Epidote pistacite - Ca2(Fe,Al)3(SiO4)3(OH)

Named in 1801 by Rene Just Hauy from the Greek "Epidosis" = "increase" because the base of the rhombohedral prism has one side larger than the other.

The Epidote structure consists of octahedral AlO6 groups sharing edges to form continuous chain parallel to b axis. In one-third of the octahedral groups, Al3+ is surrounded by four O2- and two (OH)-. The octahedral chains are linked by silicon, in SiO4 and SiO7 groups, sharing oxygen ions with aluminum. Larger Ca2+ ions fill interchain voids to produce electrostatic neutrality. The resulting basic composition is Ca2Al3O(SiO4)(SiO7)OH and the structure is dimorphous, as orthorhombic and monoclinic forms. Significant Al3+ may be replaced by Fe3+, Mn3+, or even Cr3+, and Ca2+ may be partly replaced by Ce3+, La3+, Y3+ and other rare earth elements with equal replacement of Fe3+ and Al3+ by Fe2+ to maintain electrostatic balance.

Orthorhombic varieties:

♦ Zoisite Ca2Al3O(SiO4)(SiO7)OH
♦ Thulite Ca2(Al,Fe3+, Mn3+)3O(SiO4)(SiO7)OH

Monoclinic varieties:

♦ Clinozoisite Ca2Al3O(SiO4)(SiO7)OH
♦ Pistacite Ca2Fe3+Al2O(SiO4)(SiO7)OH
♦ PiemontiteCa2(Al,Fe3+,Mn3+)3O(SiO4)(SiO7)OH
♦ Allanite (Ca,Ce,La)2(Al,Fe3+,Fe2+)3O(SiO4)(SiO7)OHOH
♦ Tawmawite Ca2(Al,Fe3+,Cr3+)3O(SiO4)(SiO7)OH
♦ Hancockite (Ca,Pb,Sr)2(Al,Fe3+)3O(SiO4)(SiO7)OH

The common epidote minerals are formed largely under conditions of low to medium grade metamorphism in association with Ca-poor plagioclase. At higher grades, epidote combines with albite to form Ca-rich plagioclase.

Optical properties:
Form: Fine to coarse aggregate are common
Color: Pale yellow, pale green
Relief: High.
Cleavage: Basal cleavage (001) perfect
Interference colors: Strong birefringence. the colors often vary within the same crystal (harlequin colors)


Bibliography



• Bucher, K., & Grapes, R. (2011). Petrogenesis of metamorphic rocks. Springer Science & Business Media.
• Fossen, H. (2016). Structural geology. Cambridge University Press.
• Howie, R. A., Zussman, J., & Deer, W. (1992). An introduction to the rock-forming minerals (p. 696). Longman.
• Passchier, Cees W., Trouw, Rudolph A. J: Microtectonics (2005).
• Philpotts, A., & Ague, J. (2009). Principles of igneous and metamorphic petrology. Cambridge University Press.
• Shelley, D. (1993). Igneous and metamorphic rocks under the microscope: classification, textures, microstructures and mineral preferred-orientations.
• Vernon, R. H. & Clarke, G. L. (2008): Principles of Metamorphic Petrology. Cambridge University Press.
• Vernon, R. H. (2018). A practical guide to rock microstructure. Cambridge university press.


