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GRANITE - A Rock with Something for Everyone

by C. Nobs, CSE PGCE MOT

Many people have commented to me or my husband that they think of rocks as "heavy", "hard" or "dull". Certainly, rocks may well be heavy or hard, depending on their size and composition. One might say the same of potatoes or rice pudding; but if you think that rocks are dull, then read this article carefully. I think you will be pleasantly surprised.
Henrietta Nobs, Penge, 1968

Granite (commercial definition): a visibly granular, igneous rock generally ranging in colour from pink to light or dark grey and consisting mostly of quartz and feldspars, accompanied by one or more dark minerals. The texture is typically homogeneous but may be gneissic or porphyritic. Some dark granular igneous rocks, though not properly granite, are included in the definition.

Granite (scientific definition): A visibly granular, crystalline rock with equigranular or inequigranular texture, normally having an essential composition of two feldspars (alkali feldspar plus sodic plagioclase or two alkali feldspars (see second paragraph)) and quartz; certain granites contain only one feldspar. Quartz may amount to 10 - 60% of the felsic (light-coloured) constituents, while alkali feldspars may constitute about 35 to 100% of total feldspars. Feldspars may be grains, or may be mutually intergrown on a megascopic to submicroscopic scale. Besides quartz and feldspars, granite typically also contains varietal minerals, commonly micas or hornblende, or both, more rarely pyroxene. Potassic feldspar, which in granites is typically orthoclase or microcline, forms a nearly complete isomorphous series with the albite end member. The albite-anorthite compositional range represents a continuous isomorphous series plagioclase known as feldspars; these have been arbitrarily subdivided according to the ration of anorthite (An) to albite (Ab) at 10, 30, 50, 70, and 90% An. The plagioclase of granite sensu stricto commonly is oligoclase, less commonly albite.

Gneiss: A foliated crystalline rock composed essentially of silicate minerals with interlocking and visibly granular texture in which the foliation is due primarily to alternating layers, regular or irregular, of contrasting mineralogic composition. In general, a gneiss is characterised by relatively thick layers as compared with a schist. According to their mineralogical compositions gneisses may correspond to other crystalline rocks with visibly granular, interlocking texture, such as those included under the definition of commercial granite, and then may be known as granite gneiss, granodiorite gneiss, etc., if strongly foliated, and gneissic granite, etc., if weakly foliated. This distinction is subjective.

Porphyritic Texture: A texture defined by relatively large grains (phenocrysts), typically of feldspar, that are distributed in a distinctly finer-grained matrix. The phenocrysts of porphyritic granites generally are rectangular or partly rounded-in.

Granites: Dark-coloured igneous rocks defined by geologists as basalt, diabase, gabbro, diorite, and anorthosite are quarried as building stone, building facings, monuments, and for special purposes and sold as granite. The chemical and mineralogical compositions of such rocks are quite different from those of true granites, but black granites nevertheless may be satisfactorily used for some of the same purposes as commercial granite. They possess an interlocking crystalline texture but, unlike granites, they contain little or no quartz or alkali feldspar. Instead, black granites are composed dominantly of intermediate to calcic plagioclase accompanied by one or more common dark rock-forming minerals such as pyroxenes, hornblende, and biotite.

Such rocks, because of their relatively high content of iron and magnesium, are designated as ferromagnesian or mafic. An exception is anorthosite which, though commonly dark, consists mostly or entirely of calcic plagioclase.

© C. Nobs, What Rock?, 352 Vol 23, Oct 1996 pp.96-8 Reproduced without permission