OMPHACITE

“Omphacite” – (Na,Ca,Fe2+,Mg)(Al, Fe2+,Fe3+,Mg)Si2O6 – is not a mineral. It is a solid solution formed by three monoclinic pyroxenes (clinopyroxenes): jadeite, augite and aegirine. It is a rock forming mineral in eclogites and macroscopically shows an intense, deep green color, resembling diopside, chromiferous zoisite and kosmochlor. It has no importance as an ore.

The solid solution has varying proportions of jadeite (25-75%), augite (25-75%) and aegirine (0-25%). The name was kept for practical reasons only. There is a variety rich in Cr and another rich in Ti. Omphacite may contain Mn and than presents a violet color (“violan”).

Omphacite occurs exclusively in high temperature and high pressure metamorphic rocks such as eclogites, granulites, pyroxenites, kimberlites, garnetites, amphiblites, migmatitic gneisses and in some metamorphic rocks of the blueschist facies, with glaucophane. May occur in metagraywackes.

In eclogites of deep origin it is associated with garnets (almandine, pyrope), corundum (ruby variety) and kyanite. In migmatitic gneisses it occurs with “hornblendes” and scapolite. In eclogites from orogenic zones it is associated with glaucophane, epidote, lawsonite, pumpellyite and zoisite. Quartz and plagioclase also occur in association.

4. Transmitted Light Microscopy

Refraction indices:  nα: 1.662 – 1.701     nβ: 1.670 – 1.712    nγ: 1.685 – 1.723

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism:Colorless to pale green, with weak pleochroism between colorless, very pale green to blue-green, more intense if it contains Fe.   

Relief: High.           

Cleavage: {110} good. In the longitudinal sections there is only one cleavage. In the basal sections there are two cleavages that intersect at angles of 87º and 93º, as in all pyroxenes. There is a partition on {100}.           

Habits: Usually anhedral grains, granular to massive. Rarely crystals as short prisms.            

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Birefringence from 0.012 to 0.028, with colors up to the end of 1st order: gray, yellow, orange, red to intense blue.           

Extinction: No information available. Probably oblique.           

Elongation sign: No information available.  

Twins: Simple, lamellar, and polysynthetic twins by {100} are common.         

Zoning: No information available.

CONVERGENT LIGHT

Character: B(+)          

2V angle: 58 – 83º         

Alterations:  No information available. 

May be confused with: calcic clinopyroxenes such as diopside, but the paragenesis is different.

Jadeite has lower birefringence, lover 2V and is often colored in PPL.