About 1% of all rocks of igneous origin present in the earth's crust are rocks of the nepheline syenite group. In this article, we will look at their main characteristics: composition, properties, genesis and existing varieties, and also find out where these breeds are used.
Systematics
Nepheline syenite is an intrusive rock. Its systematic characterization is as follows:
- Class - plutonic rocks;
- Detachment - rocks of medium composition (silicic acid content from 52 to 63%);
- Suborder - moderately alkaline rocks;
- Family - syenites;
- Type of rock - syenite.
The adjective "nepheline" refers to a rock-forming mineral. Syenites can also be enstatite, hornblende, and so on.
Mineral composition
The proportions of the minerals that form the rock can fluctuate, resulting in the existence of a large numbervarieties. In general, the mineral composition of nepheline syenite is as follows:
- Feldspars (potassium) - orthoclase or microcline - from 65 to 70%;
- Feldspathoids - nepheline - about 20%;
- Colored minerals (mainly alkaline pyroxenes, amphiboles, biotite lepidomelane) – 10 to 15%.
Feldspathoids, including nepheline, are similar in chemical composition to feldspars, but differ from them in a significantly lower silica content SiO2.
Colored minerals are mainly represented by alkaline pyroxenes and amphiboles, ferruginous biotite may be present. Sphene, apatite, zircon, perovskite and others are accessory (“additional”, not affecting the rock classification) minerals, the content of which in this type of rock is very significant.
Genesis and occurrence of rock
The formation of nepheline syenite is associated with processes of deep crystallization of magma depleted in silicic acid. It is also possible that some role in the formation of this rock belongs to alkaline metasomatic phenomena that took place in the contact zones of intrusive massifs, in particular, under the action of feldspathizing (nephelinizing) hydrothermal solutions. As a result, the rock is strongly depleted in silicon, that is, almost devoid of quartz.
This rock is very common throughout the world. Its most common form of occurrence is large stratified massifs (in the Krasnoyarsk Territory, in the Urals, abroad - in South Africa, Canada, Greenland,Brazil). Intrusive bodies in the form of laccoliths are also not uncommon (for example, in the Khibiny on the Kola Peninsula) or stocks and veins cutting through the enclosing carbonate strata.
Properties of nepheline syenite
The rock is light-colored (has a brightness of about 85.5%), has a greenish, yellowish or reddish tint. The weathered surface is sometimes bluish. The density of the rock is about 2.6 g/cm3, the Mohs hardness is 6. The compressive strength is quite high, 180–250 MPa.
The structure and texture of nepheline syenite are characteristic of plutonic rocks formed in the deep zones of the crust. The structure is full-crystalline, usually medium-grained. Sometimes the structure can be coarse-grained, but rarely enough. The texture is most often massive (homogeneous), dense, in some varieties - trachytoid (with subparallel arrangement of tabular feldspar grains) or banded.
Varieties
The variability of the mineralogical composition determines the presence of many (several dozen) varieties of nepheline syenite. The most common are the following:
- Foyaites are leucocratic (light-colored) rocks of coarse-grained structure containing albite, clinopyroxene, amphibole, less often olivine. Among the accessory minerals are rare earth, titanium and zirconosilicates.
- Lujavrites are greenish-black rocks. They also contain alkaline pyroxene and amphibole, albite, while the rock is maximally supersaturatedalkalis. Lujavrites are enriched in iron, titanium, manganese and calcium, and many of them are enriched in zirconium, niobium, and rare earth elements. They have a trachytoid or banded texture.
- Miaskites. They contain amphibole, biotite, albite, and sometimes calcite. Accessory minerals include ilmenite, zircon, apatite, titanite, garnet and corundum. The color of the breed can be varied - from light gray to pink and dark gray.
- Rishchorrites are usually coarse-grained massive rocks of yellowish or greenish-gray color, also including biotite.
The nepheline syenite sample pictured below is foyaite from a deposit in southern Portugal.
Applications
All nepheline-containing syenites are important minerals and are used in a wide variety of industries.
Rocks with a small amount of dark-colored minerals are used in ceramic and glass production. Due to the highly alkaline properties of nepheline, this rock is an excellent raw material for special formulations used in the leather, textile and woodworking industries. A high percentage of potassium in syenite allows it to be used for the production of fertilizers that can deoxidize soils.
Nepheline and feldspar are rich in aluminium. If the content of the oxide of this metal in the rock reaches more than 23%, such syenite is an ore for aluminum.
Nepheline syenite is used in the production of self-cleaninganti-corrosion coatings for steel and concrete structures. And, of course, due to its high strength and good aesthetic qualities, it serves as an excellent facing material and is very widely used in construction and finishing works.
