The silicon-oxygen backbone in island silicate is isolated tetrahedron [SiO4] 4- and double tetrahedron [Si2O7] 6-. The negative electron valences given by each silicon-oxygen tetrahedron are -4 and -3 respectively, which are the highest among all kinds of silicon-oxygen backbones.
The cations participating in the island silicate lattice also have high electricity prices, such as Zr4+, Ti4+, Al3+, Fe3+, Cr3+, etc., and divalent cations Mg2+, Fe2+, Mn2+, Ca2+, etc. are also included in large quantities. It comes into the crystal lattice, but in most cases it enters the crystal lattice together with trivalent and tetravalent cations.
The structure of island silicate is relatively tight, and the electron valence of complex anions and cations is relatively high. Therefore, the hardness (5~6), relative density (>3) and refractive index of island silicate All are relatively high. In nature, island silicates are mainly formed in endogenous and metamorphic processes, and rarely in epigenetic processes.
Island silicates often have relatively complete crystal forms. In addition to being used as identification features, sometimes the formation conditions of the same mineral can be analyzed from the different forms that appear.
Zircon family
Zircon, also called zircon ZrSiO4
[Chemical composition] ZrO267.22%, SiO232.78%, often containing Hf , Th, U and other homogeneous mixtures, those rich in Hf are called hafnium zircon. The Zr/Hf ratio of zircons produced from basic rocks to acidic rocks has a tendency to increase.
[Morphology] Tetragonal crystal system, the crystal shape is columnar, and the single shapes are often tetragonal prism {100}, {110} and tetragonal bipyramidal {111}, {101} (Figure 15-7). The crystal form of zircon changes depending on its origin, and therefore, it can be used as a standard feature (Figure 15-8).
Figure 15-7 The crystal form of zircon
Figure 15-8 The crystal form of zircon in various igneous rocks
[Physical properties] Pure ones are colorless. Often yellow, yellowish brown, brown black and other colors; streaks white; transparent; diamond luster, sometimes greasy luster. Hardness 7-8; cleavage parallel {110}, incomplete. Relative density 4.6~4.7. Often weakly radioactive, some zircons are amorphous due to strong radioactivity such as U and Th. The hardness of such zircons can be reduced to 6 and the relative density can be reduced to 3.8. Some zircons contain H2O and Other impurities are called hydrozircon. If the crystal plane is bent due to this, it is called curved spar; it sometimes fluoresces golden yellow under ultraviolet irradiation.
[Genesis and occurrence] Zircon is the most common accessory mineral in the three major rock types (magmatic rock, sedimentary rock and metamorphic rock), especially in granite, alkaline rock and related pegmatite. is common. The series of mineralogy characteristics of zircon play an important role in geological research, mainly reflected in three aspects: ① As an ideal and most commonly used dating mineral for studying magma evolution, rock formation and metamorphic age, it is currently possible to use single Granular zircon can be used for accurate uranium-lead dating; ② The morphological characteristics of zircon have typical significance, especially for tracing the origin and evolution of granite magma and judging the mineral content of granite; ③ The chemistry of zircon It is extremely stable in nature and is often preserved in drifting sand and appears as clastic material in sedimentary rocks and sedimentary metamorphic rocks. Therefore, by studying the morphology (single shape and aspect ratio), color, type and content of impurity elements, and the degree of amorphization of zircons, we can analyze the source of clastic materials in sedimentary rocks, conduct stratigraphic comparison, and explore A sign of the original rock properties of metamorphic rocks.
[Identification characteristics] Its identification characteristics include its crystal shape, hardness (large), luminescence and weak radioactivity. It is very similar to rutile and cassiterite, but the chemical properties of zircon are more stable. It is completely insoluble in hot phosphoric acid (rutile powder is soluble and reacts with Ti), and it does not react with hydrochloric acid on zinc plates (cassiterite can produce Sn film).
[Use] Mainly used as special high-grade refractory materials to prepare ZrO2-Al2O3-SiO2 series refractory materials. Zircon has high thermal stability and refractive index, and is also used as an "opacifier" in building ceramic glazes. In practical applications, the dosage and process should be controlled to avoid excessive radioactivity and environmental pollution. This mineral is the main source of zirconium and hafnium, and can comprehensively utilize impurities such as radioactive elements.
Olivine family
Olivine (Mg, Fe) 2 [SiO4]
[Chemical composition] Mg2+ and Fe2+ are completely homogeneous.
Pure forsterite contains MgO57.29%, SiO242.71%, pure fayalite contains FeO70.51%, SiO229.49%. The composition of natural olivine is between the two, and generally contains Mg. In addition, it often contains homogeneous mixed substances Ni, Co, etc.
[Crystal structure] Orthorhombic crystal system. In the structure, silicon-oxygen tetrahedrons and cations are arranged alternately, like a tetrahedral island surrounded by sea water (cations) (Figure 15-9), so it has an island shape. silicate.
[Form] The crystal shape is as shown in Figure 15-10, but it is rare. It is usually in the form of granular aggregates or scattered particles distributed among other mineral particles.
Figure 15-9 The structure of forsterite tetrahedron - [SiO4] 4-; circle - Mg2+
Figure 15-10 The crystal form of olivine
[Physical Properties] Yellow-green (olive green) or gray-yellow-green, with the increase of iron content, the color can reach dark green to black; streaks are white; transparent; glass luster. Hardness 6.5~7; cleavage parallel to {100} and {010}, incomplete. The relative density of pure forsterite is 3.22, and the relative density of pure fayalite is 4.39.
[Genesis and occurrence] Olivine is mainly formed in ultrabasic rocks and basic rocks, and is the main component of peridotite and other rocks. Forsterite can also form in the contact zone of dolomite or dolomitic limestone, a magnesium-rich carbonate rock. Fayalite is rare and may occasionally be found in some dikes or volcanic rocks.
Peridot generally does not occur with quartz. When excess SiO2 in magma makes it possible to form quartz, it first reacts with olivine to form pyroxene:
A brief tutorial on mineralogy
Therefore, olivine is formed in an environment that is unsaturated with SiO2 middle. Ultrabasic igneous rocks mainly containing olivine (including its alteration product serpentinite, especially kimberlite and lamprophyre) are the ore-bearing parent rocks of chromium, nickel, copper and diamond, so they are Look for important prospecting signs for these minerals.
Olivine is very unstable. Hydrothermal alteration can transform peridotite into serpentinite on a large scale, and can also form magnesium-containing minerals such as asbestos, talc, magnesite, and brucite. and magnetite.
During weathering, olivine rapidly decomposes to form silica colloids and other silicate minerals.
[Identification characteristics] It is easy to identify based on its granular shape, olive green color, high hardness and origin.
[Use] Forsterite can be used as refractory materials, with a melting point of 1890°C; olivine with good crystallization and high transparency can be used as gemstones.