Wollastonite

Wollastonite
Wollastonite with a view of cleavage surfaces
General and classification
other names	Tafelspat or Tafelspath
chemical formula	CaSiO 3 ; more precisely Ca 3 [Si 3 O 9 ]
Mineral class (and possibly department)	Silicates and Germanates
System no. to Strunz and to Dana	9.DG.05 ( 8th edition : VIII / F.18) 65.02.01.01a
Similar minerals	Bustamite , pectolite , sérandite , tremolite
Crystallographic Data
Crystal system	triclinic
Crystal class ; symbol	triclinic pinacoidal; 1
Space group	see crystal structure
Lattice parameters	see crystal structure
Formula units	see crystal structure
Frequent crystal faces	{001}, {540}, {100}, {101}, {102}
Twinning	often, twin axis [010] Adhesion plane (100)
Physical Properties
Mohs hardness	4.5 to 5
Density (g / cm 3 )	2.8 to 2.9
Cleavage	{100} perfect; {001} and {102} good angles in the (010) plane: (100) :( 001) = 84.5 ° (100) :( 102) = 70 °
Break ; Tenacity	uneven
colour	colorless, white; z. T. gray, yellow, red, brown
Line color	White
transparency	translucent to opaque
shine	Glass luster, mother-of-pearl luster on split surfaces
Crystal optics
Refractive indices	n α  = 1.616 to 1.640 n β  = 1.628 to 1.650 n γ  = 1.631 to 1.653
Birefringence	δ = 0.013 to 0.014
Optical character	biaxial negative
Axis angle	2V = 36 to 60 °
Pleochroism	not known
Other properties
Chemical behavior	insoluble in water, readily soluble in hydrochloric acid
Special features	sometimes fluorescent

Wollastonite (rarely also Tafelspat or Tafelspath ) is a frequently occurring mineral with the chemical composition CaSiO ₃ , more precisely Ca ₃ [Si ₃ O ₉ ]. Chemically, these are a naturally occurring calcium silicate and the calcium - salt of meta-silicic acid . Wollastonite therefore belongs to the mineral class of the silicates and Germanates . Its crystal structure consists of (SiO ₃ ) ²⁻ chains, which are linked to one another via the calcium cations . As a single-chain silicate , wollastonite is part of the inosilicates , but does not belong to the mineral group of pyroxenes , which is often incorrectly used as a synonym for single-chain silicates, but to the pyroxenoids (pyroxene-like), as the (SiO ₃ ) ²⁻ chains follow a different connection pattern in its crystal structure. Wollastonite is colorless and crystallizes in the triclinic crystal system . It is formed by contact metamorphism from limestone and is a rock-forming component of the metamorphic rock skarn .

Etymology and history

William Hyde Wollaston

The name wollastonite goes back to J. Léman, who first mentioned the name in 1818 in the Nouveau dictionnaire d'histoire naturelle appliquée aux arts, à l'agriculture when describing rocks, more precisely a skarn from Dognecea in the Romanian part of the Banat . The mineral was first described in 1793 by the Austrian mineralogist A. Stütz in Neueeinrichtung der k.-k. Natural history collection in Vienna , which named the mineral Tafelspath . For this, Stütz had handpieces from rock samples from the Banat available, but whether these also came from Dognecea is not certain. Dognecea is still considered a type locality of table spar or wollastonite . The renaming of table spar to wollastonite by Léman was intended in recognition of the scientific merits of the famous English naturalist William Hyde Wollaston (1766–1828).

