Sillimanite

The mineral sillimanite is a very common island silicate from the group of aluminosilicates and has the chemical composition Al2SiO5 or Al 2 [O | SiO4 ]. It crystallizes in the orthorhombic crystal system and forms prismatic to fibrous crystals of small size. Is not unusual to find a small percentage of Fe2O3 in sillimanite.

Sillimanite has a high hardness of 6.5 to 7.5 and a whitish-gray to green-gray color, but is sometimes colorless. The stroke color is white. Similar minerals with the same or similar chemical composition are andalusite, kyanite and mullite, which are also among the aluminosilicates.

Special Features

The Petrology of metamorphic rocks sillimanite takes as an indicator of the strength of the transformations an important position. As a so -called index minerals defined his first appearance of the Sillimanitzone or sillimanite - Isograd, its stability region is in this case limited by the thermodynamic transformations andalusite sillimanite and kyanite ⇔ ⇔ sillimanite. This range is at relatively high temperatures (> 540 ° C) and can print medium (up to ~ 1 GPa, corresponding to a depth of 36.5 km ) reach. It encompasses most of the amphibolite and granulite facies and the highly tempered contact metamorphism.

In the course of metamorphosis it comes to the formation of sillimanite by polymorphic transformation of andalusite or kyanite or conversion reactions of biotite and muscovite. As an example, the following response was given:

This reaction is very important because it divides the stability field of sillimanite in two areas - the Sillimanitzone is therefore divided into two sub-zones, in the slightly lower temperature- and pressure- stressed sillimanite - muscovite sub-zone as well as in the higher temperature- sillimanite - alkali feldspar - subzone. The reaction begins to take effect and caused the complete disappearance of muscovite from 630 ° C.

Reactions between staurolite and biotite or between staurolite and quartz.

Take place with temperatures reaching and exceeding anatectic reactions in which sillimanite is reduced again. As examples, the biotite dehydration has:

Sillimanite biotite alkali feldspar garnet ⇒ liquid or

Sillimanite biotite cordierite ± garnet ⇒ liquid

But even in the Verfauf Retromorphose sillimanite disappears gradually, with decreasing temperatures and pressure drop is eg Andalusite polymorphic regressed.

Sillimanite is quite resistant mineral weathering, but still decomposes with the formation of kaolinite and muscovite or sericite ( epizonale Serizitisierung ).

Etymology and history

Sillimanite was named after the American chemist Benjamin Silliman U.S.. A Sillimanitmineral with location in Chester, Connecticut, was described in 1824 for the first time scientifically by George T. Bowen. Sillimanite is sometimes referred to as Bucholzit - after the German pharmacists and chemists WHS Bucholz. He is also known as Glanzspat.

Classification

In the old ( 8th edition ) and new classification of minerals ( 9th Edition ) by Strunz heard sillimanite to the Department of " island silicates with tetrahedral foreign anions ( Neso - Subsilikate ) ". The new Strunz'sche Mineral classification divided here, however, precisely according to the position of the cations in the crystal, so that the mineral " in, island silicates with additional anions and cations - and / or only coordination " now the subdivision is assigned, where he served as only member is the unnamed group 9.AF.05.

The commonly used in English-speaking classification of minerals according to Dana assigns the sillimanite also in the class of silicates, there, however, in the department of " island silicates with SiO 4 Groups and O, OH, F, and H 2 O with cations in and > coordination " where he as the eponymous mineral mullite together with the " Al2SiO5 ( sillimanite - subgroup ) " with the system no. 52.2.2a forms.

Modifications and varieties

Sillimanite is the high-temperature low-pressure modification of Al2SiO5 group and trimorph with the other members of andalusite and kyanite.

Fibrolite is a tufted aggregate elongated Sillimanitkristalle ( Comte de Bournon, 1802). Fiber pebbles, however, are sub-parallel, needle-like swarms and wisps of sillimanite in quartz or cordierite (described in 1792 by Lindacker in Bohemia). Other local varieties are Monrolit ( after the town of Monroe, New York) and Bamlit (after Bamle in Brevik in Norway).

