Staurolite

Staurolite
	Cross-shaped staurolite twin in mica slate
General and classification
other names	Cross stone
chemical formula	(Fe 2+ ) 2 Al 9 Si 4 O 23 (OH)
Mineral class; (and possibly department)	Island silicates with non-tetrahedral anions (Neso subsilicates)
System no. to Strunz ; and to Dana	9.AF.30 ( 8th edition : VIII / B.03) ; 02.02.03.01
Crystallographic Data
Crystal system	monoclinic
Crystal class ; symbol	monoclinic prismatic 2 / m
Room group (no.)	C / 2 m (No. 12)
Frequent crystal faces	{110}, {101}, {010}, {001}
Twinning	often cross-shaped penetration twins (90 ° at right angles and 60 ° at an oblique angle)
Physical Properties
Mohs hardness	7 to 7.5
Density (g / cm 3 )	3.65 to 3.83
Cleavage	imperfect
Break ; Tenacity	shell-like, uneven, brittle
colour	red-brown to brown-black
Line color	White
transparency	translucent to opaque
shine	Glass, fat gloss matt
Crystal optics
Refractive indices	n α = 1.736 to 1.747 ; n β = 1.740 to 1.754 ; n γ = 1.745 to 1.762
Birefringence	δ = 0.009 to 0.015
Optical character	biaxial positive
Pleochroism	weak: colorless / light yellow / yellow-red - colorless / light yellow / yellow-red - light yellow / yellow-orange / pink-red
Other properties
Special features	typical cruciform crystal twins

The mineral staurolite ( cross stone ) is a frequently occurring island silicate with the general chemical composition M ²⁺₄ Al ₁₈ Si ₈ O ₄₆ (OH) ₂ . In this simplified structural formula, M ²⁺ stands for divalent cations , predominantly iron (Fe ²⁺ ), magnesium (Mg ²⁺ ) and zinc (Zn ²⁺ ) in any mixing ratio . According to the content of these cations, four minerals are distinguished in the staurolite group:

Staurolite (iron staurolite): (Fe ²⁺ ) ₂ Al ₉ Si ₄ O ₂₃ (OH)
Magnesiostaurolite : Mg (Mg, Li) ₃ (Al, Mg) ₁₈ Si ₈ O ₄₄ (OH) ₄
Zinc staurolite : Zn ₂ Al ₉ Si ₄ O ₂₃ (OH)

Staurolites 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 brown-black color.

Etymology and history

The name of the mineral is derived from the Greek and means cross stone (σταυρóς "cross", λíθος "stone"), thus alluding to the often found cross-shaped twinning. For this reason, larger crystals were often worn by Christians as jewelry or amulet. In the Swiss Alps in particular , they were widespread under the name Basler Taufstein .

classification

In the meanwhile outdated, but still in use 8th edition of the mineral classification according to Strunz , the staurolite belonged to the mineral class of "silicates and germanates" and there to the department of " island silicates with non-tetrahedral anions (Neso-subsilicates)", where together with gerstmannite , magnesiostaurolite and zinc staurolite formed the unnamed group VIII / B.03 .

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 staurolite to the class “Silicates and Germanates” and there in the department of “Island silicates (Nesosilicates)”. However, this section is further subdivided according to the possible presence of further anions and the coordination of the cations involved, so that the mineral is classified according to its composition in the sub-section of “Island silicates with additional anions; Cations in [4] er, [5] er and / or only [6] er coordination ”can be found, where it also forms the unnamed group 9.AF.30 together with magnesiostaurolite and zinc staurolite .

The systematics of minerals according to Dana , which is mainly used in the English-speaking world , assigns staurolite to the class of "silicates and germanates", but there in the department of "island silicates: SiO ₄ groups and O, OH, F and H ₂ O" . Here it can be found together with magnesiostaurolite and zinc staurolite in the unnamed group 52.02.03 within the subdivision of " Island silicates: SiO ₄ groups and O, OH, F and H2O with cations in [4] and> [4] coordination " .

