Calcite & Dolomite-Vishnuraayan. G.

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Calcite – Introduction

• Most common and widespread mineral on or near the Earth’s surface.

• Only stable form is CaCO3

• Principal constituent of Sedimentary limestones

• Occurs in carbonate shells as fine precipitates and as clastic materials.

• Color : Colorless or white, sometimes grey, yellow, blue, red, brown

• Luster : Vitreous

• Diaphaneity : Transparentto opaque

• Streak : White Streak

• Crystal Shape : Triagonal• Cleavage : Rhombohedral

• Fracture : Conchoidal but rarely visible due to perfect cleavage

• Hardness : 3.0

• Specific Gravity : 2.715(if pure) to 2.94• Reaction with HCl

powdered + dil. HClBrisk effervescence proving presence of carbonate group

Optical Properties

• no = 1.658

• ne = 1.486

• no – ne = 0.172 = δ• Maximum Birefringence

with fourth order orhigher colors.

• Relief : Moderate but large variations are seen

Under PPL Under XPL

• Interference FiguresA normal negative uniaxialinterference figure is seen.

• Lamellar Twinning

Uses of Calcite

• Many different uses according to its purity and character.

• Marbles and crystalline limestones – ornamental stones.

• Calcium Carbonate is used as a flux in smelting• Certain variety of calcium carbonate is used in

printing• Lime neutralize the natural acids in the soil.

Dolomite – Introduction

• Occurs in extensive beds at many geological horizons

• Dolomite may be deposited directly from sea water, but most dolomite beds have been formed by the alteration of limestones, the calcite of which is replaced by dolomite.

• Chemical Composition : (CaMg)(CO3)2

• Color : White, yellowish, brown, and sometimes red, green, or black

• Luster : Vitreous to pearly of crystals; dull of massive varieties.

• Diaphaneity : Translucent to opaque• Streak : White• Crystal Shape : Triagonal

Presence of Mg reduces the symmetry

• Cleavage : Rhombohedral• Fracture : Conchoidal

or Uneven• Hardness : 3.5 to 4.0• Specific Gravity : 2.86• Reaction with HCl

powdered + dil. HClDoes not rapidly dissolve or effervesce.

Optical Properties

• no = 1.679

• ne = 1.500

• no – ne = 0.179 = δ (nearly equal to that of calcite)

• Maximum Birefringence with fourth order or higher colors.

• Relief : Low to moderate or high• Interference Figures : A normal negative

uniaxial interference figure is seen.• Glide Twin lamellae seen on many sections

Uses of Dolomite

• Important Building material.• Making of Refractory furnace lining and

source of carbon dioxide,

How do we differentiate Calcite & Dolomite

• Dolomite differs from calcite because of the presence of MAGNESIUM.

• Calcite reacts quickly with acids and produce carbon dioxide bubbles. But dolomite, weakly reacts with acids producing bubbles very slowly.

• Dolomite is slightly harder and denser than calcite.• Calcites form scalenohedrons but dolomites never

form scalenohedrons.

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