Andy Clift: Created page with "Nepheline syenite of the Intrusive Complex of Tanguá, State of Rio de Janeiro, Brasil (Motoki et al., 2011a). The maf..."
[[File: 01A_NephelineSyenite_Tangua.jpg|thumb|right|259px| Nepheline syenite of the Intrusive Complex of Tanguá, State of Rio de Janeiro, Brasil (Motoki et al., 2011a)]]. The mafic minerals are amphibole.'''Nephelene syenite''' is a [[holocrystalline]] [[Intrusion|plutonic rock]] that consists largely of [[nepheline]] and [[alkali]] [[feldspar]]. The rocks are mostly pale colored, grey or pink, and in general appearance they are not unlike [[granite]]s, but dark green varieties are also known. [[Phonolite]] is the fine-grained extrusive equivalent.==Petrology==Nepheline syenites are silica-undersaturated and some are [[peralkaline]] (terms discussed in [[igneous rock]]). Nepheline is a [[feldspathoid]], a solid-solution mineral, that does not coexist with [[quartz]]; rather, nepheline would react with quartz to produce alkali feldspar.They are distinguished from ordinary [[syenite]]s not only by the presence of nepheline but also by the occurrence of many other minerals rich in [[alkalis]] and in [[rare earths]] and other [[incompatible element]]s. Alkali feldspar dominates, commonly represented by [[orthoclase]] and the exsolved lamellar [[albite]], form [[perthite]]. In some rocks the potash [[feldspar]], in others the soda feldspar predominates. Fresh clear [[microcline]] is very characteristic of some types of nepheline syenite.[[Sodalite]], colorless and transparent in thin section, but frequently pale blue in the hand specimens, is the principal [[feldspathoid]] mineral in addition to nepheline. Reddish-brown to black [[triclinic]] [[aenigmatite]] occurs also in these rocks. Extremely iron-rich [[olivine]] is rare, but is present in some nepheline syenite. Other minerals common in minor amounts include sodium-rich [[pyroxene]], [[biotite]], [[titanite]], [[zircon]], [[iron oxide]]s, [[apatite]], [[fluorite]], [[melanite]] [[garnet]], and [[zircon]]. [[Cancrinite]] occurs in several nepheline-syenites. A great number of interesting and rare minerals have been recorded from nepheline syenites and the [[pegmatite]] veins which intersect them.==Macroscopic aspects==Macroscopic aspects of nepheline syenite are similar to those of [[granite]]. The presence of [[nepheline]] and absence of [[quartz]] are the fundamental difference. [[Biotite]] is generally of low content and the main [[mafic]] minerals are [[clinopyroxene]] (±) and [[amphibole]] (±). The macroscopic colour is grey, being little darker than granite. There is high-grade [[metamorphic rock]] originated from [[nepheline syenite]] that is characterized by [[gneiss]] texture of very rare occurrence. It is called nepheline syenite gneiss or [[litchfieldite]]. An example is found at Canaă village, State of [[Rio de Janeiro]], [[Brazil]].==Texture==The rock is holocrystalline, generally [[equigranular]], equidirectional, and gross with grain size of 2 mm to 5 mm. In certain rare cases, the rock contains alkaline feldspar [[phenocrysts]] of 2 cm to 5 cm in length and 5 mm to 2 cm in thickness. The phenocrysts demonstrate orientation and eventually show cumulative texture.==Mineral composition==The main minerals are [[alkali feldspar]], [[nepheline]], [[clinopyroxene]] (±), [[amphibole]] (±), and [[biotite]] (±). [[Nepheline]] is the main [[feldspathoid]]. [[Quartz]] and [[orthopyroxene]] are absent. According to the IUGS classification nomenclature (International Union of Geological Sciences, Streckeisen, 1978), nepheline syenite has 10%