Download SGES1273 Lab Handout 4

SGES1273
Part 1: Minerals
Practical 4: Carbonates, SiO2 minerals, Corundum, Sulfates and Phosphates.
1. Carbonates
Carbonate minerals are most commonly found in sedimentary rocks. In most cases, they are
of biogenic origin, formed by organisms that extract carbon dioxide from the air and combine
it with calcium from seawater. The mineral formed by most of these organisms is aragonite,
which is unstable and changes into calcite. Calcite can be converted to dolomite and other
carbonate minerals by processes involving groundwater, which can also dissolve and
reprecipitate carbonates. Since there are so many processes which can affect the formation
and alteration of carbonate minerals, they exhibit many different habits. For instance, calcite
can be found in the form of (solidified) mud (micrite), as clear, rhombohedral crystals filling
veins and cracks, as stalactites and stalagmites in caves, and as the dense, massive rock with
tiny crystals we know as marble.
Carbonate minerals can be divided into two groups based on crystal symmetry. Calcite,
dolomite and siderite share the same crystal structure and are trigonal, so they tend to break
along three cleavage planes that result in rhombohedral fragments. Aragonite, on the other
hand, is orthorhombic. Since calcite and aragonite have the same chemical formula (CaCO3),
but different crystal structures, they are an example of polymorphism.
Dilute hydrochloric acid is used to identify carbonate minerals. The acid causes the release of
bubbles of CO2 gas. The reaction can be vigorous, as in the case of calcite, or very slow, as in
the case of siderite. Use your hand magnifier to observe the bubbles. You can speed up the
reaction by scratching the surface beforehand with a piece of quartz (which creates a fine
powder).
Exercise: Examine the following samples, and determine their properties (i.e. hardness,
cleavage planes, density, luster, reaction with acid, etc.)
131,160
39,182
77
144
Calcite
Aragonite
Dolomite
Siderite
Compare the different minerals, find similarities and differences between them. Take note,
especially, the presence or absence of rhombohedral cleavage, and the degree to which they
react with acid. Also compare their hardness and density.
Can you differentiate between calcite and aragonite? Between calcite and dolomite? Calcite
and siderite? Aragonite and dolomite?
2. SiO2 minerals
Minerals which have the chemical formula SiO2 are quartz, chalcedony, and high
pressure/temperature polymorphs such as coesite, stishovite and cristobalite, which are rarely
found.
Quartz is the most abundant mineral on the surface of the earth. It is common in igneous,
metamorphic and sedimentary rocks. As it is stable under surface conditions, it can survive
weathering and transport, and can be found in soils and altered rock. In igneous rocks, since it
crystallizes late compared to other minerals, it rarely develops crystal faces, instead growing
in spaces between other minerals. However, crystals with faces are formed by hydrothermal
solutions in voids, forming geodes. The familiar purple amethyst crystals one sees in souvenir
shops were formed in this manner. Quartz is found in many colors, and is often described by
color when bought and sold. Some colored varieties have special names (such as amethyst).
However, these names have limited geological significance. Quartz crystals are often twinned,
and have “left handed” and “right handed” forms (enantiomorphism).
Chalcedony is a microcrystalline (or cryptocrystalline) form of quartz. The crystals are so tiny
they are not visible even under a microscope. However, when analyzed using X-ray
diffraction, it gives the same peaks, and therefore has the same crystalline structure as quartz.
Chalcedony is, confusingly, known by many names – chert, flint, jasper, etc. As with quartz,
some of these names are based on color or patterns, but many are generic.
Examples:
101
191
190
103
105
106
107
Clear quartz
Milky quartz
Rose quartz
Smoky quartz
Amethyst (do not confuse this with fluorite)
Agate (banded chalcedony)
Flint (red, grey or black chalcedony)
Test these samples for hardness, examine them for fracture and cleavage (they should have
conchoidal fracture and no cleavage). The cryptocrystalline varieties may be a little softer
than the quartz crystals.
3. Corundum
Corundum (Al2O3) is a mineral most often found in certain metamorphic rocks. It is one of
the index minerals on Moh’s Scale, where it ranks 9 (diamond being 10). As it is very hard, it
is used as an abrasive, though in many cases it has been superceded by synthetic substances.
It has a hexagonal crystalline structure, and is therefore found as hexagonal prisms or barrelshaped crystals. Gem-quality (i.e. transparent) corundum is called ruby (if red) and sapphire
(if blue). Corundum does not have cleavage, although it exhibits parting (cleavage in a
mineral would be present in all samples, but parting will only appear in some samples) at
right angles to the hexagonal prism.
Example:
74
Corundum
4. Sulfates
Most sulfates are quite soluble in water, so sulfate minerals are not common. The two we will
examine here are barite and gypsum.
Barite (BaSO4) is formed at hydrothermal vents at the bottom of the sea, and is found with
metallic sulfides in ore deposits. It has a very high specific gravity. It is mainly used as a
weighting agent in drilling mud (when drilling oil wells). It is quite soft (3-3.5) and has two
cleavage directions.
Gypsum is formed by the evaporation of seawater. Rocks which were formed in this manner
are called evaporites, and contain minerals such as gypsum (CaSO4.2H2O), anhydrite
(anhydrous CaSO4) and halite (NaCl). It is easily recognized due to its cleavage (3 planes)
and softness (2), and can be scratched by one’s fingernail.
Examples:
38,134 Barite
72
Gypsum
5. Halides
There are not very many minerals which are halides. The most commonly encountered are
halite (NaCl) and Fluorite (CaF2).
Halite is also known as “rock salt”, and is an evaporite mineral. In certain areas of the world,
thick layers of evaporite containing halite are found, and since halite has a very low density, it
tends to rise within the subsurface to form “salt domes”. Salt domes are an important source
of salt, and one finds, associated with salt domes, deposits of anhydrite and also oil and gas.
Its cubic crystal structure results in cube-shaped crystals, and it has three perfect cleavage
planes, all at right angles to each other, forming cube-shaped fragments. It comes in several
colors - the orange colored salt lamps you see in souvenir shops are halite.
Fluorite is commonly found in hydrothermal veins and deposits. It is commonly purple, but is
also often transparent or green. Its hardness is 4 (it being one of the index minerals on the
Mohs scale). It is easily confused with quartz, but is much softer and has four cleavage planes
(an unusually high number of cleavage planes). It fluoresces under ultraviolet light.
Examples:
79
Halite
44,130 Fluorite
6. Phosphates
There are only a few phosphate minerals. The most commonly encountered is apatite
(Ca4(PO4)3(OH,F,Cl)), which can be found in tiny amounts in some igneous rocks. However,
the crystals are usually tiny, except in certain types of rare igneous rocks (carbonatites and
pegmatites). Apatite also makes up tooth enamel.
Apatite is one of the minerals on the hardness scale (5). It is also one of the three minerals
you will encounter in this course which forms hexagonal prismatic crystals (the other two
being corundum and aragonite (aragonite is not hexagonal, but one finds hexagonal prisms
due to twinning).
Compare apatite (62) with corundum (74) and aragonite (39). Can you tell them apart?