Earth Materials Lecture 9: Carbonates and Sulphates

Question 1

What is a carbonate mineral

Answer 1

A carbonate mineral is a mineral containing a carbonate ion

Question 2

List the carbonates

Answer 2

Trigonal structures:
Calcite CaCO3
Magnesite MgCO3
Dolomite Ca,Mg(CO3)2
Rhodochrosite MnCO3
Siderite FeCO3
Ankerite Ca(Fe,Mg,Mn)(CO3)2
Smithsonite ZnCO3

Orthorhombic structures:
Aragonite CaCO3
Strontianite SrCO3
Witherite BaCO3
Cerussite PbCO3

Monoclinic structures:
Malachite Cu2(OH)2CO3
Azurite Cu2(OH)2(CO3)2

Question 3

What is the most notable carbonate polymorph?

Answer 3

Calcite and Aragonite

Calcite - CaCO3 (Trigonal)
Aragonite - CaCO3 (Orthorhombic)

Question 4

Describe the solid solution between Calcite, Magnesite and Siderite

Answer 4

At the Ca (Calcium) end there is Calcite - CaCO3³

At the Mg (Magnesium) end there is Magnetite - MgCO³

At the Fe (Iron) end there is Siderite - FeCO³

Between Calcite and Magnesium there is Dolomite - Ca,Mg (CO³)²

Between Calcite and Siderite there is
Ankerite - Ca,Mg,Fe,Mn (CO³)²

Question 5

Describe the structure of Calcite crystals

Answer 5

Rhombohedron shape

True unit cell:
Retains full
symmetry whilst
giving simplest
representation of
the crystal structure

Question 6

Describe the crystal structure of Dolomite

Answer 6

trigonal-rhombohedral, with a face-centered rhombohedral cell

Very similar structure to Calcite

Question 7

Describe the physical properties of a Dolomite crystal

Answer 7

Curved faces common
on rhombic crystals in
the trigonal carbonates, Looks like a saddle.
E.g. “saddle dolomite”.

Lamellar twins common, but on
different planes.

Cleavage perfect on (10T1)

Question 8

Describe the optical properties of the trigonal carbonates

Answer 8

Uniaxial so no OAP. All uniaxial –ve.

Extreme birefringence attributed to strong polarizing effects of planar CO3 groups.

Increases with increasing Fe.

Question 9

Describe the environments/environmental conditions in which carbonates occur

Answer 9

All manner of igneous, metamorphic and sedimentary
rocks and some fossils; pearls (aragonite).

• Primary component of carbonatites, kimberlites,
  marbles; skarns.
• Secondary alteration products in igneous rocks –
  plagioclase, augite.
• Mostly sedimentary as primary deposit or cement.
• Hydrothermal/ore deposits as gangue mineral.
• Seawater precipitates – aragonite.

Question 10

What are sulphates?

Answer 10

Sulphates are minerals containing the sulphate ion.

Question 11

List the Sulphates

Answer 11

Orthorhombic structure:
Barite BaSO4
Celestine SrSO4
Anglesite PbSO4
Anhydrite CaSO4

Monoclinic structure:
Gypsum CaSO4.2H2O

Question 12

Describe the chemical composition and structure of Gypsum

Answer 12

CaSO4.2H2O

Monoclinic
Pairs of adjacent layers of
SO4 tetrahedra and CaO8 polyhedra parallel to (010).

H2O occurs between successive layers hydrogen-bonded to O of the CaO8

Weaker H-bonds leads to perfect cleavage
.

Question 13

Describe the opticale properties of gypsum

Answer 13

Usually tabular selenite(long cuboid shape);
satin spar is fibrous along the c-axis.

Colourless, low –ve relief.

The OAP is (010) – side of crystal.

Birefringence is very low, 1st order.

Question 14

Describe the optical properties of Anhydrite

Answer 14

Orthorhombic, not quite tetragonal.

Various habits; Tabular, acicular, fibrous.

Colourless in PPL.

Moderate to high relief.

Cleavage on {010} perfect, others weak.

Birefringence 3rd order blue.

The OAP is (100). Twins on (011).

Extinction will be straight

Question 15

Describe the occurrence of sulphates as a whole

Answer 15

As precipitates from saline water:
➢ Calcium carbonate CaCO3 (aragonite).
➢ Gypsum CaSO4
.2H2O / anhydrite CaSO4
.
➢ Gypsum dehydrates to anhydrite on heating to 90oC in air or
44oC in pure water (can produce the intermediate
bassanite).
* Sedimentary rocks, hydrothermal veins (ores), hot
springs; caves.