Guyana is blessed with a host of precious and semi-precious stone resources, minerals and rocks. These rocks are testimony of Guyana’s rich natural resources, which we should all cherish and not exploit.
First let’s distinguish a rock from a mineral and an ore, and then we’ll delve into the different types of rocks found in Guyana.
A rock is a naturally occurring collection of mineral or minerals, which constitute a significant part of the earth’s crust. On the other hand, minerals are naturally occurring inorganic substances with a definite and predictable chemical composition and physical properties.
An ore is a mineral or a collection of minerals from which valuable element, especially a metal, can be profitably mined or extracted.
There are three basic types of rocks – igneous, sedimentary, and metamorphic.
Igneous rocks
Igneous rocks are called ‘fire rocks’ and are formed either underground or above ground. Underground, they are formed when the melted rock, called magma, deep within the earth, becomes trapped in pockets. As these pockets of magma cool slowly underground, the magma becomes igneous rocks.
Additionally, igneous rocks are formed when volcanoes erupt, causing the magma to rise above the earth’s surface. When magma appears above the earth, it is called lava. Igneous rocks are formed as the lava cools above ground.
Sedimentary rocks
For thousands, even millions of years, little pieces of our earth have been eroded – broken down and worn away by wind and water. These little bits of our earth are washed downstream where they settle to the bottom of the rivers, lakes and oceans. Layer after layer of eroded earth is deposited on top of each. These layers are pressed down more and more through time, until the bottom layers slowly turn into rock.
Metamorphic rocks
A metamorphic rock is a result of a transformation of a pre-existing rock. The original rock is subjected to very high heat and pressure, which cause obvious physical and/or chemical changes. Examples of these rock types include marble, slate, gneiss, schist.
Remarkable rocks
Basalt is an example of igneous rock. It is actually an aphanitic igneous rock (igneous rocks that are so fine-grained that their component mineral crystals are not detectable by the unaided eye) with less than 20 per cent quartz and less than ten per cent feldspathoidal (a group of minerals) by volume, and where at least 65 per cent of the feldspar is in the form of plagioclase (a member of the feldspar group). Over half of the Earth’s crust is composed of a single group of minerals known as feldspar.
Conglomerate is a course grained clastic sedimentary rock that is composed of a substantial fraction of rounded to sub angular gravel-sized clasts such as pebbles, granules, cobbles and boulders larger than 2mm (0.079 in) in diameter.
Clasts are fragments of geological chunks and smaller grains of rock broken off other rocks by physical weathering. Conglomerates form by the consolidation and lithification (process of sediments gradually becoming solid rocks) of gravel. Conglomerates typically contain finer grained sediments such as silt, sand, clay or a combination of these, which are called ‘matrix’ by geologists.
Gabbro refers to a large group of dark, often phaneritic (coarse-grained), intrusive igneous rocks. It forms when molten magma is trapped beneath the Earth’s surface and slowly cools into a holocrystalline (made up wholly of crystals or crystalline particles) mass.
Limestone is a sedimentary rock composed largely of the minerals calcite and aragonite, which are different crystal forms of calcium carbonate (CaCO3). Calcium carbonate is a white insoluble solid occurring naturally as chalk, limestone, marble, and calcite, and forming mollusc shells and stony corals.
Most limestone is composed of skeletal fragments of marine organisms such as coral and molluscs.
Migmatite is a rock that is a mixture of metamorphic rock and igneous rock. It is created when a metamorphic rock partially melts, and then that melt recrystallizes into an igneous rock creating a mixture of the unmelted metamorphic part with the recrystallized igneous part.
Phyllite is a type of foliated (repetitive layering) metamorphic rock created from slate that is further metamorphosed so that very fine grained white mica (form of mineral) achieves a preferred orientation. It is primarily composed of quartz, sericite, mica and chlorite – all minerals.
Pink granite is a common type of igneous rock. The word ‘granite’ comes from the Latin word granum, a grain, in reference to the coarse grained structure of such a rock.
Sandstone (sometimes known as arenite) is a classic sedimentary rock composed mainly of sand-sized minerals or rock grains.
Slate is a fine grained, foliated, homogenous metamorphic rock derived from an original shale-type sedimentary rock composed of clay or volcanic ash. Shale is a fine-grained, clastic sedimentary rock composed of mud that is a mix of flakes of clay minerals and tiny fragments of other minerals. (Information by Guyana Geology and Mines Commission)
