Staurolite

• Kreuzenstein

The mineral staurolite (cross- stone) is a common island silicate having the general chemical composition M2 4 Al18Si8O46 (OH ) 2 In this simplified structural formula M2 for divalent cations, primarily iron ( Fe 2 ), magnesium ( Mg 2 ) and zinc ( Zn2 ) in any mixing ratios. After the contents of these cations four minerals are distinguished in the Staurolithgruppe:

• Staurolite ( Eisenstaurolith ): ( Fe2 ) 2Al9Si4O23 (OH )
• Magnesiostaurolith: Mg (Mg, Li ) 3 ( Al, Mg) 18Si8O44 (OH) 4
• Zinkstaurolith: Zn2Al9Si4O23 (OH )

Straurolithe crystallize in the monoclinic crystal system and develop predominantly prismatic to tabular crystals and characteristic cross-shaped - crystal twins, but also granular to massive aggregates in red-brown to dark brown color.

• 7.1 atomic positions
• 7.2 links the coordination polyhedra

Special Features

Staurolite is only imperfectly cleavable uneven conchoidal breaks and shows in pure form glass or greasy. The frequently encountered macroscopically visible crystals have a columnar appearance ( habit ). They are often larger than the crystals surrounding minerals and are then called porphyroblasts. A morphological peculiarity of Stauroliths is that it often occurs as a twin crystal in a characteristic cross shape; the angle between the crystals is either 90 or about 60 degrees.

Etymology and history

The name of the mineral is derived from the Greek and means stone cross ( σταυρóς = Cross, λíθος = stone), so playing on the cruciform twinning often found at. For this reason, larger crystals were often worn by Christians as jewelry or amulet. In particular, in the Swiss Alps they were widely used under the name Basler baptismal font.

Classification

In the now outdated but still in use 8th edition of the mineral classification by Strunz the staurolite belonged to the mineral class of " silicates and Germanates " and then to the Department of " island silicates with tetrahedral foreign anions ( Neso - Subsilikate ) " where he along with Gerstmannit, Magnesiostaurolith and Zinkstaurolith the unnamed group VIII/B.03 formed.

The 9th edition valid since 2001 and of the International Mineralogical Association (IMA ) used the Strunz'schen Mineral classification assigns the staurolite also in the class " silicates and Germanates " and then in the Department of the " island silicates ( nesosilicates ) ". This division, however, is further subdivided according to the possible presence of other anions and the coordination of the cations involved, so that the mineral according to its composition in the subdivision of " island silicates with additional anions; Cations iner, he and / or nurer coordination " is to find where it is also the untitled group 9.AF.30 ' together with Magnesiostaurolith and Zinkstaurolith.

The mainly common in English-speaking classification of minerals according to Dana assigns the staurolite in the class of " silicates and Germanates " there, however, in the department of " island silicates: SiO4 Groups and O, OH, F, and H2O" one. Here he is with Magnesiostaurolith and Zinkstaurolith in the unnamed group 52.02.03 within the subdivision of the " island silicates: SiO4 Groups and O, OH, F, and H 2 O with cations in and > coordination " to find.

Chemistry

The composition of staurolite is important because can be drawn from the occurrence of staurolite conclusions on the formation conditions of staurolithführenden rock. This is done with the objective to determine the pressure and temperature history of a rock and to reconstruct the movement of whole rock formations in the earth's crust.

For the determination of such pressure and temperature data mineral reactions must be calculated. For this, one needs firstly information about the composition of all minerals involved and other detailed knowledge of the intracrystalline distribution of the elements on the different positions of the mineral structure.

