in layers (100) , which are linked to one another by Ca - O bonds, with Ca occurring in [8] coordination compared to O. ${\displaystyle \parallel }$

Education and Locations

Gehlenite can occur naturally in plutonites , metamorphic rocks and meteorites or artificially through high-temperature firing of carbonate-containing ceramics . High-temperature metamorphism of “impure” (aluminosilicate-containing) limestone or contact metamorphism of igneous rocks with carbonates can lead to the formation of gehlenite. The mineral was also described in chondritic meteorites and is one of the first condensation products of the cooling presolar nebula . In calcium-aluminum-rich inclusions (CAI) gehlenite occurs together with Grossit , Hibonit , spinel and Fassait, a complex Klinopyroxen- mixed crystal of diopside , Kushiroit , Davisit and Grossmanit .

So far (as of 2010) Gehlenite has been found at around 60 sites around the world, for example in China , Germany , Iran , Israel , Italy , Japan , Mexico , New Zealand, Austria , Romania , Russia , Sweden , the Czech Republic , Uganda , Hungary and the United Kingdom (Great Britain) and in the United States (USA). Gehlenite was also found in the comet dust from Wild 2 .

use

Since gehlenite is relatively easy to detect by means of X-ray diffraction due to its good crystallinity and has a very limited field of formation and stability under atmospheric pressure conditions, this mineral can be used very well to determine the firing temperatures of ancient ceramics. However, this "thermometer" can only be used in carbonate-containing ceramics , since an adequate amount of reactive calcium must be available for the formation reaction of gehlenite.

See also

• List of minerals

literature

• Friedrich Klockmann : Klockmann's textbook of mineralogy . Ed .: Paul Ramdohr , Hugo Strunz . 16th edition. Enke, Stuttgart 1978, ISBN 3-432-82986-8 , pp. 689 (first edition: 1891).
• Petr Korbel, Milan Novák: Mineral Encyclopedia (=  Villager Nature ). Edition Dörfler im Nebel-Verlag, Eggolsheim 2002, ISBN 978-3-89555-076-8 , p. 211 .

Web links

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

• Mineral Atlas: Gehlenite (Wiki)
• Gehlenite search results. In: rruff.info. Database of Raman spectroscopy, X-ray diffraction and chemistry of minerals (RRUFF), accessed February 25, 2019 . 
• American-Mineralogist-Crystal-Structure-Database - Gehlenite. In: rruff.geo.arizona.edu. Retrieved February 25, 2019 . 

Individual evidence

1. ^ David Barthelmy: Gehlenite Mineral Data. In: webmineral.com. Retrieved February 25, 2019 .  
2. ↑ ^{a b c d} 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.  568 (English).
3. ↑ ^{a b c d e f} Gehlenite . 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; 67  kB ; accessed on February 25, 2019]).
4. ↑ ^{a b c} Gehlenite. In: mindat.org. Hudson Institute of Mineralogy, accessed February 25, 2019 .  
5. ↑ Richard V. Gaines, H. Catherine W. Skinner, Eugene E. Foord, Brian Mason , Abraham Rosenzweig: Dana's New Mineralogy . 8th edition. John Wiley & Sons, New York et al. 1997, ISBN 0-471-19310-0 , pp. 1143-1144 .
6. ↑ C. Tschegg, Th. Ntaflos, I. Hein, 2008, Applied Clay Science
7. ^ Lawrence Grossman: Condensation in the primitive solar nebula . In: Geochemica et Cosmochemica Acta . tape 36 , no. 5 , 1972, p. 597-619 , doi : 10.1016 / 0016-7037 (72) 90078-6 (English).
8. ^ Lawrence Grossman: Vapor-condensed phase processes in the early solar system . In: Meteoritics & Planetary Science . tape 45 , 2010, p. 7–20 ( onlinelibrary.wiley.com [PDF; 2.0 MB ; accessed on December 23, 2018]).
9. ↑ Find location list for gehlenite in the Mineralienatlas and Mindat