Since the International Mineralogical Association (IMA) was founded in 1958, wollastonite has been the internationally recognized mineral name for the naturally occurring CaSiO ₃ .

classification

Already in the now outdated, but still in use 8th edition of the mineral classification according to Strunz , wollastonite, more precisely its two modifications wollastonite-1A and wollastonite-2M , belonged to the mineral class of "silicates and germanates" and there to the department of "chain silicates and band silicates ( inosilicates) "where he together with bustamite , Cascandit , Denisovite , Ferrobustamit , foshagite , jennite , pectolite , Serandit and Vistepit the independent group / F.18 VIII 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 wollastonite-1A and wollastonite-2M to the class of “silicates and germanates” and there to the “chain and Band silicates (inosilicates) ”. This department is, however, further subdivided according to the type of chain formation, so that the wollastonites can be found according to their structure in the subsection "Chain and band silicates with 3-periodic single and multiple chains", where they give the name to the "wollastonite group" with the system -No. 9.DG.05 and the other members form bustamite, cascandite, ferrobustamite, pectolite, sérandite and tanohataite .

The systematics of minerals according to Dana , which is mainly used in the English-speaking world , assigns wollastonite-1A , wollastonite-2M and wollastonite-3A-4A-5A-7A to the class of "silicates and germanates" and there in the "chain silicate minerals" category. Here they are also in the "wollastonite group" named after them with the system no. 65.02.01 as well as the other members bustamite, ferrobustamite, pectolite, sérandite, cascandite, denisovite and tanohataite can be found in the subsection " Chain silicates: simple unbranched chains, W = 1 with chains P = 3 ".

Crystal structure

View of the unit cell of wollastonite along the b-axis

In nature, wollastonite usually occurs as wollastonite-1T. Wollastonite-1T crystallizes in the triclinic crystal system (class triclinic-pinacoidal; 1 ) in the space group P 1 (space group no. 2) with six formula units in the unit cell (Z = 6). The only symmetry element in the crystal structure is an inversion center, which multiplies the atoms by point reflection. The inversion centers are located in the corners, on the surface centers and in the center of the unit cell. The crystal structure contains three crystallographically distinguishable calcium and silicon atoms as well as nine different oxygen atoms. Crystallographically distinguishable means that these atoms cannot be converted into one another by the existing symmetry elements (in this case the inversion center). The inversion center doubles the three calcium and silicon atoms as well as the nine oxygen atoms, so that the six formula units described above (6 × CaSiO ₃ = “Ca ₆ Si ₆ O ₁₈ ”) are in the unit cell.

The crystallographic data of wollastonite-1T compared to the other two modifications are given in the table.

Crystallographic data for wollastonite
Surname	Wollastonite-1A (ex.1T) wollastonite		Wollastonite-2M Parawollastonite		Wollastonite-4A pseudowollastonite
Crystal system	triclinic		monoclinic		monoclinic
Crystal class	triclinic pinacoidal; 1		monoclinic prismatic; 2 / m		monoclinic prismatic; 2 / m
Space group	P 1 (No. 2)		P 2 1 / a (No. 14, position 3)		C 2 / c (No. 15)
Lattice constants of the unit cell	a = 794  pm b = 732 pm c = 707 pm	α = 90.03 ° β = 95.37 ° γ = 103.43 °	a = 1543 pm b = 732 pm c = 707 pm	α = 90 ° β = 95.40 ° γ = 90 °	a = 684 pm b = 1187 pm c = 1963 pm	α = 90 ° β = 90.67 ° γ = 90 °
Number of formula units in the unit cell	6th		12		8th

Coordination environment of the Ca ²⁺ and Si ⁴⁺ cations

Coordination environment of Si and Ca

As in almost all silicates , the silicon is surrounded by four oxygen atoms in the form of a tetrahedron . However, these SiO ₄ tetrahedra are not isolated in the crystal structure , but are linked to form chains (see next section). The oxygen-silicon distances are between 157 and 166  pm , which corresponds to the usual distances in silicates.
The calcium atoms are each surrounded by six oxygen atoms in the form of distorted octahedra , the calcium-oxygen distances are between 227 and 255 pm.