Chemism

The composition of staurolite is important because conclusions can be drawn from the occurrence of staurolite about the formation conditions of the stone containing staurolite. This is done with the aim of determining the pressure and temperature history of a rock and from this to reconstruct the movement of entire rock formations in the earth's crust.

Mineral reactions must be calculated to determine such pressure and temperature data. For this purpose, information is required on the one hand about the composition of all minerals involved and on the other hand detailed knowledge of the intracrystalline distribution of the elements in the various positions of the mineral structure.

Element content

The chemical formula given at the beginning gives a simplified composition of staurolite. The complexity of the crystal chemistry of staurolite only becomes apparent when the contents of elements that are difficult to analyze, such as lithium and hydrogen, and the distribution of the elements in the various cation positions are taken into account. In mineralogy, structural formulas for the notation of mineral compositions have become established because they also contain 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) ₈ O ₄₀ (O, OH) ₈ .

In this formula, the element contents of positions T2 and M4 are summarized in the first brackets (Fe, ...) _2-4 . The second bracket contains the elements of the aluminum octahedron M1,2,3 and the third bracket the elements of the silicon tetrahedron T1. O ₄₀ are the oxygen ions of the oxygen positions O2,3,4,5, while (O, OH) _{8 represents} the composition of the oxygen position O1. The latter is the oxygen ion, via which the octahedra M3 and M4 are linked and to which the hydrogen ions are bound (OH groups).

A review of almost 550 published compositions of natural staurolites provides the following picture of the element concentrations:

Si ⁴⁺ : 7 to 8 apfu (atoms per formula unit), on average: 7.72 apfu
Al ³⁺ : 16.1 to 19.5 apfu, mean: 17.8 apfu
Ti ⁴⁺ : 0 to 0.35 apfu, mean: 0.1 apfu
Cr ³⁺ : 0 to 1.4 apfu, mean: 0 apfu
Fe ³⁺ : 0 to 0.36 apfu, often not determined
Fe ²⁺ : 0.15 to 3.9 apfu, mean: 2.7 apfu
Mg ²⁺ : 0 to 3 apfu, mean: 0.7 apfu
Zn ²⁺ : 0 to 2.8 apfu, mean: 0.4 apfu
Co ²⁺ : 0 to 2.1 apfu, mean: 0 apfu
Mn ²⁺ : 0 to 0.45 apfu, mean: 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 Si ⁴⁺ ions are in the T1 position. If there are fewer than 8 silicon ions per formula unit, the remaining T1 spaces are filled with aluminum ions. The charge balance takes place via the incorporation of one hydrogen ion per aluminum ion on T1.

Almost all trivalent cations as well as Ti ⁴⁺ and approximately 10 percent of all divalent cations are incorporated in the octahedron positions M1,2,3. An exception is Zn ²⁺ , which is only incorporated in the tetrahedral position T2. The charge balance for the incorporation of a divalent cation instead of a trivalent one takes place via the incorporation of a hydrogen ion per divalent cation at position M1,2,3.

The greatest variation in the composition of staurolite is caused by the divalent cations. In nature, all compositions between pure iron staurolites and magnesium or zinc staurolites occur, but no magnesium-zinc staurolites. The majority of the divalent cations, around 80 to 90 percent, as well as lithium and small amounts of aluminum and trivalent iron are installed in the tetrahedron position T2. The charge balance for the incorporation of trivalent instead of divalent cations takes place by reducing the hydrogen ion content.

Approximately 5 to 10 percent of the divalent cations, with the exception of zinc, are incorporated in the otherwise empty M4 octahedron position. Since a simultaneous occupation of neighboring T2 and M4 positions can be ruled out, two T2 positions must be empty for each occupied M4 position. The required charge equalization takes place via the incorporation of two additional hydrogen ions per occupied M4 position.

