Smith College - Geology 222b - Petrology

Petrographic Data File

Formula Na(Al,Fe+++)Si2O6  
Crystal System monoclinic prismatic
Crystal Habit granular Generally occurs as anhedral to subhedral crystals in matrix., Fibrous - Crystals made up of fibers., Massive - Fibrous - Distinctly fibrous fine-grained forms.
Cleavage good cleavage conchoidal fracture
Color/Pleochroism Green, White, Pale bluish gray, Grayish green, Pale purple. Pleochroism- color green, yellow or yellowish green
Optic Sign Biaxial (+)   



Optic Orientation X=b 
O.A.P. = (010)
Refractive Indices
alpha = 
beta = 
gamma = 
delta = 
Max Birefringence 0.0130-0.020  
Dispersion r < v  
Distinguishing Features

In Hand Sample: Massive varities are distinguished by green color and toughness. Polished surfaces of nephrite have somewhat oily luster; jadeite is vitreous. granular material in metagraywacke may be difficult to recognize and distinguish in hand sample.

In Thin Section/Grain Mount: Jadeite has lower indices of refraction than other pyroxenes. Omphacite has higher birerefringence and aegirine and aegirine-augite have higher birerifringence and are more distinctly green. Jadeite with anomalous interference colors resembles zoisite has higher indices of refraction and parallel extinction angle(010) sections and has typical amphibole cleavage. The approximate compisition of the sodic pyroxenes can be estimated based on the value of n beta and the (211) d-spacing obtained from x-ray diffraction study.

Occurrence Jadeite is restricted to metamorphic rocks subjected to relatively high pressures and moderate temperatures. Typical occurrences are in glaucophane schist, metagraywacke, and related rocks in alpine metamorphic belts. It may occur as nearly monomineralic pods or veins or as disseminated grains. The commonly associated minerals are albite, glaucophane, lawsonite, quartz, chlorite, garnet, zoisite, tremolite, calcite, aragonite, and micas.
Editors Rachel Herrmann ('06)

Omphacite in plane polarized light. The omphacite crystals are pale green and are surrounded by purple-blue glaucophane grains and a brown rutle crystal. Click on the photo for a larger image with rollover change from ppl to xpl.

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