EPIDOTE

Epidote – {Ca2}{Al2Fe}(Si2O7)(SiO4)O(OH) – is a very common sorosilicate, very frequent as an accessory mineral in igneous and metamorphic rocks. It has no industrial use.

Epidote is part of the Epidote Group, with clinozoisite and piemontite. Forms a series with clinozoisite. May contain Al, Mn and Mg. There are 22 synonyms for “epidote”. Epidote has 7 varieties: “pistacite” is green, there are others with ETRs, with Be, with Cr, with Mn, with Y and a fibrous one.

1. Characteristics

Crystal system: Monoclinic prismatic.

Color: Citrus green to pale green. Yellowish green, yellow, greenish yellow, greenish black.

Habit: Usually long prismatic. Granular coarse to fine, fibrous, massive.

Cleavage: {001} perfect, {100} imperfect. Striations paralell to [010].

Tenacity: Brittle.

Twinning: Common on {100}, lamellar or contact.

Fracture: Irregular.

Mohs Hardness: 6

Parting: No

Streak: White.

Lustre: Vitreous to pearly.

Diaphaneity: Transparent.

Density (g/cm³): 3.38 – 3.49

 

2. Geology and Deposits

Epidote and clinozoisite are very common and well-distributed minerals. They are common in low-grade regional metamorphic rocks, greenschists, marbles, phyllites and blueschists. In medium-grade metamorphic rocks it occurs in rocks such as mica-schists and gneisses. By retrograde metamorphism, epidote and zoisite are formed by saussuritization.

Epidote is common in metamorphic contact rocks such as cornubianites with biotite, calcosilicate rocks, skarn and others.

Epidote may have magmatic origin, occurring in fractures and cavities of magmatites rich in pyroxene and amphibole, replacing the anorthite component of plagioclase. In igneous rocks it is secondary and late. It is common in cataclastic zones of granitoid rocks and may occur in albitized granites and gneisses.

 

3. Mineral Associations

In low-grade metamorphic rocks (greenschists, spilites), epidote and clinozoisite are associated with amphiboles (actinolite and others), zeolites, prehnite, byssolite, titanite, magnetite, microcline, albite, plagioclase, calcite and quartz, or chlorite, albite, vesuvianite and tremolite. With increasing metamorphic grade it occurs with albite, oligoclase, hornblende and garnet (almandine). It can occur in amphibolites and in gneisses. In the blueschist facies it is associated with glaucophane, lawsonite, pumpellyite, clinozoisite, chlorite, muscovite, quartz, albite, calcite and titanite.

In contact metamorphic rocks it occurs with vesuvianite, augite (fassaite), garnets (andradite, grossularia), actinolite, hornblende, biotite and albite. It is common in marbles, associated with garnet, diopside, vesuvianite and calcite.

In igneous rocks it occurs in granitoids. May occur detrital, both in placers and in sedimentary rocks (immature sandstones).

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1.715 – 1.751   nβ: 1.725 – 1.784    nγ: 1.734 – 1.797

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism: Pale greenish-yellow with pleochroism:

X = colorless, pale green, pale yellow;

Y = greenish yellow, greenish brown;

Z = pale green, yellow-green.

Pleochroism can be only barely noticeable.

Relief: High, increases with increasing Fe content.

Cleavage: {001} perfect, {100} imperfect. Perfect cleavage in one direction.

Habits: Aggregates of elongated to acicular crystals, parallel to the b axis, prismatic of pseudohexagonal or rhombic sections. May form divergent radial or anhedral granular aggregates. Also fibrous.

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: High birefringence, 2nd – 3rd order (0.014 – 0.045), increases with increasing Fe content. Light, intense and varied anomalous colors: yellow, green, red, blue.

Individual grains may show various colors in concentric rings (Fe-poor nuclei and Fe-rich edges) or in irregular bands and patches.

Extinction: Parallel to cleavage in prismatic sections (anomalous up to 14º).

Oblique to cleavage in pseudohexagonal sections.

Elongation sign: ES(+) or ES(-), depends on section and chemical composition. Zoned crystals may show zones with different elongation signs.

Twins: Polysynthetic, rare, with heart-shaped sections.

Zoning: Frequently zoned.

CONVERGENT LIGHT

Character: B(-)

2V angle: 64-84º

Alterations: epidote is very resistant, but can alter to chlorite. It is a product of plagioclase alteration through the saussuritization process.

May be confused with: diagnostic are the intense (anomalous) irregularly distributed interference colors.

Clinozoisite show parallel extinction and is B(+).

Olivine is similar but shows no cleavage.

Diopside and augite have more oblique extinction.

Zoisite has parallel extinction in any orientation and lower birefringence (gray colors).