Crystal structure

In almost all rock-forming silicates such as mica , pyroxenes , amphiboles and olivines , divalent cations are incorporated into octahedral gaps . The staurolite structure is interesting because it is one of the few silicate structures in which divalent cations occur predominantly in tetrahedral gaps . This has a clearly visible consequence: iron-containing staurolites are yellowish brown, while minerals with divalent iron ions in octahedral coordination are colored intensely green. What is less obvious is that staurolite is an exception to one of the rules of thumb in crystal chemistry, the pressure coordination rule: It states that with increasing pressure, the number of anions surrounding a cation, the so-called coordination number , increases. Staurolite is formed in the course of ascending metamorphosis from minerals in which the divalent cations are octahedrally coordinated, for example chloritoid . The formation of staurolite with increasing pressure is therefore associated with a decrease in cation coordination.

Atomic positions

The structure of the staurolites can be described to a good approximation as a cubic closest packing of oxygen anions (O ²⁻ ). The cations sit in the gaps between the oxygen anions. In close packing of spheres, there are two different types of such gaps, which differ in the number of adjacent spheres (oxygen anions in this case):

Tetrahedral gaps are gaps between four oxygen anions. The oxygen atoms are at the corners of a tetrahedral gap.
Octahedral gaps are gaps between six oxygen anions. The oxygen atoms are at the corners of an octahedral gap.

In the case of the staurolite structure, the cubic closest packing of spheres is distorted. The octahedron gaps are not all the same size and their shape deviates from an ideal octahedron shape. The same applies to the tertiary gaps. The symmetry of the staurolite structure is therefore not cubic , but monoclinic and is described by the 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 staurolites are distributed primarily according to their size over the various positions of the staurolite structure. The staurolite structure has two different tetrahedral gaps:

The gap T1 contains all silicon ions ( Si ⁴⁺ ) and mostly small amounts of aluminum ions (Al ³⁺ ). This tetrahedral position is always completely occupied.

The gap T2 contains the majority of all divalent cations ( Fe ²⁺ , Mg ²⁺ , Zn ²⁺ , Co ²⁺ ). This position is often not fully occupied, that is, there are empty T2 tetrahedral holes.

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

Gaps M1A and M1B contain aluminum ions (Al ³⁺ ) and small amounts of divalent cations, especially magnesium. These positions are always fully occupied.
Gap M2 contains aluminum ions (Al ³⁺ ) and very small amounts of divalent cations, especially magnesium. This position is always fully occupied.
Gaps M3A and M3B contain aluminum ions (Al ³⁺ ) and small amounts of divalent cations, especially magnesium. This position is only half filled. The distribution of cations and vacancies on the M3 octahedral positions M3A and M3B is mainly responsible for the variation of the monoclinic angle β. With complete order, i.e. H. M3A is completely occupied with cations and M3B is completely empty when β reaches its maximum value of 90.64 °. With a completely even distribution of cations and vacancies on the M3A and M3B octahedra, β goes back to 90.0 °. In this borderline case, the staurolite structure achieves orthorhombic symmetry in the space group Ccmm.
Gaps M4A and M4B contain small amounts of divalent cations and are otherwise empty.

The hydrogen ions ( protons H ⁺ ) do not lie in the gaps in the packing of the spheres, but on their bounding edges and surfaces. All protons in staurolite are bound to oxygen ions that form the tip of a T2 tetrahedron. Three H positions are known:

Positions H1A and H1B: The protons lie in the boundary surface of an empty M3 octahedron and form bifurcated hydrogen bonds to two other oxygenates.
Position H2: The protons lie on one edge of an empty T2 tetrahedron and form a linear hydrogen bond .
Position H3A and H3B: The protons lie in the boundary surface of an empty M4 octahedron and form bifurcated hydrogen bonds to two other oxygen.

