The Mineral Mosaic

Quartz (SiO2) is one of the most common and most important rock forming minerals of the silica group. Quartz is typically colourless or white, like the samples used in the display to depict the snow on the mountains. The white color is caused by fluid inclusions of gas and/or liquid that gets trapped during crystal formation. Some other varieties of quartz that appear in the display are agate, tiger’s eye, aventurine, and jasper.

Agate is a banded form of chalcedony and a cryptocrystalline variety of quartz.

Cryptocrystalline means the crystal structure is only visible under magnification. The banding associated with agate is due to deposition in cavities and is typically found parallel to the walls of the cavity. Natural agate shows colourful banding and can also be artificially dyed. This is easy to do because chalcedony is slightly porous and allows the dye to be absorbed. Agate is used in the display as part of the base of the volcano as well as the ash plume being ejected from the volcano.

Tiger’s eye is formed when blue asbestos starts to get replaced by quartz. The tiger’s eye can retain the asbestos’ blue colour as seen in the mountains of the mosaic, but is more commonly seen as a golden brown colour. This is from the oxidization of iron and can be seen as part of the base of the volcano in the mosaic.

Jasper is another type of chalcedony or cryptocrystalline variety of quartz. It is an opaque variety and is generally red, yellow or brown. The variety in our display is red, and this colour is due to included colloidal particles of iron oxides.

The last variety of quartz used in the mosaic represents grass and is aventurine.

It is characterized by its translucency and a shimmering or glistening characteristic called aventurescence. Aventurine is most commonly green, and can have fuchsite inclusions in it. Fuchsite is a chromium-rich variety of muscovite and gives aventurine a banded appearance. This can cause aventurine to be classified as a rock sometimes.

The last mineral used in the mosaic is

sodalite (Na8(AlSiO4)6Cl2). It is a commonly blue

tectosilicate mineral with a translucent

characteristic. It is most commonly found in both

volcanic and plutonic rocks of the nepheline

syenite family, and is generally associated with

nepheline.

References

Berry, L. G, Brian Harold Mason, and Richard Vincent Dietrich. Mineralogy, Concepts, Descriptions, Determinations. San Francisco: Freeman, 1983. Print.

All photos courtesy of The University of Waterloo Earth Sciences Museum.