EPIDOTE

This description is for epidote, however note that there exist several varieties of epidote, e.g., zoisite, clionzoisite, each of which have different optical properties and may all be present in the same thin section. Var. epidote represent the more Fe-rich compositions of the clinozoisite (Al-rich) -epidote series.


General Formula: Ca₂Fe³⁺Al₂O(Si₂O₇)(SiO₄)(OH)	Sample: GF-83-21A	System: Monoclinic

Epidote Equigranular epidote, with actinolite and hornblende within a metamorphosed mafic lithology. Field of view = 2.7 mm, plane light	Epidote Note the range of interference colours exhibited by the epidote. Field of view = 2.7 mm, crossed polars	Block diagram showing the relationship between the crystallographic axes and the indicatrix axes.

Optical Properties


Colour Pleochroism	higher Fe epidote produces a light yellow-green colour X = colourless to pale yellow or pale green Y = yellow-green Z = colourless to pale yellow-green	Form	found as anhedral grains or granular aggregates
Relief RI	high positive n_alpha = 1.715-1.751 n_beta = 1.725-1.784 n_gamma = 1.734-1.797	Cleavage	a poor cleavge on {100} is not usually seen
Birefringence Interference Colours	0.012-0.049 Maximum interference colours up to 3^rd order, depending on composition	Twinning	rare
Interference Figure Optic Sign 2V	biaxial negative 2V_X = 90-64°	Optic Orientation	optic orientation varies strongly with composition. in all cases Y = b and the optic plane is (010)
Composition	epidote is the Fe-rich end member of a continuous solid solution series with clinozoisite (Al-rich)	Alteration	no consistent alteration
Occurrence	common accessory mineral in a wide variety of regional and contact metamorphic rocks. A wide variety of igneous rocks contain epidote as a primary accessory mineral.	Distinguishing Features	epidote is distinguished from clinozoisite by optic sign, birefringence, and colour.

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