Download Carbonate Petrography (2).

Carbonate Petrography
GEO 431
Lab (2
(2)
Mineralogy of Carbonate Sediments
Prepared by:
Mansour Al-Hashim
Preview of Lab 1

General classification of sedimentary rocks

Simple classification of carbonate rocks

Some facts about carbonate rocks

Techniques for examining carbonate rocks
Objectives of Lab 2

Introduction to carbonate minerals

Studying the main three carbonate groups

Studying the three most important carbonate
minerals in some details

Distinguishing between calcite, aragonite, and
dolomite under the microscope (staining)
Carbonate Minerals



1.
2.
3.

Carbonate minerals contain the CO3 anion
CO3 combines with cations (Ca, Mg, Fe, Mn, and Zn)
to form most carbonate minerals
The common carbonate minerals fall into three groups:
Calcite group
Dolomite group
Aragonite group
Calcite, dolomite, and aragonite are the only
volumetrically important minerals in carbonate rocks
Carbonate Minerals

Calcite Group (Hexagonal)
The most common minerals of this group are:
1. Calcite CaCO3
2. Magnesite MgCO3
3. Siderite FeCO3
4. Rhodochrosite MnCO3
5. Smithsonite ZnCO3
Calcite CaCO3

Properties:

Crystal system: hexagonal

Cleavage: perfect rhombohedral

Color: usually white/colorless in hand specimens and
colourless in plane polarized light

Moderate to high negative relief
Calcite CaCO3
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Properties (continued):

High--order interference colors, commonly white
High

Reacts strongly and readily with cold, dilute HCl

Depending on the magnesium content, there are two
types of calcite: lowlow-Mg calcite and highhigh-Mg calcite

Dominant mineral of limestones in rocks older than the
Tertiary
Calcite under the microscope (PPL)
Calcite under the microscope (XPL)
Carbonate Minerals

Aragonite Group (Orthorhombic)
The most common minerals of this group are:
1. Aragonite CaCO3
2. Strontianite SrCO3
3. Cerussite PbCO3
Aragonite CaCO3

Characteristics:

Crystal system: orthorhombic

Cleavage: lacks the rhombohedral cleavage

Color: colorless, white, pale yellow, variously tinted in
hand specimens and colourless in thin sections

Relief changes with rotation

Effervesces vigorously in cold, dilute HCl
Aragonite CaCO3

Characteristics (continued):

High--order interference colors, commonly white
High

Alters readily to calcite and may be replaced by
dolomite or other minerals

Has a very low Mg content

Important in CenozoicCenozoic-age and modern carbonate
rocks and sediments

Fibrous (acicular or needle-like) crystals
Acicular aragonite under the microscope
From Scholle & Ulmer-Scholle (2003)
Acicular aragonite under the microscope
From Scholle & UlmerUlmer-Scholle (2003
(2003))
Acicular aragonite under the microscope
From Scholle & UlmerUlmer-Scholle (2003
(2003))
Acicular aragonite under the microscope
From Scholle & UlmerUlmer-Scholle (2003
(2003))
Carbonate Minerals

Dolomite Group (Hexagonal)
The most common minerals of this group are:
1. Dolomite CaMg(CO3)2
2. Ankerite CaFe(CO3)2
Dolomite CaMg(CO3)2

Properties:

Crystal system: hexagonal

Cleavage: perfect rhombohedral

Color: pink, white, gray, green, brown, black, or colorless

Dolomite crystals are usually subhedral to euhedral, with
planar boundaries.

Best identified through staining, and by its rhombic, often
zoned, crystals.
Dolomite CaMg(CO3)2

Properties (continued):

Only reacts with hot HCl or if powdered

Colourless in plane polarized light

Relief changes with rotation

High--order white or grey interference colors
High

Dominant mineral of dolomites

Commonly forms euhedral rhombs
Dolomite under the microscope
From Scholle & UlmerUlmer-Scholle (2003
(2003))
Dolomite under the microscope
From Scholle & UlmerUlmer-Scholle (2003
(2003))
Dolomite under the microscope
From Scholle & UlmerUlmer-Scholle (2003
(2003))
Dolomite under the microscope
From Scholle & UlmerUlmer-Scholle (2003
(2003))
Distinguishing between Carbonate
Minerals

Visual differentiation between carbonate minerals is
fundamental to studies of carbonate petrography, correlation,
and genesis.

It is difficult to distinguish between calcite, aragonite, and
dolomite in hand specimens and thin sections.

There is difficulty because their physical characteristics are
very similar.

Identification of these minerals is aided by etching and
staining techniques.
Staining (1
(1)
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Staining methods have been used in carbonate
analysis since 1887 (Lemberg).

Provides a rapid means of recognizing the
textural and compositional differences in the
rock.

Provides a means of establishing the identity of
the carbonate minerals under study.
Staining (2
(2 )
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Doesn’t require sophisticated laboratory facilities
Doesn’
and can be applied to cores and cuttings.

For identification of carbonate minerals,
alizarine red S and Harris' hematoxylin stains
are recommended because they are fast,
efficient, and dependable.
Distinguishing between Calcite and
Dolomite
Step 1
1) Etch with 1.5% HCl for 10 to 15 sec.
2) Wash with distilled water
 Result:
- Calcite and ferroan calcite show considerable
etch
- Dolomite and ferroan dolomite show negligible
etch.

Distinguishing between Calcite and
Dolomite

1)
2)
3)
4)
Step 2
Dissolve 0.2 g Alizarin red S in 100 cc of 1.5%
HCl acid (ARS)
Dissolve 2 g potassium ferricyanide in 100 cc
of 1.5% HCl acid (PF)
Mix ARS with PF in the ratio ARS:PF = 3:2
Stain for 30 to 45 sec.
Distinguishing between Calcite and
Dolomite

Result of step 2:
-
Calcite is very pale pink to red
-
Ferroan calcite is very pale pinkpink-red and pale
blue--dark blue
blue
-
Dolomite takes no stain
-
Ferroan dolomite becomes deep turquoise
Distinguishing between Calcite and
Dolomite
Step 3
- Stain with ARS solution for 10 to 15 sec.
 Result:
- Calcite and ferroan calcite become very pale
pink-red
- Dolomite and ferroan dolomite take no color
(This method is after Dickson, 1965)

The End