Epidote group

EPIDOTE GROUP

Presented ByAbhilasha Konwar.Dept. Of Applied GeologyDibrugarh University

CONTENTS

INTRODUCTION. CHEMICAL FORMULA. COMPOSITION. ENVIRONMENT. ATOMIC STRUCTURE. EPIDOTE GROUP OF MINERALS. PHYSICAL PROPERTIES. OPTICAL PROPERTIES. CRYSTALLIZATION. IDENTIFYING PROPERTIES OF

MINERALS. ASSOCIATION. PARAGENESIS. REFERENCES.

- Origin from the greek word “epididonai”, meaning “increase,” referring to the base of an epidote prism,one side of which longer than the other.

-Epidote forms a group of related minerals, with the mineral Epidote being the most prominent member of the group.

-Two uncommon members of the Epidote group are Epidote-(Pb) (which is more popularly known as Hancockite), and Epidote-(Sr).

-It is a sorosilicate group of minerals that contain a significant amount of rare earth elements. belongs to an isomorphic group & an important rock forming mineral.

INTRODUCTION

• Common in regional metamorphism (epidote-ampibolite facies).

• Isostructural group with some solid solution.

Contains both Si04 and Si207 groupsGeneral formula is X2

VIIIY3VIZ3 O12 OH

X = Ca2+, Na+ ,LREE,Y, Th,Y = A13+, Fe3+, Mn3+,

Cr3+,Ti,Mn2+,Fe2+.

Z =Si

Sorosilicate: Epidote Group:

COMPOSITION:Molecular Weight = 519.30 gm.

• Calcium :-   15.44 %  Ca   21.60 % CaO.• Aluminum:- 3.90 %  Al    7.36 % Al2O3.• Iron:-   

 24.20 %  Fe   34.60 % Fe2O3.• Silicon:-    16.22 %  Si   34.71 % SiO2.• Hydrogen:- 0.19 %  H     1.73 % H2O.• Oxygen:-    40.05 %  O.

100.00 % = TOTAL OXIDE.ENVIRONMENT:Epidote occurs in several different environments. It is

found in hornfels and skarns in contact metamorphic rocks, and in metamorphosed limestone and in schists in regional metamorphic rock. It is also found in igneous rock in basalt and diabase, and is occasionally found in granite pegmatites.

Si2O7 isolated double Tetrahedra.

SiO4 isolated singleTetrahedra.

AlO6 isolated singleOctahedra.

AlO4(OH)2 isolated singleOctahedra.

ATOMIC STRUCTUREEpidote: Ca2(A1,Fe)A12O(Si2O7)(SiO4)(OH)

EPIDOTE- Group of Minerals.

• Zoisite- Ca2Al3O(SiO4)(Si2O7)(OH).• Piedmontite-Ca2(Al,Mn3+,Fe3+)3(SiO4)(Si2O7)O(OH). • Allanite- Ca(Ce,La,Y,Ca)Al2(Fe2+,Fe3+)(SiO4)(Si2O7)O(OH).

• Clinozoisite- Ca2Al3SiO4Si2O7O(OH).• Pumpellyite-Ca2Al2(Mg,Fe2+,Fe3+,Al)(SiO4)(Si2O7)(OH)

(H2O,OH).• Lawsonite-CaAl2Si2O7(OH) 2H2O.

Cleavage: - {001} Perfect Color: Yellowish green, Brownish green, Black,

Yellow, Gray. Density: 3.3 - 3.6, Average = 3.45

Diaphaneity: Transparent to translucent to opaque Fracture: Regular - Flat surfaces (not cleavage)

fractured in a regular pattern. Habit:

Fibrous - Crystals made up of fibers. Massive - Uniformly indistinguishable crystals forming large masses. Prismatic - Crystals Shaped like Slender Prisms (e.g. tourmaline).

Hardness: 7 - Quartz Luminescence: Non-fluorescent.

Luster: Vitreous (Glassy) Streak: Grayish white

PHYSICAL PROPERTIES:

OPTICAL PROPERTIES:Color: Usually colorless but may be light green or

pink.Relief High.

Optic sign Biaxial(-ve).Pleochroism(x) Colorless, pale green, or pale yellow. Pleochroism(y) Greenish yellow or greenish brown

Extinction Parallel in elongate sections.Interference

color:Third order interference color as Fe content increases. Football shaped grains with concentric interference color rings are diagnostic.

