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Stones

What is stone? The Oxford English Dictionary defines stone as "A piece of rock or hard mineral substance (other than metal), of small to moderate size". One of the most commonly used terms related to geology it derives from stan (Old English) and steinn (Old Norse).

The formation of sedimentary rocks was described in 1802, by Scottish Professor John Playfair, uncle to architect William, and friend of James Hutton, founder of modern geology. The notable 19th century Scottish geologist Sir Archibald Geikie eloquently commented on the process of transition from unconsolidated sediment to sedimentary rock: "If you take a quantity of mud, and place it under a weight which will squeeze the water out of it, you will find that it gets firmer. You can thus harden it by pressure. Again, if you place some sand under water which has been saturated with lime or iron, or with some other mineral that can be dissolved in water, you will notice that as the water slowly evaporates it deposits its dissolved material round the grains of sand and binds them together. Were you to continue this process long enough, adding more of the same kind of water as evaporation went on, you would convert the loose sand into a solid stone."   This is essentially the process of lithification (derived from the Greek word lithos - a rock).  Thus, in a sedimentary basin through compaction by the weight of overlying strata and by cementation by mineralised fluids so sedimentary rocks are formed. In contrast, igneous rocks are usually consolidated by crystallization of minerals from molten material either within the earth's crust (e.g. granites) or at the surface (e.g. basalt lavas). As their name suggests metamorphic rocks have been transformed (recrystallised) from the original rock by a combination of heat and pressure. In these circumstances, new minerals sometimes form.

(by Andrew A McMillan and Ewan K Hyslop) http://www.sslg.co.uk/BWSS.pdf

Sedimentary rocks

Sedimentary rocks are formed by the deposition and compression of mineral and rock particles. They may form on land and in the seas and oceans. Sedimentary rocks may be clastic (e.g. conglomerates, sandstones and siltstones formed by the breakdown, transport and deposition of rocks exposed on land) or from volcanogenic sources (e.g. tuffs, and agglomerates). Principal agents of transport include wind and water. Other types of sedimentary rock include those of organic or chemical origin. The content of organic-rich sedimentary rocks is determined from organic processes such as peat accumulation or shell production from which, respectively, coal and limestone can form. Chemically produced rocks include those formed by precipitation from water of some carbonates and evaporites. Evaporites are formed from the evaporation of water in marginal salt-pans, lagoons, supra-tidal flats, and saline lakes.

 

Ripple marked sandstone formed from a river sandstone, Corsehill Quarry, Annan, England Portland roach ootitic limestone studded with fossils and shell fragments, Portland, England Corncockle quarry - Aeolian (formed from sand dunes) sandstone, Lochmaben, Scotland

 

Sandstones

Formed by the weathering and erosion of all types of pre-existing rocks, sand when lithified (consolidated) by burial forms sandstone. Sandstones typically consist of small fragments or grains held together by natural cements such as calcium carbonate (calcite), silica, iron oxide or clay minerals. Most sandstones consist of grains of quartz, feldspar and lithic (rock) fragments. The colour of sandstones is dependent on the composition. Generally quartz-rich sandstones range from white to pale grey, yellow and pink. Geywacke sandstones (sometimes colloquially referred to as ‘whin' or ‘whinstone') have a high proportion of rock fragments and are generally harder than other sandstones. Sedimentary structures and mineralogy of a sandstone may distinguish whether it is of aeolian (wind-blown dunes), fluvial (river-borne) or marine origin.

Limestones

Limestones are principally composed of calcium (calcite) and/or magnesium (dolomite) carbonate. These rocks are relatively soft in comparison to sandstones. Most limestones are formed by the accumulation on the seabed of bioclastic grains (e.g. broken shells of marine organisms), in tropical or sub-tropical settings. These grains are cemented together by natural calcium carbonate. In coarse-grained limestones, fossil shell fragments are easy to see with the naked eye. Also included in this group of rocks are the magnesium-rich or dolomitic limestones which are principally formed by the chemical alteration of an original calcium-rich limestone. This alteration process may preserve any original shelly or ooidal limestone fabric or completely destroy it to produce a crystalline rock.

Alabaster

Alabaster is a translucent, white, fine-grained variety of the mineral gypsum, a soft, hydrated calcium sulphate mineral (CaSO4.2H2O), formed from the secondary hydration of anhydrite (CaSO4), a rock forming ‘evaporite' mineral. Alabaster is may be veined typically is has a pearly to vitreous lustre.