Connections of the coordination polyhedra

Detail of the staurolite structure: T1-M1-M2 level M1A, B-octahedron: blue M2-octahedron: violet T2-tetrahedron: gray

The fully occupied aluminum octahedra M1 and M2 are linked to one another via common edges to form zigzag chains. These chains of octahedra run parallel to the crystallographic c-axis. The structure of the silicon tetrahedra is isolated, which means that they are not connected to one another via common corners, edges or surfaces; Staurolite is therefore an island silicate . The silicon tetrahedra link the aluminum octahedron chains in the direction of the crystallographic a-axis. Together with the aluminum octahedron chains, they form one of the two large structural units that make up the staurolite structure: An aluminosilicate layer parallel to the ac plane. In structure and composition it corresponds to the bc level of the kyanite structure. This is the structural explanation for the epitaxial intergrowth of staurolite and kyanite observed in nature .

Detail of the staurolite structure: T2-M3-M4 level M3 octahedron: turquoise M4 octahedron: green T2 tetrahedron: orange

The second large structural unit of the staurolite structure is an iron-aluminum-oxide-hydroxide layer, which is also parallel to the ac plane. It is built up from the M3, M4 and T2 positions as follows: The M3 octahedra are linked by common edges to form chains in the c direction, as are the M4 octahedra. Along the crystallographic a-axis, each M3 octahedron is linked to two M4 octahedra via common corners. Accordingly, each M4 octahedron is linked to two M3 octahedra via common vertices. The T2 tetrahedra lie between the M3 and M4 octahedra. Each M4 octahedron is linked to two T2 tetrahedra via common areas. Because of this surface linkage, the distances between the cation positions in M4 and T2 are so small that a simultaneous occupation of neighboring T2 and M4 positions can be excluded. All hydrogen ions (protons) are bound to the oxygen ions via which the M3 and M4 octahedra are linked in the a direction. Depending on the occupation of the adjacent cation positions M3, M4 and T2, the proton positions are either empty (M3 occupied) or one of the three positions is occupied.

Staurolite structure: alternation of T1-M1-M2-layer and T2-M3-M4-layer in b-direction

The staurolite structure can now be understood as an alternation of these two layers in the b direction. A T2-M3-M4 layer is enclosed by two aluminosilicate layers (T1-M1-M2). The aluminosilicate layers penetrate the T2-M3-M4 layer so that the M2 octahedra of the two aluminosilicate layers are connected to one another via common edges. These quite tightly packed aluminosilicate-T2-M3-M4-aluminosilicate sandwiches are only connected to one another in the direction of the crystallographic b-axis via the corners of the silicate tetrahedron T1.

properties

Staurolite is only imperfectly cleavable, breaks unevenly like a shell and in its pure form shows a glass or fat sheen. The macroscopically visible crystals that are frequently encountered have a columnar appearance ( habitus ). They are often larger than the crystals of surrounding minerals and are then referred to as porphyroblasts . A morphological peculiarity of the staurolite is that it often occurs in a characteristic cross shape as a crystal twin; the angle between the crystals is either 90 or about 60 degrees.

Modifications and varieties

Lusakite , which used to be part of the staurolite group, is no longer considered an independent mineral, but a cobalt-containing variety of staurolite. It is from blue to black in color with cobalt blue streak color and was named after its place of discovery in Lusaka in Zambia.

Education and Locations

Kyanite (light blue) and staurolite (dark red) from Pizzo del Platteo near the Bernina Pass , Canton of Graubünden, Switzerland ( total size of the step: 7.7 × 4.1 × 2.2 cm)

Cross and heart-shaped twinning of staurolite in muscovite from the Russian Kola peninsula (size: 5.7 × 5.3 × 2.1)

Iron-rich staurolite is a characteristic component of amphibolite facial metamorphic pelites , predominantly of mica schists . Here it occurs together with minerals of the mica group ( muscovite , biotite ), garnet group ( almandine ), aluminosilicates ( kyanite , sillimanite , andalusite ), quartz, as well as chloritoid and chlorite groups .

