Epidote - 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:
Color: pale yellow, pale green
Form: fine to coarse aggregate are common
Cleavage: basal cleavage (001) perfect
Interference colors: mild to strong birefringence. the colors often vary within the same crystal (harlequin colors)
Relief: high

Bibliography

• Cox et al. (1979): The Interpretation of Igneous Rocks, George Allen and Unwin, London.
• Howie, R. A., Zussman, J., & Deer, W. (1992). An introduction to the rock-forming minerals (p. 696). Longman.
• Le Maitre, R. W., Streckeisen, A., Zanettin, B., Le Bas, M. J., Bonin, B., Bateman, P., & Lameyre, J. (2002). Igneous rocks. A classification and glossary of terms, 2. Cambridge University Press.
• Middlemost, E. A. (1986). Magmas and magmatic rocks: an introduction to igneous petrology.
• 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.

Photo
Epidote and chlorite crystals in an altered trachyte. PPL image, 10x (Field of view = 2mm)	Epidote and chlorite crystals in an altered trachyte. PPL image, 10x (Field of view = 2mm)	Epidote and chlorite crystals in an altered trachyte. PPL image, 10x (Field of view = 2mm)
Epidote and chlorite crystals in an altered trachyte. PPL image, 10x (Field of view = 2mm)	Epidote and chlorite crystals in an altered trachyte. PPL image, 10x (Field of view = 2mm)	Epidote and chlorite crystals in an altered trachyte. PPL image, 10x (Field of view = 2mm)
Epidote and chlorite crystals in an altered trachyte. PPL image, 10x (Field of view = 2mm)	Epidote and chlorite crystals in an altered trachyte. PPL image, 10x (Field of view = 2mm)	Epidote and chlorite crystals in an altered trachyte. PPL image, 10x (Field of view = 2mm)
Epidote and chlorite crystals filling an amygdule. PPL image, 2x (Field of view = 7mm)	Epidote and chlorite crystals filling an amygdule. XPL image, 2x (Field of view = 7mm)	Epidote crystals filling an amygdule. PPL image, 2x (Field of view = 7mm)
Epidote crystals filling an amygdule. PPL image, 2x (Field of view = 7mm)	Epidote crystals filling an amygdule. PPL image, 2x (Field of view = 7mm)	Epidote crystals filling an amygdule. PPL image, 2x (Field of view = 7mm)
Epidote crystals filling an amygdule. XPL image, 2x (Field of view = 7mm)	Epidote crystals filling an amygdule. XPL image, 2x (Field of view = 7mm)	Epidote crystals filling an amygdule. XPL image, 2x (Field of view = 7mm)
Epidote crystals filling an amygdule. XPL image, 2x (Field of view = 7mm)	Epidote crystals filling an amygdule. XPL image, 2x (Field of view = 7mm)	Epidote crystals filling an amygdule. XPL image, 2x (Field of view = 7mm)
Epidote crystals filling an amygdule. XPL image, 10x (Field of view = 2mm)	Epidote crystals filling an amygdule. XPL image, 2x (Field of view = 7mm)	Epidote crystals filling an amygdule. XPL image, 2x (Field of view = 7mm)
Plagioclase altered by epidote. PPL image, 2x (Field of view = 7mm)	Plagioclase altered by epidote. XPL image, 2x (Field of view = 7mm)