About 1% of all rocks of igneous origin present in the earth's crust are represented by rocks of the nepheline syenite group. In this article we will consider their main characteristics: composition, properties, genesis and existing varieties, as well as find out where these rocks are used.
Taxonomy
Nepheline syenite is an intrusive rock. Its systematic characteristic is as follows:
- Class - plutonic rocks;
- Order - rocks of medium composition (silica content from 52 to 63%);
- Suborder - moderately alkaline rocks;
- Family - syenites;
- Type of rock - syenite.
The adjective "nepheline" indicates a rock-forming mineral. Syenites can also be enstatite, hornblende, and so on.
Mineral composition
The quantitative ratios of the minerals forming the rock can fluctuate, which leads to the existence of a large number of varieties. In general terms, the mineral composition of nepheline syenite is as follows:
- Feldspars (potassium) - orthoclase or microcline - from 65 to 70%;
- Feldspatoids - nepheline - about 20%;
- Non-ferrous minerals (mainly alkaline pyroxenes, amphiboles, lepidomelan biotite) - from 10 to 15%.
Feldspathoids, including nepheline, are similar in chemical composition to feldspars, but differ from them in a significantly lower content of silica SiO 2 .
Non-ferrous minerals are represented mainly by alkaline pyroxenes and amphiboles, and glandular biotite may be present. Sphene, apatite, zircon, perovskite and others are accessory (“additive” minerals that do not affect the rock classification), the content of which in this type of rock is very significant.
Genesis and occurrence of the breed
The formation of nepheline syenite is associated with the 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 influence of feldspathic (nepheline) hydrothermal solutions. As a result, the rock is very depleted of silicon, that is, practically devoid of quartz.
This rock is very common around the world. Its often occurring form 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 rods and veins cutting the host carbonate stratum.
Properties of nepheline syenite
The breed is light-colored (has a brightness of about 85.5%), has a greenish, yellowish or reddish hue. The weathered surface is sometimes bluish. The density of the rock is about 2.6 g / cm 3 , the hardness on the Mohs scale 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 a subparallel arrangement of tabular feldspar grains) or banded.
Varieties
The variability of the mineralogical composition leads to the presence of many (several tens) varieties of nepheline syenite. The following are most common:
- Foyaites are leucocratic (light) coarse-grained rocks containing albite, clinopyroxene, amphibole, and less commonly olivine. Among accessory minerals, rare earths, titanium and zirconosilicates are present.
- Luyavrites are rocks of a greenish-black color. They also contain alkaline pyroxene and amphibole, albite, while the rock is maximally supersaturated with alkalis. Luyavrites are enriched with iron, titanium, manganese and calcium, and many of them are also rich in zirconium, niobium, and rare earth elements. They have a trachoid or banded texture.
- Miaskits. They contain amphibole, biotite, albite, sometimes calcite. Among accessory minerals are ilmenite, zircon, apatite, titanite, garnet and corundum are found. The color of the breed can be varied - from light gray to pink and dark gray.
- Ricechorrites are, as a rule, coarse-grained massive rocks of yellowish or greenish-gray color, which also include biotite.
The nepheline syenite sample in the photo below is a foyaite from a deposit in southern Portugal.
Areas of use
All syenites containing nepheline are an important mineral raw material 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 compounds used in the leather, textile and woodworking industries. A large percentage of potassium in syenite allows it to be used for the production of fertilizers that are able to deoxidize the soil.
Nepheline and feldspar are rich in aluminum. If the oxide content of this metal in the rock reaches more than 23%, such syenite is an ore for aluminum.
Nepheline syenite is used in the manufacture of self-cleaning anti-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.