Rocks & Minerals
At the centre of the earth lies the core, a mass of iron and nickel. The thick shell of rocky material which surrounds the core is called the mantle. The rocky outer shell of the Earth on which all known organisms live and the oceans exist is known as the crust. The many types of rock formed in the earth's crust are a creation of one or more minerals. In many areas the rock is covered by a layer of soil. Plants and trees may grow here. Rocks do not last forever though as they are exposed to surface weathering and erosion. Plate tectonics also affect their condition. Original rock stratas are often destroyed and new rocks and formations form in their place. A spectacular example of new rocks being produced is when volcanoes erupt. Geological events of the past are evident in Namibia today. Schists, marbles and a variety of other layered rocks are evident and exposed in the Naukluft Mountains, the Kuiseb and Ugab Rivers and north of the Brandberg West mine.
Collecting rocks and minerals can be just for fun or become a passion. Many interesting rocks and minerals can be found near your home or around mines, quarries, building excavations, ocean cliffs and beaches, riverbanks and road cuts. As no 2 specimens are ever alike, even when of the same species, they can make impressive indoor or outdoor displays. Or perhaps you would simply prefer to improve your own knowledge of the earth? Accumulating rocks would essentially be an outdoor hobby unless of course you scoured the many Namibian dealers on your travels. Certain types of rocks will be around for a long time but it is possible to collect a rock specimen that may never be found again. Your collection could be of great mineralogical importance in years to come. As new species are found on a regular basis, collection and documentation contribute to the ongoing development of mineralogy. Minerals, gemstones and certain rocks are both beautiful and diverse and remember - a collection is never complete. Sources of information to collect rocks and minerals include the Internet, museums, local mineral or gemstone clubs, regional geological societies, gemstone and mineral books and visiting geology departments at universities and other similar institutions. Mining companies can also be approached for information.
The continuous changing of rocks from one type to another is called the 'The Rock Cycle' by scientists. Igneous rock is broken down by weather and gravity with the fragments accumulating elsewhere as a sediment. The smaller sedimentary rock is buried further and undergoes intense pressure and tremendous heat. New minerals now develop and a metamorphic rock forms. A section of this new metamorphic rock will then melt and it becomes molten rock which solidifies into a new igneous rock. So these 3 main rock types can be distinguished from one another by the manner in which they were formed.
- Igneous Rocks
- Sedimentary Rocks
- Metamorphic Rocks
A mineral is a naturally occurring, usually inorganic material with a specific chemical composition and a definite crystalline structure. There are over 3,500 minerals discovered so far worldwide with around 100 common to the earth's crust. In mineral-rich Namibia, some 10 groups can be found with some 105 species described on this web-site. The now closed Tsumeb Mine is known worldwide to mineral collectors and mineralogists. Gemstones tend to be the most desirable, especially diamonds in Namibia and the connection and history with the Sperrgebeit and Kolmanskop.
Identifying minerals: As minerals vary greatly in appearance and feel, here are some pointers to assist you. Some minerals have glass-like surfaces that sparkle with colour whilst others look dull and feel greasy. The hardest minerals can scratch glass and the softest ones can be scratched with a fingernail. The main physical characteristics of minerals are:
Colour: The colour of some minerals depends on the substances that make up the crystals. Other minerals get their colour from chemical impurities. Pure quartz has colourless crystals, but tiny amounts of other substances in quartz crystals can give a pink or green tint, or even black.
Streak: A streak test can identify a mineral. Although some minerals have a colourless streak, when scratched on white porcelain, some minerals leave a powdery residue, known as a 'streak'.
Lustre: May be metallic or non-metallic and refers to the light reflected off the surfaces of crystal faces of a mineral. Minerals with a metallic lustre shine like metal. Gold, galena, copper, graphite, silver, magnetite and ilmenite are all metallic minerals. The appearance of non-metallic minerals can vary in appearance. Talc has 'pearly' surface, quartz looks glossy and varieties of cinnabar appear dull and clay-like Azurite, fluorite, malachite and sulphur are other examples of non-metallic minerals. Lustres of samples can differ from sample to sample.
Transparency/Translucency: The degree that a mineral reflects light is called diaphaneity. Some minerals and gemstones are transparent, that is an object can be viewed clearly through it, others are opaque. Those minerals that are neither transparent or opaque are translucent.
Cleavage: Cleavage is the way a mineral splits or breaks when broken.
Tenacity: Tenacity is how the mineral reacts to being split or broken.
