C:\Users\Sven\Documents\škola\Škola\Books\Atlas of ignous rock\kapak.jpg •This property embraces tree different concepts: 1.what the aided and unaided eye can or cannot see 2.absolute crystal size 3.relative crystal sizes • Part 2: Granularity 1. Terms referring to what the aided and unaided eye can or cannot see -Phanerocrystalline (phanerictic taxture) – all crystals of the principal minerals can be distinguished by the naked eye1 - Aphanitic – all crystals, other than any phenocrysts present, cannot be distinguished by the naked eye2. Two sub-types exist: - (a)Microcrystalline – crystals can be identified in thin section with petrographic microscope. Crystals only just large enough to show polarization colours (less than 0.01mm) are called microlites. (b)Cryptocrystalline3 – crystals are too small to be identified even with the microscope. Globular, red-like and hiar-like crystals which are too small to show polarization colours are known as crystallites. 1 Pegmatitic texture is a variety of phanerocrytalline in which the crystals are srikingly large, bigger than 1-2cm, and in rare instances up to many meters. 2 The term aphyric is sometimes used for aphanitic rocks which lack phenocrysts. 3 Felsitic texture is sometimes a applied to siliceous rocs with illdefined, almost cryptocrystalline, grey-polarizing areas composed of more or less equigranular aggregates of quartz and alkali feldspar. The name felsite is often applied to such rocks, although this is more commonly a field term for fine-grained acid material of uncertain mode off occurrence. Phanerocrystalline granites Phanerocrystalline granites Microcrystalline olivine basalt Cryptocrystalline rock Cryptocrystalline matrix in a tuff • • 2. Terms indicating absolute ranges of grain size Coarse-grained – crystal diameters > 5mm Medium-grained – crystal diametes 1-5 mm Fine-grained – crystal diametres < 1 mm (or very fine-grained < 0.05 mm) The next six photographs were all taken at the same magnification (x27) to indicate how grain size relates to the number of crystals seen in a given field of view (4.2x3.1 mm), and hence the extent of the texture visible at that magnification. While the overall texture is recognizable in the fine-grained rock, it is not so in the coarse one and low-power objective lens would de necessary to examine it adequately. Pertographic microscopes rarely have a sufficiently low-power objective lens for examining the textures of course-grained rocks; a hend lens should be used for these, with two sheets of polaroid, if available. Fine-grained gabbro Medium-grained olivine gabbro Coarse-grained olivine gabbro • 3. Terms indicating relative size of crystals Equigranular – all crystals are of approximately the same size. Inequigranular – crystals differ substantially in size. A common variety, porphyritic texture, involves relatively large crystals (phenocrysts1) embedded in fine-grained groundmass (so the same mineral may be present as both phenocrysts and groundmass). In naming a rock with porphyritic texture the minerals present as phenocrysts should be listed and followed by the suffix – phyric, e.g. ‘hornblende-pigeonite-phyric andesite’. However, if the groundmass is glassy, the term ‘vitro-phyre’ is used, e.g. ‘olivine vytrophyre ‘. Seriate texture involves a continnuous range in size of crystals of the principal minerals; if the crystals show a broken series of sizes, the intergranular texture is said to be hiatal. Caution is necessary in thin section depend on the attitude of the intersection of the crystal in three dimensions. 1 The prefix micto-may be added to phenocrysts which have diametrs between 0.05 and 0.5 mm (e.g. ‘Olivine microphenocrysts ‘). Equigranular peridotite • Porphyritic andesite Leucite-phyric micro-ijolite Plagioclase-augite-magnetite vitrophyre Seriate-textured olivine basalt