Changing positions of sample or light source result in the apparent movement of the grey-blue lustre, giving impression of the moonlight floating on water surface. The two alternating minerals give characteristic grey-blue lustre (adularescence or Schillerʼs effect) on polished surfaces, which originates below the surface of exsolved feldspars. The K-feldspar is typically microscopically inhomogeneous to form perthitic intergrowths of K-feldspar and acid plagioclase with a near-albite composition. The K-feldspar is ternary, containing all three endmember plagioclase-orthoclase components (albite, anorthite, orthoclase). Petrographic characteristics Coarse-grained monzonite or syenite with large pronounced phenocrysts of K-feldspars, up to several cm in size. The effect of adularescence is almost indiscernible on the unpolished and altered sample surface. The structure is dominated by large phenocrystalĪlterations The sample is sericitized and probably also kaolinized. Texture Phaneritic, subhedrally grained, locally porphyric. Granularity Coarse-grained (3 mm – 1 cm) to very coarse-grained (> 1 cm) rock. Colour Greyish-blue, changing to brown colours during weathering. The official field numbers and rock definitions can be found in the description of the figures below, or see Coarse-grained ("plutonic") crystalline igneous rock or Fine-grained ("volcanic") normal crystalline igneous rock. Some of the fields are assigned to 2 or 3 rock types, which are further distinguished by their mafic mineral content or plagioclase type ( calcic or sodic). They mostly have similar field divisions, so most volcanic rocks have a corresponding plutonic rock of equivalent mineralogy (for example, rhyolite and granite). There are 2 QAPF diagrams, one for coarse grained ( plutonic) or intrusive igneous rocks, one for fine grained ( volcanic) or extrusive igneous rocks. The QAPF diagram is divided into 15 basic fields that define ranges of mineral compositions for the different classes of rocks. The corners represent cases in which only one felsic component is present, effectively 100% of either quartz, alkali-feldspar, plagioclase or foid.īecause foids and quartz are mutually exclusive in an igneous rock, the QAPF classification is always based on a maximum of 3 components, either QAP or APF, and the compositions of the rocks are plotted in either the upper or lower triangle. It is made of two ternary diagrams with the corners Q, A, P and F, A, P, adjoined to each other along their A-P edge. The composition of the rocks are plotted into a diamond-shaped coordinate system, the QAPF diagram, also known as Streckeisen diagram (named after the author of the original article). Within the QAPFM classification scheme proposed by the IUGS, rocks with less than 90 vol-% mafic minerals are classified by their content (in volume-%) of:
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