Wollastonite

• Tafelspat or panel Spath

Wollastonite (rarely also Tafelspat or panel Spath ) is a commonly occurring mineral with the chemical composition CaSiO3, more Ca3 [ Si3O9 ]. Chemically, it is a naturally occurring calcium and the calcium salt of meta-silicic acid. Wollastonite therefore belongs to the mineral class of silicates and Germanates. Its crystal structure is comprised of ( SiO 3 ) 2 - chain, which are linked by the calcium cations with each other. As a single Kettensilicat wollastonite is part of inosilicates, but does not belong to this group of minerals of the pyroxene, which is often mistakenly used as a synonym for single - Kettensilicate, but to the Pyroxenoiden ( Pyroxenähnliche ), since the ( SiO 3 ) 2 - chains in its crystal structure follow a different linkage pattern. Wollastonite is colorless and crystallizes in the triclinic crystal system. It is formed by contact metamorphism of limestone and is a rock-forming component of the metamorphic rock skarn.

Etymology and history

The name wollastonite goes back to J. Leman, the more accurately the name of a skarn from Dognecea in the Romanian part of the Banat was first mentioned in 1818 in Nouveau dictionnaire d' histoire naturelle appliquée aux arts, à l'agriculture in the description of rocks. The first description of the mineral was however in 1793 by the Austrian mineralogist A. support in setting up the New K.-K. Natural history collection at Vienna, who named the mineral board Spath. Support had this hand pieces of rock samples from the Banat available whether this is, however, also came from Dognecea not secured. Dognecea is still valid today as the type locality of the panel spar or wollastonite. Renaming the table Spath in wollastonite by Léman was designed to reward the scientific merits of the famous English scientist William Hyde Wollaston ( 1766-1828 ).

Since the founding of the International Mineralogical Association (IMA ), 1958, wollastonite is the internationally recognized mineral name for naturally occurring CaSiO3.

Classification

Already in the now outdated but still in use 8th edition of the mineral classification by Strunz was one of wollastonite, specifically its two modifications Wollastonite -1A and wollastonite -2M, the mineral class of " silicates and Germanates " and then to the Department of " chain silicates and phyllosilicates ( inosilicates ) ", where he formed an independent group VIII/F.18 together with Bustamite, Cascandit, Denisovite, Ferrobustamit, Foshagite, Jennit, Pectolite, Serandit and Vistepit.

The 9th edition used since 2001 and valid by the International Mineralogical Association (IMA ) of the Strunz'schen Mineral classification assigns Wollastonite -1A and wollastonite -2M also in the class of " silicates and Germanates " and there in the department of " chains and chain silicates ( inosilicates ) " a. This division, however, is further subdivided according to the type of chain formation, so that the Wollastonite is " with 3- periodic single and multiple chains chains and chain silicates " to find according to their construction in the subdivision where they eponymous the " Wollastonitgruppe " with the system no. Form 9.DG.05 and the other members Bustamite, Cascandit, Ferrobustamit, Pectolite, Serandit and Tanohatait.

The mainly common in English-speaking classification of minerals according to Dana classified wollastonite 1A, wollastonite -2M and wollastonite -3A -4A -5A -7A in the class of " silicates and Germanates " and there in the department of " chain silicate minerals " one. Here they are also named after them " Wollastonitgruppe " with the system no. 65.02.01 and the other members Bustamite, Ferrobustamit, Pectolite, Serandit, Cascandit, Denisovite and Tanohatait within the subdivision " chain silicates: Simple unbranched chains, W = 1 with chains P = 3" to find.

Modifications

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

The name without the addition of wollastonite describes generally the most frequent form, crystallizing in the triclinic wollastonite -1T (1 stands for " the first form of " T for triclinic ), which is sometimes referred to as wollastonite 1A or α - CaSiO3. In English literature also finds the name Wollastonite - Tc (Tc = triclinic ).

The second natural form is the monoclinic wollastonite -2M ( 2 = " second form ", M = monoclinic), which occurs much less frequently than the triclinic wollastonite -1T. Synonyms for wollastonite -2M are Parawollastonit and, actually inconsistent manner, also α - CaSiO3. The term α - CaSiO3 used for wollastonite -1T and wollastonite 2M, since both are considered to be low-temperature modifications. Wollastonite -2M usually occurs but not together with wollastonite -1T on, but is included in metamorphic rocks that have arisen at very low pressure during metamorphism.