Linkage pattern of the silicate chains

Linkage pattern of the silicate chains in wollastonite compared to the pyroxenes

Linkage pattern of the silicate and [CaO ₆ ] octahedral chains in the direction of the a- and c-axes

Although wollastonite to the single Kettensilicaten belongs (inosilicates), the linkage pattern of the SiO different ₄ - tetrahedra within the Silicatkette from that of the much more common pyroxene . The difference becomes clear when comparing wollastonite with pyroxene enstatite (MgSiO ₃ ).
The SiO ₄ tetrahedra are linked in all chain silicates via common tetrahedron corners, i.e. via common oxygen atoms. In order for a chain to be created, each silicon must share two of the oxygen atoms in its tetrahedron with the neighboring silicon atoms, so these oxygen atoms only “belong” to it half. This results in a silicon-oxygen ratio of 1: 3 in the chain, which is also reflected in the chemical formula of the chain silicates (wollastonite: CaSiO ₃ , enstatite: MgSiO ₃ ). These chains are practically infinite, they are only limited by the size of the crystal . In crystal chemistry they are therefore described as follows:

${\displaystyle \mathrm {(SiO_{3}^{2-})_{\infty }} }$or more detailed as Niggli's formula : ${\displaystyle {}_{\infty }^{1}\mathrm {\lbrace [SiO_{2/2}O_{2/1}]^{2-}} \rbrace }$

The chains can now be further distinguished from one another by the orientation of the SiO ₄ tetrahedra. While in enstatite and all other pyroxenes the same motif is repeated after two tetrahedra, the chain pattern in wollastonite is given by three tetrahedra. To put it simply, in enstatite the tetrahedra with a point point alternately "up" and "down", while in wollastonite one tetrahedron with the point points "down", the next two, however, "up". In the case of pyroxenes, one speaks of a “two-single chain”, while wollastonite has a “three-single chain”. Since the underlying motif of the chains in wollastonite consists of three tetrahedra, the chemical formula is often given tripled, Ca ₃ [Si ₃ O ₉ ]. The infinite (SiO ₃ ) ²⁻ chains run in the crystal structure of wollastonite in the direction [010], that is, in the direction of the crystallographic b-axis. The chain motif of three tetrahedra repeats after 732 pm, which corresponds exactly to the lattice constant of the unit cell in the direction of the b-axis. The more complicated arrangement of the tetrahedra in wollastonite is due to the increased space requirement of the Ca ²⁺ cations (Ca ²⁺ is larger than the Mg ²⁺ and Fe ²⁺ cations often contained in pyroxenes ) in the crystal structure.

Forest

The [CaO ₆ ] octahedra for their part also form chains in the direction of the b-axis via common edges. The octahedron chains are linked via shared oxygen atoms with the above-described silicate chains in the direction of the a- and c-axes, creating a three-dimensional structure.

Influence of the structure on the macroscopic properties

Some macroscopic properties of wollastonite can be explained on the basis of the crystal structure. Single crystals of wollastonite have a needle-like to fibrous shape ( habit ), since the crystals prefer to grow in the direction of the crystallographic b-axis, which corresponds to the orientation of the silicate chains in the crystal structure. If you break a wollastonite needle in the middle, i.e. you break the silicate chains, the result is uneven fracture surfaces , while under mechanical stress flat cleavage surfaces parallel to the b-axis ({100} perfect, {001} and {102} good cleavage) arise. This can also be explained by the chemical bonding in the crystal. While silicon and oxygen are connected to one another via covalent bonds (atomic bonds), there is an ionic bond between calcium and oxygen , which is based on a purely electrostatic interaction and thus represents the weaker bond.

Modifications

Wollastonite exists in several modifications with the same chemical formula but different crystal structures , only two of which occur in nature. Since all modifications are chemically identical, they are also known as polymorphs .

Usually the name wollastonite without addition describes the far the most common form, the triclinic crystallizing wollastonite-1T (1 stands for "first mold", T for t Riklin) which occasionally also as wollastonite 1A or α-CaSiO ₃ denotes becomes. The term Wollastonite-Tc (Tc = T ri c linic) can also be found in English literature .

