One of the most common minerals related to rock-forming is hornblende. This is the common name for amphiboles, formed from two German words - “horn” and “blind.” In a split form, the crystals of this mineral look like horn.
External description and properties
The appearance of the hornblende allows you to easily determine it among other minerals. It is distinguished by intergrown short-columned crystals with a hexagonal or rhombic cross section.
This is a fairly solid opaque mineral with a small specific gravity and unique cleavage. The hardness index is 5.5-6 on the mineralogical scale. The density of hornblende is an average of 3100 to 3300 kg / m³. Cleavage is observed in two directions at an angle of 124 degrees.
Hornblende does not differ in variety in color. It can be from light green to brownish black (usually these are basaltic rocks with a high content of alkaline compounds). Minerals of any color have an equally beautiful glass, semi-metallic with overflow shine. This breed is not exposed to acids. With strong heating, it can melt into a glass of dark green color.
Chemical composition
It is unstable and varies quite widely. The relations of aluminum to ferric iron, as well as magnesium to ferrous, are changing. Potassium may prevail over magnesium.
In the presence of a high titanium content (up to 3%), the mineral is called "basalt hornblende." The composition is generated depending on the totality of chemical elements, among which potassium oxide can be from 10 to 13%, iron oxide - from 9.5 to 11.5%, iron oxide - 3-9%, magnesium oxide - 11-14%, sodium oxide - 1.5%, silicon dioxide - 42-48%, aluminum oxide - 6-13%.
In the process of weathering, the rock decomposes into opals and carbonates. Interaction with hydrothermal solutions leads to the conversion of the mineral into chlorite, epidote, calcite and quartz.
Under the influence of various physical factors, the rock can undergo complex chemical processes leading to the formation of intermediate compositions.
Origin
Hornblende is a rock-forming mineral and the main component of amphibolites, shales and gneisses. It occurs, as a rule, in the process of exposure of pegmatites to igneous rocks. In volcanic ash, it is sometimes found in the form of single crystals. In the form of primary material in rocks poured onto the surface, this mineral is quite rare.
The ordinary hornblende described above can be transformed into basaltic. This usually happens in lava flows, under oxidizing conditions and heating to a temperature of 800 ° C. This process is quite easy to create artificially.
Place of Birth
Large crystals of hornblende are rare, so they are of great interest to collectors. They are mainly observed in gabbro pegmatites, of which there are not many. In the Urals, in the region of Mount Sokolina, well-formed crystals up to 0.5 m long were found. Very beautiful specimens of this mineral are found in the Czech Republic, Norway, as well as in the volcanic lava of Vesuvius in Italy.
Hornblende is widespread in the Ore Mountains of Germany, rich in calc-silicate rock. The Meissen syenite massif is known for its rich deposits of this mineral. Large crystal deposits are located in Burma.