Twinning Twins with (100) as twin plane are not uncommon.

Birefringence Strong, (0.0130-0.0460) .Zoisite – Orthorhombic.Clinozoisite and Epidote – Monoclinic.

CRYSTALLIATION:

Samples of Epidote in thin section.

●Color: Usually Colorless (Thin section), Greenish red to brown.●Form: Columnar aggregates.●Cleavage: Perfect One Direction (010).●Relief: High > balsam. ( 0.006-0.018).●Birefringence: Weak to moderate.●(Deep Blue).●Extinction: Parallel in most sections.●Orientation: Crystals (length-fast)●Twinning: Polysynthetic twinning●Occurrence: Rare mineral found in metamorphic rocks.●Interference Figure:-Biaxial positive with moderate axial angle.

ZOISITE

●Color: Yellow to Orange●Form: Same as Epidote. Granular or Columnar.●Cleavage: One direction {001}●Relief: High, > balsam. ●Birefringence: Very Strong. 0.061-0.082●Extinction: Parallel in elongated sections.●Orientation: The ray is difficult to determine.●Interference Figure: Biaxial (+).●Occurrence: Occurs in most part of schists and gneisses.

PIEDMONTITE

●Color: Colorless, green, pale brown, or yellow.●Form: Occurs in well formed crystals.●Cleavage: {001}, {100} moderate.●Relief: Rather high. > balsam.●Birefringence: Moderate.●Extinction: 22° On {100} cleavage.●Orientation: Parallel.●Twinning: Cruciform twins.●Occurrence: Occurs in mica schists.●Interference figure: Biaxial (+).

●Color: Colorless (Thin section). Grey green color. ●Form: Columnar, Elongated Crystals.●Cleavage: Perfect.●Relief: High.●Birefringence: Weak to rather weak. ●Extinction: Parallel.●Orientation: Some sections are length slow and length-fast. Interference Figure: Biaxial (+)●Twinning: Polysynthetic twinning●Occurrence: Product of dynamic metamorphism

CLINOZOISITE

PUMPELLYITE

●Color: Colorless (Thin section). bluish to pinkish gray. ●Form: Occurs in Euhedral crystals in varying habit.●Cleavage: Parallel.●Relief: Rather high. > balsam.●Birefringence: Moderate-0.019 (Blue)●Extinction: Parallel or symmetrical.●Orientation: Rhombic sections parallel to the slower ray.●Twinning: Polysynthetic twinning. ●Occurrence: Found in metamorphic rocks.

LAWSONITE

ALLANITE

●Color: Brown or Black.●Form: Parallel. Cerium-bearing variety.●Cleavage: Imperfect parallel to {001}.●Relief: High > balsam. ●Birefringence: Rather strong.●Extinction: Parallel.●Orientation: Difficult to obtain.●Twinning: Like Epidote.●Occurrence: Found in Granites, Syenites, and gneisses.

ASSOCIATIONEpidote is a common and widespread mineral,

characteristic of low-to –medium-grade metabasites and marbles.Associated minerals include- actinolite, chlorite and albite

in mafic rocks and diopside, grossular, and vesuvianite in marbles.Epidote is also produced by alteration of feldspar,

pyroxene, and amphibole.

PARAGENESIS• Epidote is considered as metamorphic stress mineral and is very

common in schists, slates and phyllites of low to medium grade regional metamorphism. It is to be expected to derived from limestones containing argillaceous impurities.

• In metamorphic contact zones and zones of metasomatism in limestones hosts.

• Ca-metasomatism may introduce epidote in fissures and veins.• Epidote and clino-zoisite are locally important as detrital grain in

sediments.• In igneous rocks epidote may crystallize in mafic rocks, or in

granites due to Ca assimilation. It occurs in vugs and vesicles of basalts and other volcanics or it may replace ferromagnesian minerals in granite.

REFERENCES1. OPTICAL MINERALOGY BY Paul F. Kerr.2. ROCK FORMING MINERALS By

Zussman.3. MINERALOGY By Dexter Perkins.4. OPTICAL MINERALOGY By Wm.Revell

Phillips and Dana T.Griffen.

THANK YOU.