Piemontite is pleochroic in orange.

Monazite is B(+).

 

Epidote 01: in PPL, epidote grain with high relief, a faint greenish color and a very faint pleochroism. Cleavage is present, but irregularly distributed. Around the grain, quartz and feldspars.
Epidote 01: now in XPL the same grain as of the previous image. Very colorful, intense and vivid colors are typical for epidote. The distribution of colors is very irregular, zoning is not common.
Epidote 02: in PPL, spray of epidote crystals. This is a common habit of epidote: long prismatic crystals that irradiate from a point and tend to form a fan.
Epidote 02: now in XPL, the same epidote fan of the previous image. Very colorful, with irregularly distributed colors. It is difficult to make thin sections of such epidote fans, because they are very splintery; holes in the thin section are common, as with kyanite.
Cataclastic zones with abundant epidote are common in granitic regions, forming rocks that macroscopically show an intense green color. Under a microscope, epidote in these occurences form very small grains. In XPL, it looks like colored dust and diagnostic features are almost impossible to see.
Epidote can form by alteration of biotite. The image, in PPL, is as follows: at the top of the image, colorless feldspar, “hazy” due to alteration to clay minerals. Biotite remains are in brown. Chlorite in light green. Colorless calcite (below) Epidote in citrus green. It has very clear pleochroism and very high relief.
Epidote 03: Epidote can alter to chlorite, as in this case here, in a rhyolite with granophyric texture. In PPL the epidote grain shows high relief with a “crown” (above it) of green chlorite, with lower relief.
Epidote 03: now in XPL, the same sections as in the previous image. Epidote is very colorful and chlorite shows anomalous gray-blue interference colors. Quartz and feldspars in gray/white.
Epidote can exhibit a number of habits. In this case, in a rhyolite with granophyric texture, some aggregates develop fan shapes that are characteristic for clinozosite and especially prehnite, but paragenesis indicates the presence of epidote.
In PPL, left in the image, long prismatic crystals of epidote with their typical intense interference colors. At right, rhombic basal section (almost square) with pale interference color (yellow). In gray, quartz.
Epidote 04: in the center of the image, in PPL, a large radial cluster of long prismatic crystals of epidote. When epidote achieves optimal development, it typically forms these aggregates which end up being a typical and diagnostic habit. In PPL the blank areas are holes, because the epidote splinters rather than wears out during the making of the thin section, making it difficult to make an epidote sheet without holes.
Epidote 04: the same sction of the previous image, now in XPL. Epidote with strong colors and, due to its straight extinction and the radial arrangment of the crystals, a static black cross is seen when rotating the stage.
In XPL, radial aggregate of long prismatic crystals of epidote, showing the “black cross” due to the parallel extinction of the epidote.
In XPL, several horizontally aligned radial aggregates of long prismatic crystals of epidote, each one with its “black cross” due to the parallel extinction of the epidote.
Sandstone seen in XPL with quartz and feldspar grains (white, gray, etc.) and several well-rounded epidote detrital grains (very colorful).
In XPL, in a hydrothermal vein with quartz (white, gray), epidote (colored) showing the rhombic basal section, a feature that is more difficult to find. Generally, epidotes are granular.
At XPL, prismatic epidote crystals (colored) growing perpendicularly into a vein, later filled with quartz (gray).
In the center of the image, in XPL, altered (metamict) allanite, which typically develops an epidote corona around it.

5. Reflected Light Microscopy

Reflected light microscopy is not the recommended analytical method for the identification of epidote. However, it is important to make a polished thin section or a polished section to identify the opaque minerals that occur associated with epidote.

Sample preparation: epidote, when in very small crystals, acquires a very good polishing quite easily. When in large and well-developed crystals, on the other hand, it acquires a low-quality polish due to its excellent cleavage. While quartz and feldspars are already well polished, epidote has many holes (polishing pits) but no polishing grooves.

PLANE POLARIZED LIGHT – PPL

Reflection color: When in microcrystals aggregates it presents the same gray color as quartz and feldspar. When in large crystals it shows a light gray color, almost white.

Pleochroism: No.

Reflectivity: Small crystals have low reflectivity (<10%). Large crystals show higher reflectivity (>10%).

Bireflectance: No.

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy:  No anisotropy.

Internal reflections: Widespread in pale green tones.

May be confused with: actinolite, which shows similar internal reflections and habit when in well-developed crystals.

Epidote 01: In the center of the image, a prismatic epidote crystal, with orthoclase around it. In PPL, these large epidote crystals (there is another crystal on the top left) show its very poor polishing.
Epidote 01: the same crystals as in the previous image, now in XPL. Green internal reflections characterize this epidote. Around it, orthoclase.
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