Education and Locations

Sillimanite is found in the form of columnar crystals or stalky - fibrous or solid in aluminum-rich, pelitic, regionalmetamorphen rocks. It usually occurs in two types of metamorphosis:

Kontaktmetamorph sillimanite is present in the highly tempered Sanidinit facies.

The mineral of magmatic origin, he is part of peraluminosen granitoids. Only very rarely will you find sillimanite in amphibolites and eclogites, relatively rare in pegmatites, but quite often in granulites. You meet him occasionally as detritus in sediments.

Accompanying minerals are alkali feldspar, almandine, andalusite, biotite, cordierite, enstatite ( at higher temperatures) corundum, kyanite, muscovite, plagioclase, quartz and / or spinel.

The type locality for sillimanite is Sušice in the Czech Republic. Locations in Germany are the Laacher See, the Spessart and ground corn in the Bavarian Forest. Furthermore Sellrain in Austria, the Auvergne in France, Assam in India, Myanmar, Sri Lanka, Enderby Land in Antarctica and Brandywine Springs in Delaware, USA.

Crystal structure

Sillimanite crystallized in the orthorhombic crystal system dipyramidalen in the space group P nma with the lattice parameters a = 7.484 Å; b = 7.672 Å and c = 5.77 Å and four formula units per unit cell.

The basic building blocks of Sillimanitstruktur are:

• [ SiO4 ] - and [ AlO 4 ] tetrahedra in four coordination and
• [ AlO6 ] octahedra in six- coordination.

Aluminum thus occurs in a dual role, ie in two different coordination stages, as Al and the Al, a more accurate formula for sillimanite also is this: Al [O | AlSiO4 ].

Octahedra are linked through their parallel side edges lined up in the endless chains run parallel to the c-axis. A strand is located in the center of the unit cell, four more forming the parallel to the c -axis side edges. The tetrahedra form four endless chains in the c direction, with the central atoms Si and Al regularly alternate with each other. These Si -Al -Si-Al ....- tetrahedral chains are located between the chains of octahedra and link about their oxygen atoms (OD atoms) with the chains of octahedra. However, the tetrahedral chains are not one single chains, double chains but One, that is, they link to see more about her free tips (OC- atoms) again with the opposite tetrahedron chain of the adjacent unit cell.

With this arrangement, sillimanite can also be considered as a chain silicate ( inosilicate ); this explains very well his elongated, needle-like, fibrous habit.

To illustrate the Sillimanitstruktur adjacent figure:

Shows the projection of a half unit cell ( Z = 0 to Z = 1/ 2) along the c- axis to the off - or (001 ) plane. The unit cell is outlined in red. The [ AlO6 ] octahedra are light green, those of the [ SiO4 ] - and marked [ AlO 4 ] tetrahedra beige. This form of representation has been chosen for reasons of clarity, as only the atoms of the central octahedral ( Al1 - atoms ), and the OD atoms strictly maintaining their parallelism to the c- axis; all other atoms are slightly shifted in the upper half of the unit cell in position.

The left figure is a simplified elevational view of the unit cell parallel to the c axis. It shows very clearly the link between the tetrahedral One double chains with the octahedra and the parallelism of the atomic positions Al1 and OD. Note value of the dimensional difference of the silicon tetrahedron and the aluminum tetrahedron ( 2.696 or 3.074 Å ), which summed up the dimension of the c- axis in the unit cell yield ( 5.77 Å).

The unit cell of sillimanite

The atom positions of the elementary cell of sillimanite are as follows:

This information as well as 13 subsequent symmetry operations are sufficient to completely define the unit cell.

Of note is the nearly identical dimension of the a- and b- axis, sillimanite missed a tetragonal symmetry, only marginally.

X-ray diffraction

X-ray diffraction studies of sillimanite crystals have provided the following results:

The first two maximum values ​​are quite close together and are often interchanged.

Use

Sillimanite is in good quality as a gemstone use, however, is poorly understood. Clear varieties are usually available in different facets sections as brilliant or faceted oval cut. Opaque stones and those with visual effects such as chatoyancy ( cat's eye effect ) or asterism ( star effect ), however, get a cabochonförmigen smooth finish.

Industrial sillimanite serves for the production of refractory materials (structural pipes for heating coils in electric furnaces, spark plugs, etc.).