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Geology is the Way

Wollastonite

Triclinic

Ca[SiO3]

Wollastonite is a calcium inosilicate (chain silicate), which is chemically related to pyroxenes 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 pyroxenes 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.

pyroxene and wollastonite chains

Differences in the structure of the tetrahedron chains of pyroxenes and wollastonite.

wollastonite

Radiating aggregates of fibrous wollastonite crystals. Lahore, Pakistan. Width of sample 8 cm. Photo © Siim Sepp.

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: moderate
Segno ottico:
[Mindat]

pyroxene ternary

Ternary diagram showing the relationships between wollastonite and quadrilateral pyroxenes (ortho- and clinopyroxenes) Modified after Morimoto (1989).

Caratteristiche di terreno

wollastonite crystal morphology

Sketch of a wollastonite crystal, showing the position of crystal axes and refractive indices. Modified after Optical Mineralogy: Principles and Practice.

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 amphibole, talc, or pyroxenes, considering also that it is commonly found intergrown with these minerals.

prismatic wollastonite

Prismatic crystals of wollastonite (colorless). Fuka mine, Okayama Prefecture, Japan. Photo © Marcus Origlieri via RRUFF.

wollastonite skarn

Radiating aggregates of wollastonite (brown) in a metasomatized limestone (skarn) from Norsi, Island of Elba, Italy. [BLOG POST]

wollastonite crystals

A close-up of the wollastonite aggregates shown above. The microphotos shown in this page come from these rocks. Norsi, Island of Elba, Italy. [BLOG POST]

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 amphiboles 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 amphibole is similar to wollastonite at plane polarized light, but at crossed polars shows much higher second-order interference colors.

CPL + AP
CPL + AP
CPL + AP
CPL
NX
PPL
N//

Above: 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.

CPL
CPL
NX
PPL
N//

Above: 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 mm. Norsi, Island of Elba, Italy.

CPL
CPL
NX
PPL
N//

Above: fibrous and prismatic wollastonite crystals, intergrown in a skarn. Width: 1 mm. Norsi, Island of Elba, Italy.

CPL
CPL
NX
PPL
N//

Above: 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.
Below: the video highlights the difference in angle of extinction between wollastonite and diopside.

[BLOG POST]: The wollastonite skarn of Norsi, shown in thin section below.

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.

Bibliografia

        

The information displayed on this page is after Introduction to the Rock-Forming Minerals and Optical Mineralogy: Principles and Practice.

Mineral Properties
Minerali

 

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