The second is the natural form monoclinic wollastonite 2M (2 = "second mold", M = m onoklin), which significantly less than the triclinic wollastonite 1T occurs. Synonyms for wollastonite-2M are parawollastonite and, actually inconsistently, also α-CaSiO ₃ . The term α-CaSiO ₃ is used for wollastonite-1T and wollastonite-2M, as both are considered to be low temperature modifications. However, wollastonite-2M does not usually occur together with wollastonite-1T, but is contained in metamorphic rocks that formed at very low pressure during metamorphosis .

The high-temperature modification is called pseudowollastonite (sometimes also wollastonite-4A) or β-CaSiO ₃ and is only stable at temperatures above 1120 ° C. Pseudowollastonite also crystallizes monoclinically, but has a pseudo- orthorhombic structure due to the β angle of its unit cell , which is very close to 90 ° . While wollastonite-1T and wollastonite-2M belong to the single chain silicates (inosilicates), the silicates in pseudowollastonite form ring-shaped structures. Pseudowollastonite thus belongs to the group of ring silicates (cyclosilicates). The arrangement of the SiO ₄ - tetrahedron is rather with the structure of Benitoit (BaTi [Si ₃ O ₉ comparable]).

Further modifications were obtained from wollastonite-1T in high-pressure experiments; they all crystallize triclinic, which in turn results in changes in the crystal structures. The high pressure modifications include wollastonite-3T, wollastonite-4T, wollastonite-5T, and wollastonite-7T.

Education and Locations

Needle wollastonite (white) from Libušin , Central Bohemia , Czech Republic (field of view 8 mm)

Wollastonite often occurs in metamorphic rocks that are formed from rocks containing carbonate and is a rock-forming component of skarn . It typically arises during contact metamorphosis through the contact of limestone with silica-containing magma . At temperatures of more than 600 ° C, the so-called wollastonite reaction occurs :

Calcite + quartz wollastonite + carbon dioxide ${\displaystyle \rightleftarrows }$

${\displaystyle \mathrm {CaCO_{3}+SiO_{2}\;\rightleftarrows \;CaSiO_{3}+CO_{2}\uparrow } }$

Since the reaction CO ₂ escapes as gas, have to shift the balance of the Le Chatelier's principle following the side of the products , that is, the reaction is complete and is in nature not reversible practical. The wollastonite reaction is therefore a classic example of metasomatosis . During a normal metamorphosis , the structure and mostly also the mineral content of the rock changes, but the minerals created by the metamorphosis have largely the same chemical composition as the minerals originally present. In metasomatosis, as here in the case of the wollastonite reaction , the chemistry of the rock also changes.

Wollastonite can have traces or even larger amounts of iron and manganese in the form of divalent cations in the places of the Ca ²⁺ cations in the crystal lattice. High proportions of iron and manganese are particularly noticeable when looking at the mineral in thin sections of the rock on the polarizing microscope through higher refractive indices . Magnesium (Mg ²⁺ ), aluminum (Al ³⁺ ) or sodium (Na ⁺ ) and potassium cations (K ⁺ ) are found less frequently in the calcium positions. With iron contents of more than 10% (Ca _0.9 Fe _0.1 SiO ₃ ) or manganese contents of more than 25% (Ca _0.75 Mn _0.25 SiO ₃ ) wollastonite crystallizes in the structure of bustamite ((Mn, Ca , Fe) [SiO ₃ ]). The structurally related minerals of wollastonite include bustamite, pectolite (NaCa ₂ [Si ₃ O ₈ (OH)]) and sérandite (Na (Mg, Ca) ₂ [Si ₃ O ₈ (OH)]).

Accompanying minerals ( parageneses ) of wollastonite are typically diopside , various garnets (especially grossular and andradite), tremolite , Vesuvian (idokras), microcline and calcite .