The high-temperature modification (sometimes also wollastonite -4A) called pseudowollastonite or β - CaSiO3 and is only stable at temperatures above 1120 ° C.. Pseudowollastonite also crystallizes monoclinic, but has due to the lying very close to 90 ° β angle of its unit cell a pseudo- orthorhombic structure. While wollastonite -1T and wollastonite -2M belong to the single Kettensilikatenc ( inosilicates ), form the silicates in pseudowollastonite annular structures. Pseudowollastonite thus belongs to the group of ring silicates ( cyclosilicates ). The arrangement of the SiO4 tetrahedra is more comparable with the structure of Benitoite ( BaTi [ Si3O9 ] ).

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

Education and Locations

Wollastonite often occurs in metamorphic rocks, which are originated from carbonate rock, and is a rock-forming part of the skarn. It typically arises in the contact metamorphism by the contact of limestone with siliceous magma. At temperatures higher than 600 ° C there is the so-called Wollastonitreaktion:

Since the reaction CO2 escapes as a gas, the equilibrium shifts to the principle of Le Chatelier following the side of the products, ie the reaction proceeds completely, and is practical in nature is not reversible. The Wollastonitreaktion is therefore a classic example of the metasomatism. In a normal metamorphosis does tend to alter Although the structure and in most cases the mineral content of the rock, the minerals formed by the metamorphosis largely, however, have the same chemical composition as the originally existing minerals. When metasomatism, as here in the case of Wollastonitreaktion, also the chemistry of the rock changes.

Wollastonite may contain traces up to larger amounts of iron and manganese in the form of divalent cations have on the places of Ca2 cations in the crystal lattice. High iron and manganese interests manifest themselves as higher refractive indices, especially in the consideration of the mineral in rock thin sections on the polarization microscope. Rare also magnesium ( Mg2 ), aluminum ( Al3 ) or sodium (Na ) and potassium cations are (K ) on the calcium positions. With iron contents of more than 10% ( CA0 9Fe0, 1SiO3 ) and manganese contents in excess of 25 % ( Ca0, 75Mn0, 25SiO3 ) crystallized in the structure of wollastonite Bustamite ( (Mn, Ca, Fe) [ SiO3 ] ). Among the structurally related mineral deposits of wollastonite include not only Bustamite also Pectolite ( NaCa2 [ Si3O8 (OH)] ) and Serandit (Na (Mg, Ca ) 2 [ Si3O8 (OH)] ).

Accompanying minerals ( mineral assemblages ) of wollastonite are typically diopside, various grenade (especially grossular and andradite ), tremolite, vesuvianite ( Idocrase ), microcline and calcite.

Wollastonite occurs worldwide at numerous localities and is also mined for industrial use. The Production information relating to the World Mineral Report of the British Geological Survey, 2005:

• China (main producer on the world market ), 350,000 t
• India, 128 285 t
• USA in Willsboro (New York) and Governor (City) (New York) 120,000 t
• Mexico, State of Chiapas, 27 123 t
• Finland, 15,950 t
• Namibia, 253 t

Locations with no commercial mining:

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

The Wollastonite from Franklin (New Jersey) are frequently characterized by a blue to white fluorescence when exposed to UV light. This, as well as the fluorescence in the mineral fluorite, caused by very small amounts of europium cation ( Eu2 ) for calcium positions in the crystal lattice.

Crystal structure

In nature, wollastonite usually occurs as wollastonite -1T. Wollastonite -1T crystallizes in the triclinic crystal system in the crystal class 1 or the space group P1 with six formula units in the unit cell (Z = 6). The only element of symmetry in the crystal structure is a center of inversion, which multiplies the atoms through point mirroring. The inversion centers are located in the corners, on the face centers and the center of the unit cell. The crystal structure contains three crystallographically distinct calcium and silicon atoms as well as nine different oxygen atoms. Crystallographically distinct means that these atoms are not the existing elements of symmetry (in this case the center of symmetry ) are interconvertible. By the inversion center, the three calcium and silicon atoms as well as the nine oxygen atoms are doubled, so that overall, the above described six formula units (6 x CaSiO3 = " Ca6Si6O18 " ) are in the unit cell.

Crystallographic data of 1T wollastonite are given in comparison with the other two modifications in the table.