Element contents

The above- mentioned chemical formula is a simplified composition of staurolite again. The complexity of the crystal chemistry of staurolite becomes apparent only when the contents difficult analyzable elements such as lithium and hydrogen, and the distribution of elements is taken into account on the different cation positions. In mineralogy structural formulas have prevailed for the notation of mineral compositions, because they still contain additional structural information. A simplified structural formula for staurolite is:

(Fe, Mg, Zn, Co, Ni, Mn, Li, Al) 2-4 ( Al, Cr, Ti, Mg, Fe ) 18 ( Si, Al ) 8O40 ( O, OH ) 8

In this formula, the element contents of the positions T2 and M4 are given in the first bracket (Fe, ...) 2-4. The second bracket contains the elements of M1 Aluminiumoktaeder, 2.3, and the third bracket, the elements of the silicon tetrahedron T1. O40, the oxygen ions of the oxygen positions O2, 3,4,5, during (O, OH) 8 reflects the composition of the oxygen O1 position. The latter is the oxygen ion, through which the octahedron M3 and M4 are connected and to which the hydrogen ions are bound ( OH groups).

A review of nearly 550 published compositions of natural Staurolithe provides the following picture of the element concentrations:

• Si4 : 7 to 8 apfu (atoms per formula unit ), in the Middle: 7.72 apfu
• Al3 : 16.1 to 19.5 apfu, on average: 17.8 apfu
• Ti4 : 0 to 0.35 apfu, on average: 0.1 apfu
• Cr3 : 0 to 1.4 apfu, on average: 0 apfu
• Fe3 : 0 to 0.36 apfu, often not determined
• Fe2 : 0.15 to 3.9 apfu, on average: 2.7 apfu
• Mg2 : 0 to 3 apfu, on average: 0.7 apfu
• Zn2 : 0 to 2.8 apfu, on average: 0.4 apfu
• Co2 : 0 to 2.1 apfu, on average: 0 apfu
• Mn2 : 0 to 0.45 apfu, on average: 0.06 apfu
• Li : 0 to 1.6 apfu, often not determined
• H : 1.8 to 4.6 apfu, often not determined

Element distributions

All Si4 ions are located on the T1 position. If less than 8 silicon ions per formula unit present, the remaining T1 - spaces are filled with aluminum ions. The charge equalization takes place via the installation of a hydrogen ion per aluminum ion T1.

Almost all trivalent cations and Ti4 and about 10 percent of all divalent cations are incorporated in the octahedral positions M1, 2,3. An exception is the Zn2 which is installed only on the tetrahedral position T2. The charge compensation for the incorporation of a divalent cation, a trivalent instead takes place via the installation of a hydrogen per divalent cation in the position M1, 2,3.

The largest variation in the composition of staurolite cause the divalent cations. In nature, all compositions between pure iron Staurolithen as well as magnesium or zinc Staurolithen occur, but no magnesium zinc Staurolithe. The predominant fraction of the divalent cations, about 80 to 90 percent, and lithium, and small amounts of aluminum and trivalent iron can be installed on the tetrahedral position T2. The charge compensation for the installation place of the divalent cations of trivalent via a reduction in the hydrogen ion concentrations.

About 5 to 10 percent of the divalent cations, with the exception of zinc is incorporated into the otherwise empty M4 octahedral. Since a simultaneous occupation of adjacent T2 and M4 positions can be excluded, two T2 positions must be blank for each occupied M4 position. The required charge compensation is done via the installation of two additional hydrogen ions per occupied M4 position.

Modifications and varieties

The previously counted also for Staurolithgruppe Lusakit is now regarded not as an independent mineral but as cobalt-containing variety of staurolite. He is from blue to black in color with cobalt blue stroke color and was named after its place of discovery Lusaka in Zambia.

Education and Locations

Iron -rich staurolite is a characteristic constituent amphibolithfazieller metamorphic pelitic rocks, mainly of mica schists. Here he meets with minerals of the mica group ( muscovite, biotite ), Garnet ( almandine ), aluminosilicates ( kyanite, sillimanite, andalusite ), quartz and chloritoid and chlorite.

