WOLLASTONITE

Wollastonite – CaSiO3 or Ca3(Si3O9) – is an inosilicate, a pyroxenoid. Its occurrence is more restricted, a mineral typical of metamorphic and metasomatic rocks. It is an industrial mineral with many important uses.

It is classified in the Wollastonite Group, there are several monoclinic and triclinic polytypes, in addition to three varieties (impure, with Fe, with Mn). In addition, it may contain Mg. May show yellow cathodoluminescence.

Macroscopically, it can be confused with tremolite and pectolite, which occur in the same paragenesis. It is very similar to dalnegorskite (pyroxenoid with the composition of wollastonite + Mn).

1. Characteristics

Crystal system: Triclinic pinacoidal, pseudo-monoclinic.          

Color: White, gray-white, colorless, more rarely light green, pink, brown, yellow or red.     

Habit: Tabular, lamellar, columnar, short to long prismatic, more rarely fibrous or acicular.       

Cleavage: {001} perfect, {001} good, {-102} good.

Tenacity: Very brittle.        

Twinning: Twins are common, either by the twin axis [010] or by the twin plane {100}. 

Fracture: Irregular.       

Mohs Hardness: 4.5 – 5

Parting:  No.        

Streak: White.         

Lustre: Vitreous, pearly.          

Diaphaneity: Transparent.           

Density (g/cm³): 2.86 – 3.09 

 

2. Geology and Deposits

Wollastonite typically occurs abundantly in impure carbonate rocks (usually marbles) that have undergone thermal metamorphism, in this case generating high-grade wollastonite marbles. This metamorphism can be generated by an intrusive igneous (generally granitic) rock which, in contact with the limestone rock, forms skarns and buchites. Similar situations occur when metamorphism occurs due to a dike intruded in limestone and in sandstone sequences. Wollastonite can also be formed by a regional metamorphism of low to high degree (amphibolite and granulite facies).

More rarely and sparsely, it occurs in phonolites, ijolites, nepheline It can occur in meteorites (chondrites).

It is common in slags and glasses in the form of pseudowollastonite, which is a cyclosilicate, a high-temperature phase, dimorphic with wollastonite, which has very rarely been found in natural geological situations.

 

3. Mineral Associations

Typical associated minerals are carbonates (calcite, dolomite, aragonite), clinopyroxenes (diopside, fassaite (low Fe augite)), plagioclases, garnets (grossular, andradite), olivines (forsterite), epidote and vesuvianite.

It also occurs with scheelite, monticellite, minehillite, akermanite, spurrite, larnite, merwinite and amphiboles.

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1.616 – 1.640     nβ: 1.628 – 1.650     nγ: 1.631 – 1.653

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Colorless to grayish white, without pleochroism.          

Relief: Moderate to high.           

Cleavage: (100) perfect, (001) good, (-10-2) good. Fragments controlled by (100) and (001). It can show good cleavage, the quality of which is slightly inferior to that of micas, but better than that of pyroxenes.

In the basal section, cleavages {100} and {001} intersect at 84.5º. This angle is very similar to the angle formed by the two cleavages in the basal sections of the pyroxenes, but the cleavages of wollastonite are better and consequently the lattice in the basal sections of wollastonite is more defined.

The {-102} cleavage intersects the {100} cleavage at 70°. As this cleavage is of much lower quality, generally the visible angle is that of 84.5°.           

Habits: Lamellar, columnar, tabular, more rarely fibrous or acicular. Subparallel or divergent or brush-shaped aggregates are common. It can be tabular in {100}. With almost rectangular sections, poikiloblastic.            

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Low birefringence, 0.013 – 0.017 (or 0.012 – 0.014), higher 1st order: gray to at most 1st order yellowish-orange colors.

Birefringence increases a little with increasing Fe+Mn contents.

Extinction: Always oblique, but in longitudinal sections parallel to “b” almost parallel. In other sections with angles of 0 – 5º (β,b) or 30-44º (α,c). 

Elongation sign: ES(+) or ES(-), its not diagnostic.

Twins:  In lamellae, on (100).        

Zoning: Frequently zoned.             

CONVERGENT LIGHT

Character:  B(-).         

2V angle: 36 – 60º, usually around 40º.         

Alterations: to pectolite, apophyllite, calcite, quartz, chalcedony and opal.          

May be confused with: several other Ca and Ca-Mg silicates that occur in marbles.

Diopside has higher relief, higher 2V and always has pyroxene cleavage, which is coarser and slightly worse than that of wollastonite.

Zoisite and epidote have higher relief, higher 2V, and much more intense interference colors, and they often show anomalous interference colors.

Tremolite has typical amphibole cleavage, a larger 2V angle, higher birefringence.

Pectolite has higher birefringence.

Cancrinite has the same interference colors, but has low relief and is U(-).

An indication of the occurrence of wollastonite is the presence of holes in the thin section, which are places where the minerals of the thin section were stripped during the making of the thin section. The holes are due to the fact that wollastonite has good quality cleavages and presents “very brittle” tenacity, which causes it to shatter easily during the making of the thin section. The same goes for kyanite.         

 

Wollastonite 01: In PPL, longitudinal sections of wollastonite with its typical good cleavage, praralell to the elongation. The relief is moderate.
Wollastonite 01: The same section as of the previous image, now in XPL. Wollastonite shows relatively low interference colors. Twin and zoning are rare, a feature that is also diagnostic.
Wollastonite 02: In PPL, a basal section with two cleavages intersecting at 84.5º and a longitudinal section with only one cleavage.
Wollastonite 02: The same section as of the previous image, now in XPL. Interference colors always are of first order.
In XPL, wollastonite with granular habit, showing interference colors between yellow and orange. In these grains the cleavages are not well developed.
In XPL, wollastonite with lamellar twin. Colorful grains are of clinopyroxene, most probably diopside.
Wollastonite 03: In the center of the image is a hole in the thin section generated because wollastonite has good cleavages and because it shatters easily. It is still a feature that helps in the recognition of wollastonite, because few minerals generate holes so easily.
Wollastonite 03: The same section as of the previous image, now in XPL. Kyanite is another mineral that shatters very easily during the thin section preparation process, creating holes mainly in the basal sections.

5. Reflected Light Microscopy

Reflected light microscopy is not the recommended analytical method for the identification of wollastonite. However, it is important to make a polished thin section or a polished section to identify the opaque minerals that occur associated with wollastonite, like pyrite.

Sample preparation: Wollastonite acquires a good polish without difficulty. The quality of the polish, however, varies with the orientation of the sections: sections parallel to the main cleavage acquire an excellent polish, while sections oblique to the cleavages show many triangular polishing pits or show well-developed cleavage in one direction.

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray, like quartz and feldspars.       

Pleochroism: No.      

Reflectivity: Low (<10%).        

Bireflectance: No.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: Anisotropy was not observed.        

Internal reflections: Generalized in light colors, sometimes multicolored.      

May be confused with: many other colorless minerals.       

General Characteristics: 

Triangular polishing pits may be present.

Cleavage, well developed, may be visible in sections perpendicular to the main cleavage direction.

Wollastonite 01: In PPL, aggregate of wollastonite crystals. There are grains with great polishing and others with lots of polishing and cleavage figures. There are also holes generated by the wollastonite being too brittle (it shatters during preparation of the polished section).
Wollastonite 01: The same section as of the previous image, now in XPL. Internal reflections are generalized and, depending on the orientation of the main cleavage with the plane of the polished section, colored reflections may appear as in the isolated crystal in the image.