TO CLAIM A FIND FOR THIS EARTHCACHE
1) Explain in your own words what elements, minerals and rocks are.
2) In which direction do the dark rocks above the mine protrude? Why is it in this particular direction?
3) Approximately 8m to the left of the mine (when facing it), under a ~2m high overhanging rock is a great example of one of the textures or mineral habits of psilomelane. Which is it?
Note: Please e-mail your answers to questions 1-3 to the cache owner.
4) Optional: Please share your experience with the caching community by uploading photos of the site and/or your caching party with your log. |
OVERVIEW
This is the THIRD of three earthcaches in the area that introduce geocachers to some of the most fundamental concepts of geology – “Earthcaching 101” if you would. This earth cache attempts to clarify some of the basic issues regarding the origin and composition of rocks and minerals, while manganiferous minerals and a safe, easily accessible historic mining site are used to illustrate the geological features discussed.
Important notes:
1) You can only park at the designated parking coordinates ~120m from the mine.
2) The site is right next to a road. Although it is usually quiet and has a low speed limit it is fairly narrow, so PLEASE BE CAUTIOUS – especially when caching with children.
3) You don’t have to clamber into the mine to see the relevant geological features or complete the tasks.
ELEMENTS, MINERALS & ROCKS
A chemical element is a substance that cannot be broken down into more basic components, i.e. it is a substance consisting of only one type of atom as represented on the periodic table. Atoms combine to form molecules, e.g. one sodium (Na, a soft, highly reactive metal) atom can combine with one chlorine (Cl, a poisonous gas) atom to form a molecule of sodium chloride, i.e. table salt (NaCl, an innocuous compound we eat every day!). Ninety-four (94) different elements have been found to occur naturally. These can interact in specific ways with each other to form countless unique and often complex molecules. If a significant amount of a specific naturally occurring, inorganic molecule “pack” together to form a solid crystal it is called a mineral. Or defined more formally, a mineral is any substance with all five of the following qualities:
1. Minerals are natural - they form without any human intervention.
2. Minerals are inorganic – they are not derived from living organisms.
3. Minerals are pure – they have a definite chemical composition.
4. Minerals are solid - they are firm; don't droop, melt or evaporate under normal conditions.
5. Minerals are crystalline – their atoms are arranged in distinct patterns (crystalline structure).
So, if the sodium chloride (NaCl) molecules we discussed above associate to form crystals (commonly known as rock salt), it complies with all five these qualities and can then also be called halite - the mineral form of sodium chloride.
Minerals range in composition from pure elements, e.g. gold (Au), and simple salts, e.g. halite (NaCl), to very complex compounds containing various elements, e.g. tanzanite [Ca2Al3O(SiO4)(Si2O7)(OH)]. Every mineral on earth is a unique substance with its own chemical formula and a unique set of characteristics as defined by its lustre, density, hardness, streaking characteristics, crystalline structure (and associated cleavage patterns) and colour.
In contrast, rocks are aggregates of one or more minerals (and they may also contain organic remains and mineraloids). Some rocks are predominantly composed of just one mineral. For example, limestone is a sedimentary rock composed almost entirely of the mineral calcite (CaCO3). Other rocks contain many minerals of which the composition, relative amounts and crystal sizes can vary widely, depending on the conditions under which the rock formed. Granite, for example, generally consists of the minerals orthoclase, plagioclase, quartz and biotite; discernible as the individual “grains” with different colours and shapes in the rock. The minerals can form relative large individual crystals, i.e. coarse “grains”, if they crystallise over a relative long time, e.g. when the magma from which they form cools down slowly. Conversely, if the magma sets quickly there is not enough time for large crystals to form and the resulting granite has an apparent fine texture.
The vast majority of the ~4000 known minerals are very rare. Only eight groups of minerals account for the great bulk of Earth’s crust and upper mantle and are called the rock-forming minerals. These are the silicates, native elements, carbonates, oxides, sulphides, sulphates, phosphates and halides. Silicates (all containing silicon and oxygen) are the largest class of rock-forming minerals, constituting approximately 90% of the crust and are further divided into groups based on their Si:O ratio. The most common silicate minerals are the feldspars, quartz, the pyroxenes, olivine, the clay minerals and the micas. Important non-silicates include native elements like gold and silver, calcite, dolomite, hematite, pyrite, gypsum and halite (see figure below).
