Magnesiostaurolite
| Magnesiostaurolite | |
|---|---|
 |
|
| General and classification | |
| chemical formula | Mg (Mg, Li) 3 (Al, Mg) 18 Si 8 O 44 (OH) 4 |
|
Mineral class (and possibly department) |
Silicates and Germanates |
|
System no. to Strunz and to Dana |
9.AF.30 ( 8th edition : VIII / B.03) 02.02.03.02 |
| Crystallographic Data | |
| Crystal system | monoclinic |
| Crystal class ; symbol | monoclinic prismatic; 2 / m |
| Space group | C 2 / m (No. 12) |
| Lattice parameters |
a = 7.8706 Å ; b = 16.5411 Å; c = 5.6323 Å β = 90.007 ° |
| Formula units | Z = 1 |
| Physical Properties | |
| Mohs hardness | 7 to 7.5 |
| Density (g / cm 3 ) | calculated: 3.54 |
| Cleavage | Well |
| colour | colorless, pale yellow |
| Line color | White |
| transparency | transparent |
| shine | Glass gloss to resin gloss |
| Crystal optics | |
| Refractive index | n = 1.709 at 592 nm |
| Birefringence | δ <0.010 |
| Optical character | biaxial |
Magnesiostaurolite is a very rarely occurring mineral belonging to the mineral class of " silicates and germanates ". It crystallizes in the monoclinic crystal system with the chemical composition Mg (Mg, Li) 3 (Al, Mg) 18 Si 8 O 44 (OH) 4 and has so far only been found in the form of inclusions in other minerals.
Etymology and history
Magnesiostaurolite was first found in 1992 in the Gilba Valley in the Italian municipality of Brossasco and recognized as an independent mineral under the number IMA 1992-035 . The mineral was first described by Christian Chopin, Bruno Goffe, Luciano Ungaretti and Roberta Oberti, who named the mineral based on its close relationship with staurolite and its magnesium content and described and named it in 2003 in the European Journal of Mineralogy 15, 167-176 published.
classification
In the old (8th edition) and new systematics of minerals (9th edition) according to Strunz , magnesiostaurolite belongs to the department of " island silicates with non-tetrahedral anions (Neso-subsilicates)". The new Strunz'sche mineral classification now subdivides here more precisely according to the position of the cations in the crystal, so that the mineral is now part of the subdivision of "island silicates with additional anions and cations in [4] -, [5] - and / or only [6 ] Coordination ", where it forms the unnamed group 9.AF.30 together with zinc staurolite and staurolite .
The systematics of minerals according to Dana , which is common in the English-speaking world , also assigns magnesiostaurolite to the class of silicates, but there in the division of “ island silicates with SiO 4 groups and O, OH, F and H 2 O with cations in [4] and > [4] coordination ”, where it also forms the unnamed group 52.2.3 together with zinc staurolite and staurolite .
Crystal structure
Magnesiostaurolite crystallizes monoclinically in the space group C 2 / m with the lattice parameters a = 7.8706 Å ; b = 16.5411 Å; c = 5.6323 Å and β = 90.007 ° and one formula unit per unit cell .
Magnesiostaurolites from the Dora-Maira region contain not only magnesium but also significant amounts of lithium and some iron. The full structural formula is:
M4 (Fe 2+ 0.16 Mg 0.72 vac 3.12 ) T2 (Mg 1.86 Li 0.94 Zn 0.02 vac 1.18 ) M1.2 (Al 15.96 Ti 0.04 ) M3 (Al 1.58 Mg 0.45 vac 1.97 ) T1 (Si 7.96 Al 0.04 ) O 44.02 (OH) 3.98
In this structural formula, spaces are indicated as vac (for vacancy ).
A good 1/4 of the cations on the lattice positions of the divalent cations are occupied by lithium (Li + ). This makes the Dora Maira staurolites one of the most lithium-rich staurolites in literature. The reason for the high Li content is not an unusually high Li content of the entire rock, but rather the presence of large tetrahedral gaps (T2) in the staurolite structure compared to other rock-forming minerals . This means that staurolites absorb the entire amount of cations in a rock for which such a large tetrahedral gap is particularly favorable in terms of energy (e.g. Li + and Zn 2+ ).
The generally empty M4 octahedral gap is almost 1/4 occupied by cations (0.88 apfu). Each M4 octahedron is connected to two T2 tetrahedra by common faces. The distance between an M4 and a T2 gap is so small that a common occupation of neighboring T2 and M4 positions can be excluded. With an even distribution of the cations on the M4 positions, approximately two T2 positions should be empty for each occupied M4 position. In fact, there are significantly fewer (1.34 vacancies on T2 per occupied M4 position). This indicates that clusters with high and low occupancy of the M4 position are formed in the staurolite lattice.
Education and Locations
Magnesiostaurolite is a pure high pressure mineral. Experimental studies show that pure magnesia staurolite is stable at pressures between 12 and 60 kbar and temperatures between 600 ° C and 900 ° C.
In nature, magnesiostaurolite occurs in the ultra-high pressure rocks (white slate) of the Italian western Alps ( Dora-Maira massif ). There it occurs together with talc , clinochlor and kyanite as an inclusion in pyrope . Furthermore, Magnesiostaurolite could still be found near Zhimafang in the Chinese district of Donghai .
See also
literature
- Thomas Fockenberg: An experimental investigation on the stability of PT Mg staurolithe in the system MgO-Al 2 O 3 -SiO 2 -H 2 O . In: Contributions to Mineralogy and Petrology . tape 130 , 1998, pp. 187–198 , doi : 10.1007 / s004100050359 (English).
- Christian Chopin, Bruno Goffe, Luciano Ungaretti, Roberta Oberti: Magnesiostaurolite and Zincostaurolite: mineral description with a petrogenetic and crystal-chemical update . In: European Journal of Mineralogy . tape 15 , no. 1 , 2003, p. 167-176 , doi : 10.1127 / 0935-1221 / 2003 / 0015-0167 (English).
- John Leslie Jambor , Andrew C. Roberts: New Mineral Names . In: American Mineralogist . tape 88 , 2003, p. 1626–1629 (English, minsocam.org [PDF; 71 kB ; accessed on November 29, 2018]).
Web links
Individual evidence
- ↑ a b IMA / CNMNC List of Mineral Names; November 2018 (English, PDF 1.7 MB)
- ↑ a b Webmineral - Magnesiostaurolite (English)
- ↑ American-Mineralogist-Crystal-Structure-Database - Magnesiostaurolite (English)
- ↑ a b c d Stefan Weiß: The large Lapis mineral directory. All minerals from A - Z and their properties. Status 03/2018 . 7th, completely revised and supplemented edition. Weise, Munich 2018, ISBN 978-3-921656-83-9 .
- ^ A b c d Christian Chopin, Bruno Goffe, Luciano Ungaretti, Roberta Oberti: Magnesiostaurolite and Zincostaurolite: mineral description with a petrogenetic and crystal-chemical update . In: European Journal of Mineralogy . tape 15 , no. 1 , 2003, p. 167-176 , doi : 10.1127 / 0935-1221 / 2003 / 0015-0167 (English).
- ↑ Find location list for magnesiostaurolite at the Mineralienatlas and at Mindat