Epidote-Clinozoisite


Occurrence and Compositon
Epidote and clinozoiste are common in many igneous rocks and low- to medium-grade metamorphic rocks. They also form as alteration products in rocks of many sorts. They may be dominant or abundant in a rock, but are more often accessory minerals. Clinozoisite and epidote form a solid solution series related by substitution of Fe3+ for Al. Clinozoisite has composition close to Ca2Al3Si3O12(OH). Epidote has general formula Ca2(Al,Fe3+)3Si3O12(OH). The most Fe-rich epidotes have about 35% of the Al replaced by Fe3+.

Identification
These minerals may be hard to identify if present as small accessory grains. When larger, identification is simpler. High relief, anomalous interference colors or (for epidote) grains showing many interference colors, are keys to identification. Relief and birefringence increase with increasing Fe-content.

Important properties
 ·Relief - Relief is high.
 ·Interference colors - Clinozoisite has first order, often somewhat anomalous yellow-green, interference colors. Epidote has higher order interference colors, typically upper second- to third-order. Individual grains show multiple colors, sometimes in concentric rings. First order white interference colors are rarely seen; anomalous interference colors may replace them, especially at grain edges.
 ·Color - Clinozoisite is colorless. Epidote has a characteristic yellowish green (sometimes weakly pleochroic) color.
 ·Extinction - Extinction is parallel in elongate grains (not in grains in other orientations).

Similar minerals
 ·Epidote is distinguished from clinopyroxenes by having only a single cleavage, yellow-green color, parallel extinction, anomalous interference colors, and by its optic sign.
 ·Clinozoisite has lower birefringence than epidote. Epidote is optically negative; clinozoisite is positive.
 ·Zoisite (not the same mineral as clinozoisite) and olivine have parallel extinction in all orientations.
 · Epidote may sometimes be confused with olivine, but olivine shows no cleavage.


Hornblende Replaced by Chlorite, Epidote and Magnetite in a Granite

The diamond-shaped original crystal was hornblende but it has been replaced by other minerals. In PP light, chlorite and epidote both appear light green (but the chlorite has a more "micaceous" character). Under crossed polars (XP) they are distinguished because the chlorite shows anomalous green-gray interference colors while the epidote shows upper second order interference colors. Th opaque mineral is magnetite; quartz and feldspar surround the amphibole grain.

This sample comes from near Garfield, Colorad. The field of view is about 2 mm.


Epidote in a Highly Altered Basalt

The photos above show epidote that has filled an amygdule in a highly altered basalt. The color of the epidote is typical: a sort of off-color yellow-green. The interference colors, too, are classic: various shades of pastels within individual grains. The opaque material around the amygdule is a mixture of glass, hematite and chlorite. Note several bubbles introduced when the thin section was made.

The field of view is about 3.5 mm. This specimen comes from Keweenaw County, Michigan.


Epidote and Hornblende in a Mafic Schist from near Panoche Pass, California

The photos show pleochroic hornblende (green hues; PP) and epidote (clear; PP) in a mafic schist. Note the epidote grains show multiple interference colors creating zones or concentric rings in most grains (XP). Also note that some of the hornblende shows a hint of a diamond shape and of amphibole's characteristic 60o-120o cleavage angle.

The field of view is about 2.5 mm.


Chlorite, Glaucophane and Epidote in a Blueschist

Green chlorite, blue glaucophane (an amphibole), clear white mica flakes (bottom center and right) and a number of small high-relief epidote grains (most are wedge-shaped) are visible in PP light. In XP light the chlorite shows anomalous interference colors, the glaucophane shows 2nd order interference colors, in places somewhat masked by the blue color of the mineral. The mica flakes show obvious mottled 2nd order interference colors.The epidote grains are hard to pick out in XP light

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.


Epidote/Clinozoisite in a Schist from Western Massachusetts

This view contains high relief clinozoisite surrounded by lower relief quartz (and plagioclase, but the plagioclase cannot be distinguished in this view). Note that some of the clinozoisite shows anomalous blue-green interference colors. The patchy interference colors are typical of epidote/clinozoisite. The high birefringence of one grain suggests that it is epidote. It was not possible to determine the optic sign of that grain, but the optic sign of a different grain is positive -- suggesting it is clinozoisite. It is possible that both epidote and clinozoisite are present in this rock, but chemical analysis is needed to tell for sure.