Wollastonite occurs in numerous locations around the world and is also mined for industrial use. The following information on production in 2016 is based on the World Mineral Report of the British Geological Survey 2012-2016:

• China , 950,000 t
• India , 166,168 t
• Mexico , 63,683 tons
• USA , 60,000 t
• Spain , 13,553 t
• Finland , 10,000 t
• Canada , 6,000 t
• Australia , 1,797 t

Locations without commercial mining:

• Saxony (Germany)
• Banat (Romania)
• Vesuvius and Monte Somma (Italy)
• Franklin (New Jersey, USA)

The wollastonites from Franklin (New Jersey) are often characterized by a blue to white fluorescence when exposed to UV light . Like the fluorescence in the mineral fluorite , this is caused by very small amounts of europium cations (Eu ²⁺ ) on the calcium positions in the crystal lattice.

use

Due to its fibrous to needle-like crystals and its high melting point (1540 ° C), wollastonite offers a wide range of technical uses. Its production takes place via the reaction of calcium oxide (CaO, quicklime) with silicon dioxide (SiO ₂ , quartz or silica gel ):

${\displaystyle \mathrm {CaO+SiO_{2}\longrightarrow CaSiO_{3}} }$

One of the main areas of application for wollastonite is in the ceramic industry , where it is used to improve the mechanical properties of white ceramic goods.

Due to its high melting point, wollastonite is used as a substitute for asbestos fibers . It is typically used in welding electrodes , insulation materials (see calcium silicate board ) and fire-resistant protective clothing . While asbestos fibers are among the carcinogenic substances, wollastonite fibers do not pose a health risk, as they dissolve in the body within a few days to a few weeks.

In the plastics industry , wollastonite is mainly used as a filler in thermoplastics . Among other things, it serves to improve the rigidity and flexural strength of polyesters , polyamides and polypropylenes . It is also used with reactive resins such as epoxy resins in order to avoid stress cracks caused by shrinkage.

There are numerous trade names for wollastonite, including Kemolit , Hycon and Tremin .

See also

• Systematics of the minerals
• List of minerals

literature

• NL Bowen, JF Schairer, E. Posnjak: The system CaO-FeO-SiO ₂ . In: American Journal of Science . Series 5, No. 26 , 1933, pp. 193-283 (English).
• MJ Buerger, CT Prewitt: The crystal structures of wollastonite and pectolite . In: Proceedings of the National Academy of Sciences, USA Volume 47 , 1961, pp. 1884-1888 (English).
• WA Deer, RA Howie, J. Zussman: An Introduction to the Rock Forming Minerals . Prentice Hall, Harlow 1992, ISBN 0-582-30094-0 (English).
• RI Harker, OF Tuttle: Experimental data on the P (CO ₂ ) -T curve for the reaction: calcite + quartz ↔ wollastonite + carbon dioxide . In: American Journal of Science . tape 254 , 1956, pp. 239-256 (English).
• U. Müller: Inorganic Structural Chemistry . Teubner, Stuttgart 2004, ISBN 3-519-33512-3 .
• Y. Ohashi: Polysynthetically-twinned structures of enstatite and wollastonite . In: Physics and Chemistry of Minerals . tape 10 , 1984, pp. 217-229 (English).
• Martin Okrusch, Siegfried Matthes: Mineralogy. An introduction to special mineralogy, petrology and geology . 7th, completely revised and updated edition. Springer, Berlin et al. 2005, ISBN 3-540-23812-3 .
• H.-X. Yang, CT Prewitt: On the crystal structure of pseudowollastonite (CaSiO ₃ ) . In: American Mineralogists . tape 84 , 1999, pp. 929-932 (English).

Web links

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

• Extensive data on wollastonite on webmineral.com (English)
• Mineral atlas: wollastonite (general) and polytypes mineral atlas: wollastonite-1A , mineral atlas: wollastonite-2M , mineral atlas: wollastonite-3A , mineral atlas: wollastonite-4A , mineral atlas: wollastonite-5A , mineral atlas: wollastonite-7A - data and crystals
• Pictures of wollastonite in thin section from the GeoMuseum of the TU Clausthal
• Overview of the chain silicate structures (University of Freiburg)

Individual evidence

1. ↑ World Mineral Production 2012–2016. (PDF; 1.9 MB) BGS , February 2018, p. 74 , accessed on May 23, 2018 (English). 

This version was added to the list of articles worth reading on August 20, 2006 .