Specific Gravity (SG): Refers to the atomic weight of a mineral's ingredient elements and how they are arranged together.
Hardness: This can be performed by scratching one mineral against another. The harder mineral scratches the softer one. A scale of hardness is used, Mohs' Scale of Hardness, invented by Friedrich Mohs, a German mineralogist, in 1822. The scale lists 10 minerals from the softest to the hardest, each of which can scratch the one below it on the scale. Therefore corundum can scratch all other minerals except diamond. Apatite can scratch all minerals softer than its 5 hardness value. A fingernail has a hardness of about 2!
Mohs' Scale of Hardness
- 10: Diamond
- 9: Corundum
- 8: Topaz
- 7: Quartz
- 6: Feldspar
- 5: Apatite
- 4: Fluorite
- 3: Calcite
- 2: Gypsum
- 1: Talc
Shape and form: Minerals take many shapes and forms when created, always dependant on the temperature which the crystallization occurred, the chemical solution it occurred from and the amount of space the growing crystal was allowed.
Habit: Habit refers to the general appearance of a mineral. For instance gold is found in the form of nuggets, diamonds are found as crystals. Halite are in grains, clumps of crystals or large chunks. Other crystalline shapes or forms are; shaped like a cauliflower (cauliform), radiating from a central point (stellate), shaped like a bunch of grapes (botryoidal), kidney-shaped (reniform), prism-shaped (prismatic), fibrous, bladed, needle-like ( acicular), flat (tabular), snowflake (reticulated), tree branches (arborescent) and like a fern leaf or dendritic.
Crystal Systems: There are 6 crystal systems which all known mineral crystals are divided into. They are:
Materials that were once part of older rocks, materials, animals or plants are called sedimentary rocks. They are deposited by water, wind or ice and accumulate in layers (strata). The majority of deposits occur on the ocean floors although some form in fresh water and on the land. In time, the loose materials harden into solid rocks. Sedimentary rocks fall into 2 categories; clastic or non-clastic. Clastic sedimentary rocks are are formed through a number of weathering and movement processes and are classified according to the size of the mineral grains making up that particular rock. The finest clastic particles are clay, followed by silt and sand. Gravel (conglomerate) is also a type of sedimentary rock. Non-clastic sedimentary rocks are formed by chemical or organic processes.
An extremely complex group of minerals with 6 amphibole species found in Namibia. Riebeckite and grunerite form the 'asbestos' minerals, the common name for fibrous minerals that were used for decades as fire-proofing agents.
Some species of apapite resemble quartz and beryl, hence the Greek apatan meaning 'to deceive'. Fluorapatite is the most common member of this group. Fluorapatite crystals are usually 6-sided, prismatic and can be either short and stubby or elongated. Colours can range from white to grey, blue, violet, red, brown, purple or yellow. Pendants and earrings are often made from this mineral. There are 19 species of the apapite group found in Namibia. They are:
Chlorite crystals are very small, usually less than 1mm. Chlorite is still the general term for this group to aid in general identification, but clinochlore is now the name that has replaced chlorite. It is extremely difficult to distinguish group species without analytical procedures. There are 16 species of chlorite group found in Namibia. They are:
- Columbite series
Feldspars make up the main components of igneous and metamorphic rocks. Sedimentary rocks also contain a certain degree of feldspar. There are 2 main feldspar groups: Plagioclase feldspars and the alkall (potassium) feldspars. There are 3 species in this group found in Namibia. They are:
This is a group chemically related to the feldspar Group with less silica. This feature means they are found in rocks rich in aluminium but not in the same rock as quartz. There are 5 species of the feldspathoid group found in Namibia. They are:
The most common species in the garnet group are almandine, grossular, pyrops and spessartine. There are 26 species found in Namibia. They are:
The olivine group is the name for a collection of 4 related silicates. Iron-rich fayalite and magnesium-rich forsterite are listed below. There are 9 species of the olivine group found in Namibia. They are:
Minerals that occur in ultramafic and mafic igneous rocks in gneisses, granulates and charnockites belong to the pyroxene group. They are:
Spinal is a mineral species itself but there are a number of the spinal group of cubic oxide minerals. The most common 9 species found in Namibia are:
Elbaite and schorl are the most common members of the Tourmaline group. It is a popular gemstone widespread in the jewellery industry, due to its hardness, striking colours and transparency. Colours range from green to orange, peach, mauve, pink, red and blue. More than one colour in 1 gemstone is not unusual. There are 9 different types of tourmaline found in Namibia. They are: