TYPES OF GRANITE USED IN METROLOGY
Types of granite used in metrology include Diorite, Gabbro, Rhyolite Porphyry, Intrusion Breccia, Diabase Dykes and Dolerite.
Diorite is a medium-grained intrusive igneous rock consisting mostly of andesine plagioclase and pyroxene, and small amounts of hornblende and biotite. It is the intrusive equivalent of andesite. Therefore, a medium-grain diorite contains black hornblende and white plagioclase feldspar.
Gabbro is a dark, medium to coarse-grained intrusive igneous rock composed of calcium plagioclase, pyroxene, and possibly olivine, but no quartz. It is the intrusive equivalent of basalt and the deep intrusive equivalent of diabase. A coarse-grained gabbro also consists of black pyroxene and/or hornblende and white plagioclase feldspar.
Rhyolite is a very fine-grained to glassy extrusive igneous rock (i.e., volcanic), usually light in color but not always, consisting essentially of quartz (silica) and alkalic feldspar. It is the extrusive equivalent of granite. Porphyry is an igneous rock of any composition that contains conspicuous crystals (Phenocrysts) in a fine-grained matrix. A fine-grained rhyolite contains larger pink feldspar Phenocrysts, which gives a porphyritic appearance.
Breccia is a rock consisting of angular fragments in a matrix of finer particles. An intrusion breccia consists of older host rocks (wall rocks) surrounded by younger intrusive igneous rocks.
Diabase Dykes (intrusive relationships)
A dyke is a sheet of intrusive rock that cuts across layering or bedding in the surrounding rocks. Therefore diabase is a dark-colored intrusive rock, found as dykes or sills, composed of mainly feldspar, pyroxene +/- olivine. It is the intrusive equivalent of basalt and the shallow intrusive equivalent of gabbro. The linear, black, diabase dyke shows cuts across (is intrusive into) the layering / banding in older gneisses.
Dolerite is composed of Plagioclase and Pyroxene. As the result, it is three times as stiff as Granite. And it deflects only 1/3 rd as much as Granite, under an equal load. Dolerite is quite uniform in hardness. Its Scleroscope readings averaging >Hs 95. Density of Dolerite is more than offsets the 10% greater hardness of the few very hardest Granite. As the result Dolerite can provide greater wear resistance than compared with other types of granite used in metrology . The Co-efficient of linear expansion hold by the Dolerite is 2.4 X10-6/ oC. Granite holds bits higher in this regard as 4.16 X 10 -6/ oC.
When we consider the moisture absorption rate in form of Porosity percentage, Porosity percentage of Dolerite is NIL. But, Granite holds upto 1.5% of porosity in nature. And relatively Chinese granite holds 0.13% in form of porosity percentage. Besides, the porosity plays the role in the Granite surface plates, absorbing moisture from the air and there by deflecting the plate, causing deviation on Flatness.
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