Geocaching

Granite is a common and widely occurring type of intrusive, felsic, igneous rock. Granite has a medium to coarse texture, occasionally with some individual crystals larger than the groundmass forming a rock known as porphyry. Granites can be pink to dark gray or even black, depending on their chemistry and mineralogy. Outcrops of granite tend to form tors, and rounded massifs. Granites sometimes occur in circular depressions surrounded by a range of hills, formed by the metamorphic aureole or hornfels.

Granite is nearly always massive (lacking internal structures), hard and tough, and therefore it has gained widespread use as a construction stone. The average density of granite is 2.75 g/cm³ and its viscosity at standard temperature and pressure is ~4.5 • 10¹⁹ Pa·s.

The word granite comes from the Latin granum, a grain, in reference to the coarse-grained structure of such a crystalline rock.

Mineralogy    Granite is classified according to the QAPF diagram for coarse grained plutonic rocks (granitoids) and is named according to the percentage of quartz, alkali feldspar (orthoclase, sanidine, or microcline) and plagioclase feldspar on the A-Q-P half of the diagram. True granite according to modern petrologic convention contains both plagioclase and alkali feldspars. When a granitoid is devoid or nearly devoid of plagioclase the rock is referred to as alkali granite. When a granitoid contains <10% orthoclase it is called tonalite; pyroxene and amphibole are common in tonalite. A granite containing both muscovite and biotite micas is called a binary or two-mica granite. Two-mica granites are typically high in potassium and low in plagioclase, and are usually S-type granites or A-type granites. The volcanic equivalent of plutonic granite is rhyolite. Granite has poor primary permeability but strong secondary permeability.

Chemical composition    A worldwide average of the average proportion of the different chemical components in granites, in descending order by weight percent, is:

• Silicon Dioxide SiO₂ — 72.04%
• Aluminium Oxide Al₂O₃ — 14.42%
• Potassium Oxide K₂O — 4.12%
• Sodium Oxide Na₂O — 3.69%
• Calcium Oxide CaO — 1.82%
• Iron (II) Oxide FeO — 1.68%
• Iron (III) oxide Fe₂O₃ — 1.22%
• Magnesium Oxide MgO — 0.71%
• Titanium Dioxide TiO₂ — 0.30%
• Phosphorus Pentoxide P₂O₅ — 0.12%
• Manganese (II) Oxide MnO — 0.05%

    Based on 2485 analyses

Occurrence    Granite is currently known only on Earth where it forms a major part of continental crust. Granite often occurs as relatively small, less than 100 km² stock masses (stocks) and in batholiths that are often associated with orogenic mountain ranges. Small dikes of granitic composition called aplites are often associated with the margins of granitic intrusions. In some locations very coarse-grained pegmatite masses occur with granite.

Granite has been intruded into the crust of the Earth during all geologic periods, although much of it is of Precambrian age. Granitic rock is widely distributed throughout the continental crust of the Earth and is the most abundant basement rock that underlies the relatively thin sedimentary veneer of the continents.

Origin    Granite is an igneous rock and is formed from magma. Granitic magma has many potential origins but it must intrude other rocks. Most granite intrusions are emplaced at depth within the crust, usually greater than 1.5 kilometres and up to 50 km depth within thick continental crust. The origin of granite is contentious and has led to varied schemes of classification. Classification schemes are regional; there is a French scheme, a British scheme and an American scheme. This confusion arises because the classification schemes define granite by different means. Generally the 'alphabet-soup' classification is used because it classifies based on genesis or origin of the magma.

Geochemical origins    Granitoids are a ubiquitous component of the crust. They have crystallized from magmas that have compositions at or near a eutectic point (or a temperature minimum on a cotectic curve). Magmas will evolve to the eutectic because of igneous differentiation, or because they represent low degrees of partial melting. Fractional crystallisation serves to reduce a melt in iron, magnesium, titanium, calcium and sodium, and enrich the melt in potassium and silicon - alkali feldspar (rich in potassium) and quartz (SiO₂), are two of the defining constituents of granite.