Photo
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Epidote (pistacite) crystals. PPL image, 2x (Field of view = 7mm)
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Epidote (pistacite) crystals with high interference colors. XPL image, 2x (Field of view = 7mm)
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Epidote (pistacite) crystals with high interference colors. XPL image, 2x (Field of view = 7mm)
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Epidote (pistacite) crystals with high interference colors. XPL image, 2x (Field of view = 7mm)
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Epidote (pistacite) crystals. PPL image, 2x (Field of view = 7mm)
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Epidote (pistacite) crystals. PPL image, 2x (Field of view = 7mm)
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Epidote (pistacite) crystals with high interference colors. XPL image, 2x (Field of view = 7mm)
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Epidote (pistacite) crystals. PPL image, 10x (Field of view = 2mm)
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Epidote (pistacite) crystals with high interference colors. XPL image, 10x (Field of view = 2mm)
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Epidote (pistacite) crystal. PPL image, 10x (Field of view = 2mm)
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Epidote (pistacite) crystal with high interference colors. XPL image, 10x (Field of view = 2mm)
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Epidote (pistacite) crystal with high interference colors. XPL image, 10x (Field of view = 2mm)
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Epidote (pistacite) crystal with high interference colors. XPL image, 10x (Field of view = 2mm)
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Epidote (pistacite) crystal with high interference colors. XPL image, 10x (Field of view = 2mm)
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Epidote (pistacite) crystal with high interference colors. XPL image, 10x (Field of view = 2mm)
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Epidote (pistacite) and chlorite crystals. PPL image, 10x (Field of view = 2mm)
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Epidote (pistacite) crystals with high interference colors in a Quartzite. XPL image, 10x (Field of view = 2mm)
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Epidote (pistacite) crystals with high interference colors in a Quartzite. XPL image, 10x (Field of view = 2mm)
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Epidote (pistacite) crystals. PPL image, 10x (Field of view = 2mm)
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Epidote (pistacite) crystals with high interference colors in a Quartzite. XPL image, 10x (Field of view = 2mm)
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Epidote (pistacite) crystals with high interference colors in a Quartzite. XPL image, 10x (Field of view = 2mm)
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Epidote (pistacite) crystals with high interference colors in a Quartzite. XPL image, 10x (Field of view = 2mm)
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Epidote (pistacite) crystals with high interference colors in a Quartzite. XPL image, 10x (Field of view = 2mm)
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Epidote (pistacite) crystals with high interference colors in a Quartzite. XPL image, 10x (Field of view = 2mm)
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Epidote crystals (colorless) in a Orthogneiss. PPL image, 2x (Field of view = 7mm)
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Epidote crystals (with high interference colors) in a Orthogneiss. XPL image, 2x (Field of view = 7mm)
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Epidote crystals (with high interference colors) in a Orthogneiss. XPL image, 10x (Field of view = 2mm)
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Ehuedral Epidote crystal (colorless) in a Orthogneiss. PPL image, 10x (Field of view = 2mm)
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Ehuedral Epidote crystal in a Orthogneiss. XPL image, 10x (Field of view = 2mm)
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Epidote crystal in a Orthogneiss. XPL image, 10x (Field of view = 2mm)
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Epidote crystals in a slate. XPL image, 2x (Field of view = 7mm)
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Epidote crystals in a slate. XPL image, 2x (Field of view = 7mm)
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Epidote crystals in a slate. XPL image, 2x (Field of view = 7mm)
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Pistacite crystals. XPL image, 10x (Field of view = 2mm)
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Pistacite crystals. XPL image, 10x (Field of view = 2mm)
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Pistacite crystals. PPL image, 10x (Field of view = 2mm)
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Pistacite crystals. XPL image, 10x (Field of view = 2mm)
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Pistacite crystals. PPL image, 10x (Field of view = 2mm)
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Pistacite crystals. XPL image, 10x (Field of view = 2mm)
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Pistacite crystals. PPL image, 10x (Field of view = 2mm)
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Pistacite crystals. XPL image, 10x (Field of view = 2mm)
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Pistacite crystals. XPL image, 10x (Field of view = 2mm)
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Pistacite crystals. XPL image, 10x (Field of view = 2mm)
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Pistacite crystals. XPL image, 10x (Field of view = 2mm)
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Pistacite crystals. XPL image, 10x (Field of view = 2mm)
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Pistacite crystal. Anzasca Valley, Piedmont, Italy. XPL image, 10x (Field of view = 2mm)
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Pistacite crystal. Anzasca Valley, Piedmont, Italy. XPL image, 10x (Field of view = 2mm)
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Pistacite crystal. Anzasca Valley, Piedmont, Italy. XPL image, 10x (Field of view = 2mm)
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Pistacite crystal. Anzasca Valley, Piedmont, Italy. XPL image, 10x (Field of view = 2mm)
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Pistacite and biotite crystal. Anzasca Valley, Piedmont, Italy. XPL image, 10x (Field of view = 2mm)
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Pistacite crystal. Anzasca Valley, Piedmont, Italy. XPL image, 10x (Field of view = 2mm)
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Pistacite crystal. Anzasca Valley, Piedmont, Italy. XPL image, 20x (Field of view = 1mm)
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Pistacite and white mica crystals. Anzasca Valley, Piedmont, Italy. XPL image, 20x (Field of view = 1mm)
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Pistacite crystal. Anzasca Valley, Piedmont, Italy. XPL image, 20x (Field of view = 1mm)
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Pistacite crystal. Anzasca Valley, Piedmont, Italy. XPPL image, 20x (Field of view = 1mm)
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Pistacite and white mica crystals. Anzasca Valley, Piedmont, Italy. XPL image, 20x (Field of view = 1mm)
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Pistacite crystal. Anzasca Valley, Piedmont, Italy. XPL image, 20x (Field of view = 1mm)
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Epidote, quartz (colorless) and green alkali amphiboles. PPL image, 2x (Field of view = 7mm)
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Epidote (high interference colors), quartz and alkali amphiboles. XPL image, 2x (Field of view = 7mm)
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Epidote, quartz (colorless) and green alkali amphiboles. PPL image, 2x (Field of view = 7mm)
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Epidote (high interference colors), quartz and alkali amphiboles. XPL image, 2x (Field of view = 7mm)
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Epidote (high interference colors), quartz and alkali amphiboles. XPL image, 2x (Field of view = 7mm)
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Epidote, quartz (colorless) and green alkali amphiboles. PPL image, 2x (Field of view = 7mm)
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Epidote (high interference colors), quartz and alkali amphiboles. XPL image, 2x (Field of view = 7mm)
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Epidote, quartz (colorless) and green alkali amphiboles. PPL image, 2x (Field of view = 7mm)
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Epidote (high interference colors), quartz and alkali amphiboles. XPL image, 2x (Field of view = 7mm)
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Epidote, quartz (colorless) and green alkali amphiboles. PPL image, 2x (Field of view = 7mm)
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Epidote (high interference colors), quartz and alkali amphiboles. XPL image, 2x (Field of view = 7mm)
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Epidote (high interference colors), quartz and alkali amphiboles. XPL image, 2x (Field of view = 7mm)
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Epidote, quartz (colorless) and green alkali amphiboles. PPL image, 2x (Field of view = 7mm)
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Epidote (high interference colors), quartz and alkali amphiboles. XPL image, 2x (Field of view = 7mm)
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Epidote (high interference colors), quartz and alkali amphiboles. XPL image, 2x (Field of view = 7mm)
epidoto2016(16).jpg

Epidote (high interference colors), quartz and alkali amphiboles. XPL image, 10x (Field of view = 2mm)
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Epidote (high interference colors), quartz and alkali amphiboles. XPL image, 10x (Field of view = 2mm)
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Epidote (high interference colors), quartz and alkali amphiboles. XPL image, 10x (Field of view = 2mm)
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Epidote (high interference colors), quartz and alkali amphiboles. XPL image, 10x (Field of view = 2mm)
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Epidote (high interference colors), quartz and alkali amphiboles. XPL image, 10x (Field of view = 2mm)
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Epidote (high interference colors), quartz and alkali amphiboles. XPL image, 10x (Field of view = 2mm)