With increasing metamorphosis, staurolite is formed from chloritoid from around 500 ° C via various mineral reactions, for example according to the reaction equation

Chloritoid + aluminosilicate = staurolite + chlorite + water

At temperatures between 600 ° C and 750 ° C, staurolite is broken down again via various mineral reactions, for example according to the equation

Staurolite + muscovite + quartz = garnet + biotite + aluminosilicate + water

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

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

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

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

Findings are within Europe in Styria in Austria and in the Italian South Tyrol , there in particular near Sterzing , next to Monte Campione in Switzerland , in Brittany in France , in the Spessart and in Scotland. In America, staurolite can be found in the US states of Georgia , Maine , Montana , New Hampshire , New Mexico , North Carolina , Tennessee and Virginia , in Africa it is found in Zambia and Namibia , and in Russia, for example in the Keivy Mountains on the Kola Peninsula.

use

Staurolite in various gemstone cuts

Staurolith rarely forms crystals in good gemstone -quality, which then in various cut shapes are offered mainly to collectors. In North Carolina, the typical cross-shaped crystal twins are regionally sold as amulets under the name Elfenstein (fairy stone) .

The Lusakite variety is mined in African Zambia and used as a blue pigment .

literature

MA Marzouki, B. Souvignier, M. Nespolo: The staurolite enigma solved . In: Acta Crystallographica . A70, no. 4 , 2014, p. 348-353 , doi : 10.1107 / S2053273314007335 .
FC Hawthorne, L. Ungaretti, R. Oberti, F. Caucia, A. Callegari: The crystal-chemistry of staurolites I: Crystal structure and site populations. In: Can. Mineral. 31, 1993, pp. 551-582. (PDF)
FC Hawthorne, L. Ungaretti, R. Oberti, F. Caucia, A. Callegari: The crystal-chemistry of staurolites II: Order-disorder and the monoclinic orthorhombic phase transition. In: Can. Mineral. 31, 1993, pp. 583-596. (PDF)
FC Hawthorne, L. Ungaretti, R. Oberti, F. Caucia, A. Callegari: The crystal chemistry of staurolites III: Local order and chemical composition. In: Can. Mineral. 31, 1993, pp. 597-616. (PDF)
JDH Donnay, G. Donnay: The staurolite story . In: Tschermaks mineralogical and petrographic communications . tape 31 , no. 1-2 , 1983, pp. 1-15 , doi : 10.1007 / BF01084757 .
Martin Okrusch, Siegfried Matthes: Mineralogy. An introduction to special mineralogy, petrology and geology . 7th, completely revised and updated edition. Springer Verlag, Berlin et al. 2005, ISBN 3-540-23812-3 , pp. 86 .
Petr Korbel, Milan Novák: Mineral Encyclopedia (= Villager Nature ). Nebel Verlag, Eggolsheim 2002, ISBN 3-89555-076-0 , p. 204 .
Walter Schumann: Precious stones and gemstones. All species and varieties in the world. 1600 unique pieces . 13th revised and expanded edition. BLV Verlag, Munich et al. 2002, ISBN 3-405-16332-3 , p. 228 .

Web links

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

Individual evidence

↑ ^a ^b ^c ^d IMA / CNMNC List of Mineral Names (English, PDF 1.8 MB)
↑ Hans Jürgen Rösler : Textbook of Mineralogy . 4th, revised and expanded edition. German publishing house for basic industry, Leipzig 1979, ISBN 3-342-00288-3 , p. 490 .
^ Helmut Schrätze, Karl-Ludwig Weiner: Mineralogie. A textbook on a systematic basis . de Gruyter, Berlin / New York 1981, ISBN 3-11-006823-0 , p. 700 .

[IMAList-1] IMA / CNMNC List of Mineral Names (English, PDF 1.8 MB)

[2] Hans Jürgen Rösler : Textbook of Mineralogy . 4th, revised and expanded edition. German publishing house for basic industry, Leipzig 1979, ISBN 3-342-00288-3 , p. 490 .

[3] Helmut Schrätze, Karl-Ludwig Weiner: Mineralogie. A textbook on a systematic basis . de Gruyter, Berlin / New York 1981, ISBN 3-11-006823-0 , p. 700 .