This process operates regardless of the origin of the parental magma to the granite, and regardless of its chemistry. However, the composition and origin of the magma which differentiates into granite, leaves certain geochemical and mineral evidence as to what the granite's parental rock was. The final mineralogy, texture and chemical composition of a granite is often distinctive as to its origin. For instance, a granite which is formed from melted sediments may have more alkali feldspar, whereas a granite derived from melted basalt may be richer in plagioclase feldspar. It is on this basis that the modern "alphabet" classification schemes are based.

Granitization    An old, and largely discounted theory, granitization states that granite is formed in place by extreme metasomatism by fluids bringing in elements e.g. potassium and removing others e.g. calcium to transform the metamorphic rock into a granite. This was supposed to occur across a migrating front. The production of granite by metamorphic heat is difficult, but is observed to occur in certain amphibolite and granulite terrains. In-situ granitisation or melting by metamorphism is difficult to recognise except where leucosome and melanosome textures are present in gneisses. Once a metamorphic rock is melted it is no longer a metamorphic rock and is a magma, so these rocks are seen as a transitional between the two, but are not technically granite as they do not actually intrude into other rocks. In all cases, melting of solid rock requires high temperature, and also water or other volatiles which act as a catalyst by lowering the solidus temperature of the rock.

Azul Noche (Spain)

Santa Cecilia (Brazil)

Gran Violet (Brazil)

Lavanda Blue (Brazil)

Natural radiation    Granite is a natural source of radiation, like most natural stones. However, some granites have been reported to have higher radioactivity thereby raising some concerns about their safety.

Some granites contain around 10 to 20 parts per million of uranium. By contrast, more mafic rocks such as tonalite, gabbro or diorite have 1 to 5 ppm uranium, and limestones and sedimentary rocks usually have equally low amounts. Many large granite plutons are the sources for palaeochannel-hosted or roll front uranium ore deposits, where the uranium washes into the sediments from the granite uplands and associated, often highly radioactive, pegmatites. Granite could be considered a potential natural radiological hazard as, for instance, villages located over granite may be susceptible to higher doses of radiation than other communities. Cellars and basements sunk into soils over granite can become a trap for radon gas, which is formed by the decay of uranium. Radon can also be introduced into houses by wells drilled into granite. Radon gas poses significant health concerns, and is the #2 cause of lung cancer in the US behind smoking.

There is some concern that materials sold as granite countertops or as building material may be a hazardous to health. One expert, Dr. Dan Steck of St. Johns University, has stated that approximately 5% of all granites will be of concern, with the caveat that only a tiny percentage of the tens of thousands of granite slabs have been actually tested. Various resources from national geological survey organizations are accessible online to assist in assessing the risk factors in granite country and design rules relating, in particular, to preventing accumulation of radon gas in enclosed basements and dwellings.

A study of Granite Countertops was done (initiated and paid for by the Marble Institute of America) in November 2008 by National Health and Engineering Inc of USA , and found that 18 of the 39 full size granite slabs that were measured for the study failed to meet the European Union safety standards (section 4.1.1.1 of the National Health and Engineering study).

PAARL'S GRANITE    Granite is the most common rock in the earth's crust but very little of it can be used for building stone, monumental stone, decorative stone and dimension stone. Only granite that is free from cracks and unaffected by weathering can be used.

Granite is an igneous rock, that is, it formed (and still forms) below the surface of the earth by crystallisation of a molten rock known as magma. When molten, granite magma has a temperature well above 1000 degrees centigrade - which is nearly white hot! -- and by the time it has cooled to about 600 degrees centigrade, and still glowing a dull red, it is already almost solid. The entire process of intrusion of the granite magma, including the cooling process, takes place underground. Hot springs and boiling mud pools and geysers may develop at the surface of the earth over the cooling granite magma.

During the crystallisation process the characteristic chemical composition of all granite magmas produces basically three distinctive minerals common to nearly all granites world-wide, namely feldspar, quartz and mica. In most granites the mineral grains tend to be less than about 6mm across and tightly locked into one another. On average granites contain about 2/3 by volume of feldspar, about ? of quartz, and the rest is mica and a smattering of other minerals.

For any rock to be properly called "granite", it must it must be an igneous rock; it must contain sodium/potash feldspar, quartz and some minor minerals (mostly mica); and its chemical analysis must indicate about 70% silica (of which about 25% will be in the form of the mineral quartz). Anything else, and is not granite.

