Metamorphic Minerals

Texture and Microstructure

Metamorphic Rocks

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 O^2- 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.

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

Metamorphic Minerals

Texture and Microstructure

Metamorphic Rocks

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

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