Sodic Amphibole

Occurrence and Compositon
There are a number of Na-amphiboles, all typcally found in high pressure (especially blue schist facies) metamorphic rocks. The most important members of this group are glaucophane, crossite and riebeckite. Glaucophane has general formula Na2Mg3Al2(Si8O22)(OH)2. Crossite is similar, but in crossite Fe2+ has replaced some Mg, and Fe3+ has replaced some of the Al. Riebeckite contains little Mg; it is essentially an Na-Fe amphibole.

Blue amphiboles are generally identified by mineral association, and distinctive color and pleochroism.

Important properties
 ·Appearance and habit - blades or long slender crystals are typical; diamond shaped cross section are less common.
 ·Color - pleochroic in various shades of blue or violet depending on composition.
 ·Interference colors - Maximum interference colors are upper first order but they are frequently modified by the color of the mineral.

Similar minerals
 ·Blue amphibole may be confused with blue troumaline, but tourmaline is uniaxial.
 ·The blue color (seen in PP light) of sodic amphiboles may look a lot like the blue "interference colors" (seen in XP light) of sodic pyroxenes, which sometimes leads to confusion.

Glaucophane (and epidote) in a Blueschist

Most of this thin section consists of blades of the blue amphibole glaucophane. In XP light, its interference colors are somewhat anomalous because the color of the mineral has added to the normal upper first-order interference colors. The light green mineral with relatively high relief is epidote (PP). Note that its interference colors are high-order in some grains and appear anomalous in others.

This specimen comes from Sonoma County, California. The field of view is 3.5 mm.

Chlorite, Glaucophane and Epidote in a Blueschist

Green chlorite, blue glaucophane, white mica flakes (bottom center and right) and a number of small high-relief epidote grains are visible in PP light. In XP light the chlorite shows anomalous interference colors, the glaucophane shows 2nd order interference colors, not generally typical of this mineral. The epidote grains are hard to pick out in XP light but the white mica flakes show obvious mottled interference colors typical of micas.

This sample comes from near Panoche Pass, Callifornia. The field of view is about 2.5 mm.

Glaucophane and Epidote with Jadeite (Blueschist)

This section contains abundant blue glaucophane, some showing the classic diamond-shaped amphibole cross section and cleavage. Many small grains of high relief epidote are visible in PP light but get lost in the XP view. The nearly clear matrix material that encloses the glaucophane and epidote is jadeite. It displays low order inteference colors, anomalous in some grains, in the XP view.

This sample comes from near Panoche Pass, Callifornia. The field of view is about 2.5 mm.