MANGANESE
Manganese is a chemical element, designated by the symbol Mn and is found as a free element in nature, often in combination with iron, and in many minerals. It constitutes approximately 0.1% of the Earth's crust, making it the 12th most abundant element. Manganese minerals are widely distributed, with oxides [e.g. psilomelane - (Ba,H2O)2Mn5O10 and pyrolusite - MnO2], silicates [e.g. braunite - Mn2+Mn3+6 (O8)(SiO4)] and carbonates [e.g. rhodochrosite - MnCO3] being the most common.
Depending on their oxidation state, manganese ions have various colours. Several oxides of manganese are abundant in nature and have been used as the black/dark blue pigment in rock paintings since the Stone Age. These and other manganese minerals are still used as industrial pigments to this day. As a free element, manganese is a metal with important industrial metal alloy uses, particularly in stainless steels. Manganese is essential to iron and steel production by virtue of its sulphur-fixing, deoxidising, and alloying properties - adding manganese to iron makes it harder, without making it any more brittle. About 80% of the known world manganese resources are found in South Africa. Manganese is also an essential trace nutrient in all forms of life as many classes of enzymes have manganese as a cofactor. The human body contains about 12mg of manganese, which is stored mainly in the bones.
THE ROOIELS MANGANESE MINE
When driving along the scenic Clarence Drive on False Bay’s eastern coast you may be excused for not noticing the relative small patch of dark rocks, which are dissected by the road, approximately 3km north of Rooiels. The outcrop is clearly visible from the nearby parking area and whale watching spot, Blousteen (“Blue stone”, named after the bluish Malmesbury Group siltstone also exposed here at sea level and NOT the manganese exposure), as illustrated in the photo below. The dark blue/black staining of the rocks next to the road is due to manganese minerals impregnating the sandstone and/or high-grade manganese mineral enrichment in and along this porous fracture zone.
Manganese enrichment is present in the quartzitic sandstones of the Table Mountain Group in several places in the Cape Town area – usually identifiable as conspicuous bluish-black outcrops. It mostly takes the form of either surface impregnation of sandstone over large areas, or medium- to low-grade manganese enrichment in and along fault breccias (a porous, clastic rock type formed by tectonic movement along the zone of brittle deformation, i.e. the fault zone, in a rock formation). A few small, higher-grade deposits, hosted in the Peninsula Formation, can be found in the vicinity of Constantiaberg (the old Hout Bay mine) and along the eastern False Bay coast (this site). These higher grade deposits are contained in vertical fissures that coincide with the principal north-west joint and fault direction in the Cape Peninsula and contain minimal host-rock material. Percolating groundwater, which had leached manganese minerals from the surrounding sandstone, accumulates in these fissures and then the minerals precipitate out of the solution, leading to localised mineral enrichment.
This Rooiels deposit consists of interdigitating (“intertwined”) manganiferous and siliceous (sandstone) rocks, which grades laterally into manganese-impregnated sandstone. The manganiferous rock consists primarily of psilomelane and minor pyrolusite. Psilomelane commonly has a botryoidal habit (a globular external form resembling a bunch of grapes) with a smooth shining surface, but can also adopt a dendritic habit. Manganese containing dendrites are often confused for fossils as they resemble the small branches of a plant.
The mine itself consists of a shallow horizontal tunnel (~10m in length, 2m in height and 1.5m wide) next to the current road and an inaccessible vertical tunnel (~2x2x2m) above it. Regrettably little is known of the history of the mine. A brief reference to the mine in 1914 reports that "Ground has been pegged for Manganese at Koegels Bay and it is proposed to commence active operations at an early date." No evidence of a syndicate or registered company which was responsible for mining the ore has ever been found. Given the fact that the there is still quite a lot of ore above the mine (calculated to be ~4000 tons), the reasons why the mine closed down probably have something to do with profitability and/or the low quality of the ore. If you visit the site during low tide you may also still see the remains of an old jetty in the sea below from which the ore was loaded onto boats.
REFERENCES
Cole DI (2001). The metallogeny of the Cape Town area. Geological survey of South Africa, Council for Geosciences.
MacGregor D (2007). Manganese and the Mountain: Origin, Deposition and Geochemistry. UCT BSc(Hons) report.
http://geology.about.com/od/mineralsresources/a/whatsamineral.htm
http://en.wikipedia.org/wiki/Manganese
ACKNOWLEDGEMENTS
Thank you to John Rogers, John Compton, Duncan MacGregor, Peter Spargo and Doug Cole for pointing me in the right direction, sharing your personal insights with me and/or contributing directly to the cache description through documents you have authored. Your time and efforts are much appreciated!
FTF: 