The feldspars in granite tend to be enriched in sodium and potassium, the calcium feldspars being quite unimportant. Variations in the feldspars are responsible for most of the textural and colour variations in granite. Those granites that have cooled at a relatively slow rate tend to have large feldpar crystals compared to granites that cooled more quickly; this leads to a conspicuous difference in grain size. It's not uncommon to find granites with feldspar crystals several centimetres long instead of the usual few millimetres; such granites tell us their magma had a complex and variable cooling process.

Feldspar crystals, viewed individually, have the shape of a slightly squashed match box, and the way the crystals align themselves in the flowing magma while it cools produces a very important grain or texture or rift in the granite. This alignment, albeit nearly invisible to the ordinary eye, is fundamentally important to the quarryman, because it determines the safe direction in which to split the rock along a predetermined direction in much the same way an axe splits a wooden log along the grain rather than across it.

The feldspars can also record in their colours minute variations in chemical composition in the magma. Pure white feldspar, which is a pure sodium feldspar, gives a very pale grey colour to granite. Most mixed sodium/potassium feldspars tend to have pastel shades (mostly creamy pale yellow to light apple green). Such granites are tinted accordingly. More rarely, some feldspars are brick red because of an excess of iron in the magma that tends to accumulate in the feldspars, eventually tinting the feldspar reddish because of the included red-pigmented iron minerals.

Feldspar is an igneous mineral, that is, it is stable under high temperatures, and less so at the surface of the earth; therefore it is the first to be attacked by rain water, plant acids, oxygen and such like, and weathered into clay. In most cases the granite soils so produced are clayey, and stained yellow to reddish by the little bit of iron present in most granites anyway. Where the weathering process has been very thorough the granite clays are leached to pure white kaolin clay, used, amongst others, in the manufacture of porcelain and other ceramics.

Paarl Mountain is a granite mountain, and a very beautiful example at that. Paarl Granite belongs to a group of granites known as the Cape Granite, known to have intruded into the crust of the earth between about 548 million years ago, up to the last events at about 488 million years ago; most of the intrusive action seems to have been in the time period 525-501 million years ago, and the cooling period must have taken many millions of years. As a whole the Cape Granite intrusion is distributed from Saldanha Bay in the north to George in the east.

In plan view the oval outline of Paarl Mountain traces the sides of the large granite magma intrusion, also known as a batholith - one of many similar intrusions of the Cape Granite. Of all these intrusions Paarl Mountain is the most pristine in its topographic and geological expression. Paardeberg is a nearby granite mountain neither as symmetrical nor as well exposed as Paarl Mountain. All the other batholiths of the Cape Granite family have to a larger or lesser extent been weathered away, and have lost all signs of their original glory.

The huge rounded granite domes that crown Paarl Mountain are magnificent examples of the geological process known as exfoliation, that is, the tendency for granite to develop onion-skin-like cracks as the surface layers of the earth's crust are successively eroded away, releasing the pressure that compressed the granite, allowing the granite to expand, and causing huge concentric cracks as a result. As the process continued through the millennia the huge rounded granite rocks, capped here and there with remaining "skins", remain to crown the mountain. Of course, the exfoliation process has not stopped, but continues, albeit too slowly for human eyes to see. The huge granite domes on top of Paarl Mountain have therefore not been pushed up into their present position, but they have been opened up and exposed there where they are seen today through the processes of natural weathering, erosion and exfoliation. In fact, the topographic prominence of the 645 metre high Paarl Mountain is almost entirely the result of the erosive powers of the Berg River.