Igneous rocks

Igneous rocks are hard and crystalline rocks composed of the primeval material of the earth. These rocks formed directly by the cooling of hot molten magma of varying composition and under variable conditions of temperature and pressure that consequently produce a very wide spectrum of rock types. Igneous rocks may be broadly divided into plutonic, formed and crystallized within the earth's crust, or extrusive, brought to the earth's surface by volcanic processes.  Acid igneous rocks contain about 60 % or more silica by weight. A basic igneous rock is one that contains a high proportion of calcium, magnesium and iron and between 45 and 53 % silica by weight. Ultrabasic rocks have less than 45 % silica by weight.

 

Corrennie pink granite with basalt graining, Kemnie, Scotland Obsidian - a naturally occurring volcanic glass formed as an extrusive igneous rock, Mexico Basalt rock, Midlothian, Scotland

 

Granite and other plutonic rocks

Widely used as a source of building and ornamental stone many coarse-grained igneous rocks are commonly termed ‘granites' by the trade. Scientifically, however, igneous rocks show a range from acid, pale-coloured, coarsely-crystalline, quartzo-feldspathic varieties, that include the true granitic rocks and coarse-grained granodiorites, together with diorites, gabbros (dark grey basic igneous rocks), syenites and monzonites (larvikite).

Basalt

Basalt is a dark-coloured, fine-grained, basic extrusive igneous rock. Basalt and related rocks typically occur hard and blocky weathering rocks in lava flows which have rapidly cooled at the earth's surface. Used extensively world-wide for building, and for street-making, much basalt is today crushed as a source of hard rock aggregate.

Tuff and pumice

Tuff is a typically lithified, light, volcanic ash deposit, formed in water. Pumices is a vesicular, volcanic frothy silica glass.

Porphyry

Medium- to coarse-grained rocks with more than 25% feldspar phenocrysts (large crystals clearly distinguishable from the groundmass) are commonly called porphyry.

Metamorphic rocks

Metamorphic rocks are comprise aggregates of minerals formed by the recrystallization of pre-existing rocks in response to changes of pressure, temperature, or volatile content. Regionally metamorphosed rocks, formed in response to changes leading to high presusure and temperature include gneiss, schist, and psammite. The metamorphic grade is a measure of the relative intensity of the metamorphism and can be determined by the mineral assemblage of the rock. Contact metamorphic rocks formed in response to changes leading to high temperature with low pressure, include pre-existing sedimentary rocks surrounding igneous intrusions.

 

Laminated siltstone and fine grained sandstone (flagstone).  Initially formed in shallow lakes, Spittall Quarry, Caithness, Scotland Gneiss river boulder, Sichuan, China Ministravia Marble Quarry, Monte Altissimo, Carrara, Italy 

 

Gneiss

Gneiss is a coarse-grained, banded rock formed during high-grade regional metamorphism. The banding is a result of the separation of dark silicate minerals (biotite, hornblende and pyroxene) from the light-coloured quartz and feldspar.

Schist

Schist a coarse-grained, foliated rock formed during regional metamorphism. The principal minerals include muscovite (white mica), biotite, and quartz. Basic igneous rocks, when metamorphosed can produce hornblende-schists. The alignment of the micas produces a distinctive fabric or ‘schistosity'.

Quartzite and psammite

Quartz-rich sandstones, when metamorphosed produce hard, blocky quartzites. Psammites contain a proportion of feldspar, and may exhibit a blocky weathering. They may be tickly or thinly bedded (flaggy) depending to some extent on the bedding of the original sedimentary rock.

Serpentinite

Serpentinite is an altered rock formed from an ultrabasic rock through interaction with water under low pressures. Typically compact, variously coloured, these rocks have been much used as ornamental stone. Serpentine is a soft, secondary hydrated mineral, an alteration product of olivine (a magnesium, iron silicate mineral). It has been used extensively as a facing stone and for ornamental purposes.

Slate

Slates are fine-grained, low-grade metamorphic rocks showing strong fissility (i.e. slaty cleavage) which allows the rock to be split into thin sheets of consistent lithology. Slates are formed by the recrystallization of fine-grained sedimentary (mudstone) or igneous rocks (usually volcanic ash). The alignment of new minerals, most notably micas, produces the  characteristic slaty cleavage that enables slates to be easily split into thin sheets.

Marble

Geologists apply the term ‘Marble' only to limestones that have been recrystallized during metamorphism. However, the building trade uses the term to cover any hard limestone capable of taking a polish.  Marbles, being principally composed of calcium or magnesium carbonate, are commonly white. Chemical and mineralogical variations produce texturally and colourfully distinctive examples (e.g. green Connemara, Iona marbles). The hardest and attractive marbles have been employed in building and statuary from earliest times.


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