In ascending metamorphosis staurolite forms from about 500 ° C from Chloritoid about various mineral reactions, for example, according to the reaction equation

At temperatures between 600 ° C and 750 ° C staurolite is degraded via various mineral reactions again, as according to the equation

The stability range of iron-rich Staurolithen is therefore limited to a narrow temperature range (500 ° C - 750 ° C ) limits. Rocks, whose metamorphosis has this temperature range is not reached or exceeded this, do not contain staurolite. This makes iron-rich staurolite to an index mineral for moderate metamorphism of pelites ( clayey sediments ).

The equilibrium positions of the staurolite -forming and staurolite -degrading reactions intersect at about 600 ° C and 15 kilobars. This means that iron-rich Staurolithe above this pressure, corresponding to a depth of about 50 kilometers, no longer occur.

The stability of staurolite strongly depends on its composition. Incorporation of magnesium instead of iron shifts the stability field of staurolite to higher pressures and temperatures, incorporation of zinc instead of ferrous Staurolithstabilität extends to higher pressures and lower temperatures.

In addition, staurolite is due to its great hardness and resistance to weathering in river sediments as a mineral soap before.

Localities lie within Europe in Styria in Austria and South Tyrol, Italy, where especially in Sterzing, next at Monte Campione in Switzerland, in Brittany, France, in the Spessart and in Scotland. In America, staurolite can be found among others in the U.S. states of Georgia, Maine, Montana, New Hampshire, New Mexico, North Carolina, Tennessee and Virginia, in Africa it occurs in Zambia and Namibia, and in Russia it can be, for example, prove on the Kola Peninsula.

Crystal structure

In almost all rock-forming silicates such as mica, pyroxenes, amphiboles, olivine divalent cations are incorporated in octahedral sites. The Staurolithstruktur is interesting because it is one of the few silicate structures, mainly occurring in the divalent cations in tetrahedral holes. This has a clearly visible consequence: Ferruginous Staurolithe are yellowish brown, while minerals with divalent iron ions are intensely colored green in octahedral coordination. Less obvious is that staurolite is an exception to one of the thumb rules of crystal chemistry, the pressure - coordination rule: It says that with increasing pressure, the number of surrounding a cation anion, called the coordination number increases. Staurolite forms in the course of ascending metamorphosis of minerals, in which the divalent cations are octahedrally coordinated, for example chloritoid. The formation of staurolite with increasing pressure thus goes hand in hand with a decrease in the cation coordination.

Atomic positions

The structure of the Staurolithe can be described to a good approximation as a cubic close packing of oxygen anions (O2 - ). The cations are sitting here in the gaps between the oxygen anions. In close packing of spheres there are two different types of such gaps, which differ ( oxygen anions in this case) in the number of adjacent balls:

• Tetrahedral holes are gaps between four oxygen anions. The oxygen atoms are located at the corners of a tetrahedron- shaped gap.
• Octahedral holes are gaps between six oxygen anions. The oxygen atoms are located at the corners of an octahedron-shaped gap.

In the case of Staurolithstruktur the cubic close packing is distorted. The octahedral sites are not all the same size and its shape deviates from an ideal octahedron. The same applies to the tert Raeder gaps. The symmetry of the Staurolithstruktur is not cubic, but rather is described by the monoclinic space group C2 / m. The monoclinic angle β varies between 90.0 ° and 90.64 °.

The various cations that make up the composition of the Staurolithe, distributed primarily according to their size on the different positions of Staurolithstruktur. The Staurolithstruktur has two different tetrahedral sites:

• The gap T1 contains all the silicon ion ( Si4 ) and generally have small amounts of aluminum ions ( Al3 ). This tetrahedral position is always fully occupied.
• The gap T2 contains the majority of all divalent cations ( Fe2 , Mg2 , Zn2 , Co2 ). This position is often not fully occupied, that is, there are empty T2 tetrahedral interstices.