Paarl Mountain consists of several separate granites, not just one; they represent minor phases of intrusion of the same main event. Five varieties are recognised. On most of the mountain is found a variety known as Bretagne Granite after its type area. Its feldspar grains are relatively coarse-grained, giving the granite of Bretagne Rock its distinctive appearance. The De Hoop granite quarry, located on the southeastern slopes of Paarl Mountain, mines Laborie Granite; its trade name is Paarl Grey Granite. Laborie Granite, which also makes Paarl Rock, has medium-sized grains all more or less the same size, giving it good characteristics as a dimension stone. This granite was also quarried on Diamant on the southwestern slopes of Paarl Mountain. The remaining three granite types on Paarl Mountain are less common. Bethel Dam Granite, a relatively fine-grained variety, occurs only near the western shores of Bethel Dam. Montvue Granite, mostly fine-grained but with conspicuous individual large feldspar crystals, occurs mostly along the northeastern and northern parts of Paarl Mountain. The fifth granite is known simply by its scientific name "quartz porphyry", and occurs as a number of very narrow fissures, or dykes, ranging from the southern end of Paarl Mountain near Zandwyk all along the western foothills of the mountain to well north of Hoogstede. Though only a few metres wide each, these quartz porphyry dykes may well have connected the Paarl Mountain granite magma to a Mount St Helens-type volcano way above the magma chamber itself but we'll never know this for sure because all of the overlying part of the earth's crust of more than 500 million years ago has long since been eroded away, and no direct evidence remains of a possible granite volcano in the Paarl area.

Various kinds of granite are mined in South Africa. Most of these are much older than the Paarl Granite.

The highly decorative granite of the Vredefort astrobleme, for example, mined near the town of Parys in the Free State, owes its origin to a most unusual event, namely the impact of a very large meteorite that dealt the earth what must have been a devastating blow some 2020 million years ago. One of the by-products of this huge meteorite impact was a granite magma that crystallised into a distorted reddish granite with pitch black veins.

From several quarries in the Transvaal (Mpumalanga), come a very popular dark grey to almost black "granite" that takes a high polish, but it's not a granite at all, but an igneous rock known scientifically as "norite". In the trade this norite is widely known as "black granite", using, for example, trade names such as "Rustenburg Black Granite" (a type of salt-and-pepper norite with a uniform very dark grey colour), and "Belfast Black" (a uniform fine-grained, very dark grey norite, ranging into an almost black colour). All of the "black granites" belong to the igneous Rustenberg Layered Suite of the Bushveld Complex, a most unusual geologic feature of the Transvaal Highveld that formed some 2050 million years ago.

Near Bitterfontein a coarse-grained greenish-tinted granite belonging to the Namaqua old granites is mined; its age is approximately 1100 million years.

Near Saldanha the Cape Granite produces a coarse-grained, pinkish granite with a swirley texture, quite unlike the Paarl Granite, which formed more or less at the same time as part of the same igneous event, but in a different place in the magma chamber. Granite quarries around Cape Town, also mining the Cape Granite but now no longer in production, were the source of, amongst others, the distinctive kerb stones lining the older city streets, the coarse-grained greyish granite being characterised by very large individual crystals of potash feldspar.

These are only a few examples of South African granites quarries commercially.

In the trade granite is known as "dimension stone". This simply means that granite is a type of rock that can be produced in very large blocks, some of which can be several meters to a side, allowing them to be cut up into very large slabs or worked in other ways. Dimension stone differs, for example, from natural paving stone, that can only be produced in relatively thin slabs of uniform thickness, and not in blocks. Of course, granite can also be cut up, using large diamond saws, into paving slabs, but more commonly the huge dimension stone blocks are cut up into a carefully matched set of slabs subsequently polished and used as coherent wall cladding or floor paving in prestigious buildings. Granite dimension stone can be quarried in large blocks precisely because of the inherent "grain" or "rift" developed in the granite as the feldspar crystals align themselves during the complicated process of intrusion flowage and cooling of the magma. Once the quarryman has identified the "grain" or "rift" in the granite, not to mention the exfoliation crack system and other nearly invisible zones of weakness, the process of breaking out individual large blocks of dimension stone is facilitated. Long before diamond-tipped tools were available quarrymen used the locked-in grain of the granite to help them split the rock along precisely controlled predetermined planes using very simple hand tools.

In general granite is a very durable stone. It is reasonably hard-wearing and takes a good polish. It has very low porosity, which means that it does not absorb water to any significant extent. Its minerals are in general quite resistant to weathering; however, feldspar in granite remains susceptible to acid rain attack but granite is not nearly as vulnerable as, for example, sandstone, limestone and marble. Compared to almost all other dimension stone granite is superior in terms of it durability and strength.