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 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:
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



• Deer, W.A., Howie, R.A., Zussman, J. (1998) Rock-forming Minerals.
• Optical Mineralogy : The Nonopaque Minerals by Phillips / Griffen
• E. WM. Heinrich (1956): Microscopic Petrografy. Mcgraw-hill book company,inc
• Nesse, William D., 1986, Introduction to Optical Mineralogy: New York, Oxford University Press

Photo
epidotialterazionetrachite(2).jpg

Epidote and Chlorite crystals in an altered Trachyte. PPL image, 10x (Field of view = 2mm)
epidotialterazionetrachite(3).jpg

Epidote and Chlorite crystals in an altered Trachyte. PPL image, 10x (Field of view = 2mm)
epidotialterazionetrachite(4).jpg

Epidote and Chlorite crystals in an altered Trachyte. PPL image, 10x (Field of view = 2mm)
epidotialterazionetrachite(5).jpg

Epidote and Chlorite crystals in an altered Trachyte. PPL image, 10x (Field of view = 2mm)
epidotialterazionetrachite(6).jpg

Epidote and Chlorite crystals in an altered Trachyte. PPL image, 10x (Field of view = 2mm)
epidotialterazionetrachite(7).jpg

Epidote and Chlorite crystals in an altered Trachyte. PPL image, 10x (Field of view = 2mm)
epidotialterazionetrachite(8).jpg

Epidote and Chlorite crystals in an altered Trachyte. PPL image, 10x (Field of view = 2mm)
epidotialterazionetrachite(10).jpg

Epidote and Chlorite crystals in an altered Trachyte. PPL image, 10x (Field of view = 2mm)
epidotialterazionetrachite(9).jpg

Epidote and Chlorite crystals in an altered Trachyte. PPL image, 10x (Field of view = 2mm)
epidotivescicole(3).jpg

Epidote and Chlorite crystals filling an Amygdule. PPL image, 2x (Field of view = 7mm)
epidotivescicole(2).jpg

Epidote and Chlorite crystals filling an Amygdule. XPL image, 2x (Field of view = 7mm)
epidotivescicole(4).jpg

Epidote crystals filling an Amygdule. PPL image, 2x (Field of view = 7mm)
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Epidote crystals filling an Amygdule. PPL image, 2x (Field of view = 7mm)
epidotivescicole(10).jpg

Epidote crystals filling an Amygdule. PPL image, 2x (Field of view = 7mm)
epidotivescicole(7).jpg

Epidote crystals filling an Amygdule. PPL image, 2x (Field of view = 7mm)
epidotivescicole(6).jpg

Epidote crystals filling an Amygdule. XPL image, 2x (Field of view = 7mm)
epidotivescicole(8).jpg

Epidote crystals filling an Amygdule. XPL image, 2x (Field of view = 7mm)
epidotivescicole(9).jpg

Epidote crystals filling an Amygdule. XPL image, 2x (Field of view = 7mm)
epidotivescicole(14).jpg

Epidote crystals filling an Amygdule. XPL image, 2x (Field of view = 7mm)
epidotivescicole(17).jpg

Epidote crystals filling an Amygdule. XPL image, 2x (Field of view = 7mm)
epidotivescicole(18).jpg

Epidote crystals filling an Amygdule. XPL image, 2x (Field of view = 7mm)
epidotivescicole(19).jpg

Epidote crystals filling an Amygdule. XPL image, 10x (Field of view = 2mm)
epidotivescicole(22).jpg

Epidote crystals filling an Amygdule. XPL image, 2x (Field of view = 7mm)
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Epidote crystals filling an Amygdule. XPL image, 2x (Field of view = 7mm)
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Plagioclase altered by Epidote. PPL image, 2x (Field of view = 7mm)
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Plagioclase altered by Epidote. XPL image, 2x (Field of view = 7mm)