Smith College - Geology 222b - Petrology

Petrographic Data File

Formula CaSiO3 Pyroxenoid group. Usually pure, but Mn and Fe2+ can substitute for Ca.
Crystal System Triclinic  
Crystal Habit Columnar and fibrous elongate grains, often with twinning  
Cleavage Perfect cleavage on {100}, good cleavages on {001} and {-102} Splitery cleavage fragments. Angles of cleavage: 84.5 degrees, and 70 degrees.
Color/Pleochroism Colorless, white, greyish, often with yellowish or brownish tint. Vitreous. No pleochroism  
Optic Sign Biaxial (-)  
2Valpha 36-60 degrees

Most wollastonite has a 2V of ~40º. This can be higher for Fe-bearing samples.

Optic Orientation X^c = -30 to -44 degrees
Y^b ~ 0 degrees
z^a = +35 to +49 degrees
O.A.P. ~ (010)
Cleavage fragments are length slow or length fast depending on orientation due to Y being parallel to fiber length.

Refractive Indices

alpha: 1.616-1.645
beta: 1.628-1.652
gamma: 1.631-1.656
delta: 0.013-0.017

Increase with Fe and Mn content.
Max Birefringence 0.013-0.017 Wollastonite resembles tremolite and pectolite, but both have a higher birefringence.
Elongation Grains elongate parallel to b  
Extinction Parallel   Elongate crystals display parallel extinction.
Dispersion r > v  
Distinguishing Features Colorless to grey in thin sectionwith moderate to moderatly high relief. First order interference color yellow-orange. One perfect cleavage and two good cleavages producing splintery cleavage fragments. H = 4.5-5. G = 2.86-3.09. Streak is colorless or white. 
Occurrence Occurs commonly as a product of contact and/or regional metamorphism in limestone and dolomite. Associated minerals include calcite, and grossular in hornfels, tremolite, epidote group members, diopside, and other Ca-Mg silicates.

Deer, W.A., Howie, R.A., Zussman, J. (1963) Rock-Forming Minerals, Chain Silicates, p. 94-100. Longmans, Green and Co LTD, London.

Editors Sarah Martell (AC '06) , Carolyn Tewksbury ('07), Jessica Chang ('17)

Photomicrograph of wollastonite in plane and crossed polarized light (roll over image). Wollastonite has parallel extinction and has two good cleavages that intersect at an angle of 84º.
Photomicrograph of wollastonite in plane light (roll over image). Wollastonite typically has no color, and has distinct splintery cleavage in an end view.

Photomicrograph of wollastonite in plane and crossed polarized light showing elongated crystals having parallel extinction.

WWW Oxford University [images 74 and 75]

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