Wollastonite
Triclinic
Ca[SiO3]
Wollastonite is a calcium inosilicate (chain silicate), which is chemically related to pirosseni but is characterized by a different structure. Wollastonite is a common product of high temperature metamorphism or metasomatism of limestones that occurs predominantly within contact aureoles. The mineral was named in 1818 in honor of the English chemist and mineralogist William Hyde Wollaston, who discovered the elements Rh and Pd.
Struttura e chimica
Wollastonite is closely related to pirosseni from a chemical point of view, in particular quadrilateral pyroxenes like enstatite (Mg[SiO3]) and diopside (CaMg[SiO3]2). However, the geometry of the silicon tetrahedral chains differs significantly in the wollastonite structure from that of pyroxenes, since wollastonite tetrahedral chains consist of two tetrahedrons pointing in one direction alternating with one tetrahedron pointing in the opposite direction and with one edge oriented parallel to the chain, repeating infinitely along the b-axis (while in pyroxenes, the chains are oriented along c). Tetrahedral chains alternate with octahedral sites (M1, M2, and M3), which are larger than those of the pyroxenes and are all able to accommodate Ca. Wollastonite has different polytypes that differ in the way tetrahedron chains are stacked: wollastonite-1A is the high-temperature (triclinic) form, whereas wollastonite-2M is the low-temperature (monoclinic) form. Both occur in rocks, but wollastonite-1A is the most common.
Proprietà
Abito: fibrous, acicular, lamellar, more rarely prismatic or tabular
Durezza: 4.5 – 5
Densità: 2.9 – 3.1 g/cm3
Sfaldatura: {100} perfect; {001}, {102} good (3 cleavage planes intersecting at 70° and 84° visible on basal sections)
Geminazione: {100} composition plane – [010] twin axis: common
Colore: white, colorless, very pale green
Lucentezza: vitreous to dull or pearly
Struscio: bianco
Alterazione: talc, diopside, stevensite, calcite
In sezione sottile...
α(α^c = 30-44°): 1.616-1.640
β(//b): 1.628-1.650
γ(γ^c = 70-56°): 1.631-1.653
2Vα: 36-60°
Colore: incolore
Birifrangenza (δ): 0.013-0.014 (first order yellow/red)
Rilievo: moderato
Segno ottico: –
[Mindat]
[HoM]
Caratteristiche di terreno
Wollastonite occurs in carbonate rocks that were affected by contact metamorphism or high temperature fluid circulation, like hornfelses, marbles, and skarns. Wollastonite is found mostly as fibrous or bladed crystals (rarely prismatic). In the field, it is very difficult to distinguish wollastonite from the other calc-silicates that are common in these geologic environments, like anfibolo, talc, or pirosseni, considering also that it is commonly found intergrown with these minerals.
Wollastonite in thin section
Under plane polarized light, wollastonite is colorless and shows moderate relief, and three different sets of sfaldatura planes, intersecting at 70° and 84°. However, wollastonite is commonly fibrous or acicular, making it difficult to find appropriate basal sections where to observe the intersection between cleavage planes. At crossed polars, wollastonite shows first-order grey to red interference colors and straight extinction. Wollastonite can be easily confused with orthopyroxene, clinopyroxene, or colorless anfiboli like tremolite, all minerals that can be found in contact metamorphic environments.
Wollastonite can be distinguished from orthopyroxene (enstatite) because of the higher birefringence (enstatite reaches first-order yellow interference colors at most). Ferrous varieties of orthopyroxene are also pleochroic (wollastonite is never pleochroic). Diopside (clinopyroxene) is very similar to wollastonite but shows oblique extinction. Tremolite is similar to wollastonite at plane polarized light, but at crossed polarizers shows much higher second-order interference colors.
CPL + AP
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⇔ slider. Fibrous wollastonite crystals (right) radiating within a metasomatized calcite marble (skarn). Tiny grains of other calc-silicates are visible within the calcite crystals to the left. Width: 3 mm. Norsi, Island of Elba, Italy.
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⇔ slider. Prismatic crystals of wollastonite, like the one shown above, are quite rare. These crystals show well-developed cleavage parallel to their long axes (the three distinct sets are visible only on basal planes). Width: 1.2 mm. Norsi, Island of Elba, Italy.
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⇔ slider. Fibrous and prismatic wollastonite crystals, intergrown in a skarn. Width: 1.2 mm. Norsi, Island of Elba, Italy.
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⇔ slider. Wollastonite (center) with diopside rim, surrounded by calcite in a marble. Associations of this type are common, because diopside may grow together with or replace wollastonite in skarns and high grade metamorphic rocks. Width: 3 mm. Norsi, Island of Elba, Italy.
Video. Wollastonite core with diopside rim. Note the difference in extinction angle. CPL. Width: 3 mm. Norsi, Island of Elba, Italy.
Examples of wollastonite-bearing rocks
Wollastonite skarns
Calcsilicate skarns (containing abundant wollastonite) formed in sequences of limestone and shale, due to the reaction between limestones and hot hydrothermal fluids originated from a nearby pluton (monzogranitic rocks).
Campione: calcsilicate skarns, wollastonite skarns
Associazione mineralogica: wollastonite, diopside, calcite, tremolite, hydrogrossular, adularia, talc
Località: Norsi, Isola d’Elba, Italy
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Ritrovamento
Wollastonite is a typical mineral of high-temperature metamorphic and metasomatic environments. It occurs in contact-metamorphosed limestones, in skarns, and associated aureole rocks. Wollastonite forms in response to the decarbonation of calcite [CaCO3] in the presence of SiO2, either in the form of quarzo, as in impure limestones, or supplied by metamorphic fluids, following the reaction:
CaCO3 + SiO2 ⇌ CaSiO3 + CO2
This reaction may take place at very high temperatures (T > 1000 °C) or at much lower temperatures if silica-bearing metasomatic fluids are involved. Indeed, wollastonite is found not only in the highest grade part of contact aureoles, at the contact between marbles and intrusive rocks, but also in the outer parts or even outside aureoles, where metamorphic fluids reacted with carbonates to produce calc-silicate rocks. Wollastonite is also a product of high-temperature regional metamorphism of marbles at upper amphibolite- to granulite-facies conditions.
Wollastonite is extremely rare in igneous rocks but it occurs in some phonolites and in alkaline igneous complexes.
Daval, D., Martinez, I., Corvisier, J., Findling, N., Goffé, B., & Guyot, F. (2009). Carbonation of Ca-bearing silicates, the case of wollastonite: Experimental investigations and kinetic modeling. Chemical Geology, 265(1-2), 63-78.
Ohashi, Y. (1984). Polysynthetically-twinned structures of enstatite and wollastonite. Physics and Chemistry of Minerals, 10(5), 217-229.
Ribas, R. G., Campos, T. M. B., Schatkoski, V. M., de Menezes, B. R. C., do Amaral Montanheiro, T. L., & Thim, G. P. (2020). α-wollastonite crystallization at low temperature. Ceramics International, 46(5), 6575-6580.
Tolliday, J. (1958). Crystal structure of β-wollastonite. Nature, 182(4641), 1012-1013.
Risorse
An introduction to the Rock-Forming Minerals. Deer, Howie & Zussmann.
Optical Mineralogy: Principles & Practice. Gribble & Hall.
Transmitted Light Microscopy of Rock-Forming Minerals: An Introduction to Optical Mineralogy (Springer Textbooks in Earth Sciences, Geography and Environment). Schmidt.
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