Coordination environment of the Ca2 - and Si4 cations

As in almost all silicates silicon is surrounded by four oxygen atoms in the form of a tetrahedron. This SiO4 tetrahedra are not isolated in the crystal structure before, but are linked into chains ( see next section). The oxygen-silicon distances are 157-166 pm, which corresponds to the usual distances in silicates. The calcium atoms are in each case surrounded by six oxygen atoms in the form of distorted octahedrons, calcium - oxygen distances are 227 to 255 pm.

Linkage pattern of Silicatketten

Although wollastonite is one of the single Kettensilicaten ( inosilicates ), the linkage pattern of the SiO4 tetrahedra is different within the Silicatkette from that of the much more common pyroxenes. The difference is evident in the comparison of wollastonite with the pyroxene enstatite ( MgSiO3 ). The linking of SiO4 tetrahedra occurs in all Kettensilicaten on common tetrahedron corners, so the sharing of oxygen atoms. In order for a chain that each silicon has to share two of the oxygen atoms of its tetrahedron with neighboring silicon atoms, these oxygen atoms "belong" to him so only half right. For a silicon - oxygen ratio of 1:3, which is also reflected in the chemical formula of Kettensilicate arises in the chain ( wollastonite: CaSiO3, enstatite: MgSiO3 ). These chains are virtually endless, they are only limited by the size of the crystal. In the crystal chemistry they are therefore described as follows:

Or detailed as Niggli formula:

The chains can now be further distinguished by the orientation of SiO4 tetrahedra with each other. While repeated in enstatite and all other pyroxenes the same subject by two tetrahedra, the chain pattern is defined by three tetrahedra in wollastonite. Put simply show in enstatite the tetrahedron with a peak alternating "up" and " down " while in the wollastonite a tetrahedron with the top " down " shows, the next two, however, "up". In the case of pyroxene is therefore also referred to as a " two single chain ", wollastonite has a "triple single chain ". Since the underlying motive of the chains consists in wollastonite from three tetrahedra, the chemical formula is often tripled, Ca3 [ Si3O9 ] stated. The endless ( SiO 3 ) 2 - chains run in the crystal structure of wollastonite in the direction, i.e. in the direction of the crystallographic b axis. The chain motif consisting of three tetrahedra repeats itself after 732 pm, which corresponds exactly to the lattice constant of the unit cell along the b- axis. The complicated arrangement of the tetrahedra in wollastonite occurs due to the increased space requirement of Ca2 cations (Ca2 greater than Mg2 often contained in pyroxene - and Fe2 cations ) in the crystal structure.

Forest

The [ CaO6 ] octahedra form themselves on common edges also in chains along the b axis. The octahedra are linked by the sharing of oxygen atoms with the Silicatketten described above in the direction of the a- and c- axis to form a three-dimensional structure.

Influence of the structure on the macroscopic properties

Based on the crystal structure of some macroscopic properties of wollastonite can be explained. Single crystals of wollastonite have a needle-like to fibrous form ( habit ), since the crystals grow preferentially in the direction of the crystallographic b-axis, which corresponds to the orientation of the Silicatketten in the crystal structure. If you break a wollastonite needle in the middle, that is, one breaks the Silicatketten, resulting uneven fracture surfaces while under mechanical stress parallel to the b - axis planar cleavage surfaces ({ 100} perfect, { 001 } and { 102 } good cleavage ) arise. This can be explained by the chemical bonding in the crystal. While silicon and oxygen through covalent bonds ( atomic bonds ) are interconnected between calcium and oxygen, an ionic bond, which is based on a purely electrostatic interaction, and thus is weaker bond.

Use

Wollastonite offers a variety of technical applications due to its fibrous to acicular crystals and its high melting point (1540 ° C). His preparation is carried out on the reaction of calcium oxide ( CaO, lime ) with silica (SiO2, quartz or silica gel):

One of the main applications of wollastonite is the ceramics industry, where it is used to improve the mechanical properties of white ceramics.

Due to its high melting point wollastonite serves as a substitute for asbestos fibers. Typically, it is used in welding electrodes, insulating materials (see calcium silicate board ) and fire resistant protective clothing used. While asbestos fibers are among the most carcinogenic substances, is of wollastonite from a health risk, because they dissolve within a few days to a few weeks in the organism.

In the plastics industry, wollastonite is mainly used as a filler in thermoplastics. It is used, inter alia, to improve the rigidity and bending strength of polyesters, polyamides and polypropylenes. Similarly, one uses it at the reaction resins, such as epoxy resins, in order to avoid tension cracks caused by shrinkage.

For wollastonite, there are numerous trade names, including Kemolit, Hycon and Tremin.