Lytelton Dolomite EarthCache

Dolomite

Dolomite in northern areas of SA.

What is dolomite?

Dolomite is an anhydrous carbonate mineral composed of calcium magnesium carbonate CaMg(CO₃)₂. The word dolomite is also used to describe the sedimentary carbonate rock, which is composed predominantly of the mineral dolomite (also known as dolostone).

History

Most probably the mineral dolomite was first described by Carl Linnaeus in 1768. In 1778, it was described by the Austrian naturalist Belsazar Hacquet as the "stinking stone" in 1791, it was described as a rock by the French naturalist and geologist, Déodat Gratet de Dolomieu (1750–1801) first from buildings in the old city of Rome and later as samples collected in what is now known as the Dolomite Alps of northern Italy. The mineral was given its name in March 1792 by Nicolas de Saussure, naming it after De Dolomieu. Hacquet and Dolomieu met in Laibach (Ljubljana) in 1784, which may have contributed to De Dolomieu's work.

Properties

The mineral dolomite crystallizes in the trigonal-rhombohedral system. It forms white, tan, gray, or pink crystals. Dolomite is a double carbonate, having an alternating structural arrangement of calcium and magnesium ions. It does not rapidly dissolve or effervesce (fizz) in dilute hydrochloric acid as calcite does. Crystal twinning is common.

Solid solution exists between dolomite, iron rich ankerite and the manganese rich kutnohorite.  Small amounts of iron in the structure give the crystals a yellow to brown tint. Manganese substitutes in the structure also up to about three percent MnO. A high manganese content gives the crystals a rosy pink color. Lead, zinc, and cobalt also substitute in the structure for magnesium. The mineral dolomite is closely related to huntite Mg₃Ca(CO₃)₄.

Because dolomite can be dissolved by slightly acidic water, areas of dolomite are important as aquifers and contribute to karst terrain formation.

Formation

Recent research has found modern dolomite formation under anaerobic conditions in supersaturated saline lagoons along the Rio de Janeiro coast of Brazil, namely, Lagoa Vermelha and Brejo do Espinho. It is often thought that dolomite will develop only with the help of sulfate-reducing bacteria (e.g. Desulfovibrio brasiliensis).  However, promising new research on low-temperature dolomite formation indicates that low-temperature dolomite may occur in natural environments rich in organic matter and microbial cell surfaces. This occurs as a result of magnesium complexation by carboxyl groups associated with organic matter.

Vast deposits of dolomite are present in the geological record, but the mineral is relatively rare in modern environments. Reproducible, inorganic low-temperature syntheses of dolomite and magnesite were published for the first time in 1999. Those laboratory experiments showed how the initial precipitation of a metastable "precursor" (such as magnesium calcite) will change gradually into more and more of the stable phase (such as dolomite or magnesite) during periodical intervals of dissolution and re-precipitation. The general principle governing the course of this irreversible geochemical reaction has been coined "breaking Ostwald's step rule".

 Weathering Process

Dolomite is a calcium-magnesium carbonate rock with a distinctive “elephant skin” texture. As a result of this chemical composition, the dolomites are susceptible to dissolution resulting from the percolation of rainwater and the flow of sub-surface water. This dissolution results in the formation of underground caves and cavities with well known examples being the Sudwala Caves and Wonder Cave in the Cradle of Humankind.

The long process of solution weathering also results in the formation of a complex residual soil mantle known as “Wad”, overlying the dolomite bedrock. Wad is a very low density, very weak material that is highly erodible and highly compressible and therefore completely unsuitable for foundations of any type.

Interspersed in the wad are layers of very hard chert (silica) in discrete bands ranging from several millimetres to over a metre thick. These bands are discontinuous and, as they are contained within the soft wad, also unsuitable for supporting foundations.

Lyttelton Centurion Mine

The mine has been in operation since 1936, initially developed to supply metallurgical grade dolomite for iron and steel plants. Today the mine has increased its product line to include all sizes of construction dolomite required for the construction market and a milling operation that supplies powders into the coal and agricultural markets.
The dolomite deposit is divided into two distinct layers sloping at 15 to 20 degrees eastwards - the upper formation yielding an aggregate with less than 3% silica content (SiO2) and the lower being a chert-rich formation with a silica content greater than 3%.
Current life of the mine is in excess of 30 years. The pit has a length of 1300m and will eventually have a width of approximately 350 metres. Current depth of the mine is 80 metres.
The mine is situated in the suburb of Lyttelton in Centurion, between Botha Avenue and the main railway linking Pretoria and Kempton Park. A large percentage of the metallurgical product is loaded using its own wagon loading facility for delivery by rail to Middelburg.
The mine is ideally placed to serve the local Gauteng construction market with a superior concrete aggregate from 19mm stone down to a super sand which is strictly washed and grade controlled.
Flexibility has been created in the overall beneficiation plant with the introduction of satellite crushing, washing and screening plants having been established over the past period. The plant in its current configuration is able to produce 1.6 million tons of saleable product per annum.
The milling facility grinds the low silica content material down to a powder which in the main is used in the stone dusting operation in the underground coal mines for the prevention of coal dust explosions.
The beneficiation plant also has a bagging plant for the powders to cater for those clients who prefer to receive the product in 25kg bags.
Due to the mine’s close proximity to urban areas, an environmental committee has been successfully in operating with the objective of bringing all role players in the area together in order to monitor (on behalf of the residents and local commerce), the mine development and environmental management measures.

Products supplied by the mine:

• Metallurgical dolomite aggregate for the iron and steel making industry;
• Fine and course aggregates for the building and construction industry;
• Aggregates for road surfacing;
• Powders (stonedust) for the underground coal industry;
• Powders used in industrial markets, i.e. filler in asphalt mixes, tiling industry, making of calcium carbide;
• Powders as an agent in clay brick manufacturing to prevent discolouring;
• Dolomite powder for the fertilizer industry in the making of LAN.

NOTE:  DO NOT TRY TO GAIN ACCESS TO THE MINE SITE.  NO PUBLIC ACCESS ALLOWED.  The Lytelton Centurion Dolomite Mine is an active mine and access to mining areas are enforced by legislation.

Please answer the following to qualify your find:

1. At the listed coordinates you will find a stone structure.  What is this called in mining terms?
2. Describe the look and feel of this structure.
3. Describe the grain structure and colour of the structure.  Find a recent chip mark to do this.
4. In your own words describe what causes sinkholes in dolomite areas.
   
5. Optional:  Post photographs of the structure.
   

Sources:

• Wikipedia
• The geology of an area south of Pretoria with specific reference to dolomite stability by Nicole Yvette-Marie Ghislaine Trollip
• Lytelton Centurion Dolomite Mine

F

Rescent Publications:

National Geographic published 24 July 2014