In addition to the tetrahedral sites there are four different octahedral positions:

• M1A and M1B gaps contain aluminum ions ( Al3 ) and small amounts of divalent cations, especially magnesium. These positions are always fully occupied.
• Gap M2 contains aluminum ions ( Al3 ) and very small amounts of divalent cations, especially magnesium. This position is always fully occupied.
• M3A and M3B gaps contain aluminum ions ( Al3 ) and small amounts of divalent cations, especially magnesium. This position is only open half way. The distribution of cations and spaces on the M3 octahedral M3A and M3B is primarily responsible for the variation of the angle β monoclinic. In perfect order, that is, M3A is fully occupied with cations and M3B is completely discharged, β reaches its maximum value of 90.64 °. In completely uniform distribution of cations and vacancies on the M3A - M3B and octahedron β goes back to 90.0 °. In this limit, the Staurolithstruktur reached orthorhombic symmetry in space group CCMM.
• Gaps M4A M4B and contain small amounts of divalent cations and are otherwise empty.

The hydrogen ions (protons, H ) do not lie in the interstices of the packing of spheres, but on their bounding edges and surfaces. All protons in staurolite are bonded to oxygen ions, forming the tip of a T2 - tetrahedron. H three positions are known:

• H1A and H1B positions: The protons are located in the periphery of an empty M3 octahedron and form bifurcated hydrogen bonds to two other oxygens.
• Position H2: The protons lie on an edge of an empty T2 tetrahedron and form a linear hydrogen bond.
• H3A and H3B position: The protons are located in the periphery of an empty M4 octahedron and form bifurcated hydrogen bonds to two other oxygens.

Linkages of the coordination polyhedra

The fully occupied aluminum octahedra M1 and M2 are linked by common edges to form zigzag chains. These octahedra are parallel to the crystallographic c -axis. The silicon tetrahedra are isolated in the structure, that is, they are not connected by common corners, edges or faces; Staurolite, therefore, is an island silicate. Silicon tetrahedra link the aluminum octahedra in the direction of the crystallographic a- axis. They form together with the aluminum octahedra one of the two major building blocks that make up the Staurolithstruktur: A Alumosilikatschicht parallel to the ac plane. It corresponds in structure and composition of the bc plane of the Kyanitstruktur. This is the structural explanation for the observed in nature epitaxial intergrowth of staurolite and kyanite.

The second major assembly of the Staurolithstruktur is an iron- aluminum oxide hydroxide, which is also parallel to the ac plane. It builds from the M3, M4 and T2 positions as follows: The M3 octahedra are linked via common edges to form chains along the c direction, as is the M4 octahedron. Along the crystallographic a - axis of each M3 octahedra linked through common corners with two M4 octahedra. Accordingly, each M4 octahedra linked through common corners with two M3 octahedra. The T2 tetrahedra are located between the M3 and M4 octahedra. M4 each octahedra linked via two common surfaces T2 tetrahedra. Because of this Flächerverknüpfung the distances of the cation positions in M4 and T2 are so small that a simultaneous occupation of adjacent T2 and M4 positions can be excluded. All hydrogen ions ( protons) are bound to the oxygen ions through which the M3 and M4 octahedra in the a- direction. Depending on the occupation of the adjacent cation positions M3, M4 and T2 are the proton positions either empty (M3 busy) or one of the three positions is occupied.

The Staurolithstruktur can now be regarded as a removable storage of these two layers in the b direction. A T2 -M3 -M4 layer is enclosed by two Alumosilikatschichten (T1 -M1 - M2). The Alumosilikatschichten penetrate the T2 -M3 -M4- layer, so that the M2 of the two octahedron Alumosilikatschichten are connected by common edges. This rather densely packed aluminosilicate T2 -M3 -M4- aluminosilicate sandwiches are connected in the direction of the crystallographic b- axis only on the corners of the silicate tetrahedra T1 together.

Use

Staurolite rarely forms crystals of good gem quality, which are then available in different shapes cut especially for collectors. In North Carolina, the typical cross-shaped crystal twins regionally under the name Elfenstein ( fairy stone) are sold as amulets.

The variety Lusakit is mined in Africa, Zambia and used as a blue pigment.