Formed as aggregates of crystals

Mineralogists interesting not only the sequence of growth of one mineral species to another, but also to highlight each form of the mineral. Often a substance crystallizes in the form of a completely different, sometimes bizarre, aggregates.
Why is the same mineral can form aggregates of different species? Development of individuals and their splice depends on the conditions of education. The units are composed of a mineral called homogeneous or monomineral. Having grown up in the nature of the crystals can be placed in the space randomly or in order.
Irregular intergrowths
The name refers to units with a disorderly arrangement of crystals.
Granular
This is the most basic units of irregularities. These include, for example, include marble, composed of calcite grains.
Geodes and Druzes
In geode crystals cover the inside of the cavity. To friends and brushes crystals growing on an exposed surface. In the form of friends and geodes crystallizes many minerals, including widespread, such as calcite and quartz.
Rounded and branched
Frequent grozdevidnoy units, round and kidney-shaped, formed by individual spherulites. Each spherulite is a spherical selection consisting of the finest crystals that grow from a central location in all directions, firmly adhering to each other. Aggregate of many spherulites has burgistuyu surface. Such forms of isolation are common in goethite and marcasite. Spherulitic structure characteristic of stalactites with stalagmites.
Dendritic aggregates, or dendrites, shaped like a tree branch. Such
type of education is common in fast crystallization with a sharp decrease in temperature. Often in the form of dendrites crystallized native metals (silver, gold and copper) and some oxides and hydroxides (pyrolusite, psilomelane).
More correct
In fibrous aggregates thin crystals (fibers) are oriented parallel to each other. These growths are more ordered and are particularly characteristic of chrysotile, crocidolite, tremolite and other minerals, united by the common term "asbestos".
Lamellar aggregates characteristic of minerals with a very perfect cleavage, such as mica and gypsum. In foliated aggregates individual grains resemble leaves. So stand barite and some zeolites.
Among the rarer units - such bizarre forms as "cockscomb" marcasite. It consistently crystals grow in one plane. Roses and rosettes (depending on size) in the form of aggregates called a rose. Thus, the famous "Desert Rose" folded gypsum mixed with sand, because they are formed in desert conditions.
Regularities splices
When you first look at these units in their appearance captured some geometric pattern. Location of individuals subject to the peculiarities of the crystal structure of the mineral. Mesh aggregates are formed at the intersection of the crystals at an angle given by the geometry of the crystal structure. These units are characteristic sagenita - needle varieties of rutile. In the case of radiating aggregates prismatic crystals grow from one point to the sides like spokes on a bicycle wheel. This category also includes rolling aggregates, which are incomplete circle. In the sheaf-like aggregates of prismatic crystals as if in a bun at the base, and their peaks freely apart, like ears of corn into sheaves. So crystallized natrolite and stilbite. Avtoepitaksicheskimi called aggregates formed by different generations of a single mineral. Crystallographic axes previously formed and re-growing crystal coincide, so the "new" individual is a natural continuation of the "old." This often occurs with quartz vein. Separate cases are auto-epitaxy scepter quartz, amethyst which grows on rock crystal. In parallel units crystals fused together the edges. Such splice formed tabular hexagonal crystals vanadinita looks like a mosaic. The regular units called twins grow together if individuals can mentally combine with each other in mirror image in the plane or a rotation around an axis
Doubles
Minerals form twins by different laws. How will the twinning splice depends on the characteristics of the crystal structure of the mineral.
In nature, the formation of twins - a common phenomenon typical of 80% of chemical species. Specific patterns of aggregates can serve as a reliable diagnostic of the mineral. Therefore, gathering a collection, it is necessary to get acquainted with common types of twins.
What is a double?
Twins called legitimate units, individuals who can mentally combine with each other by reflection in the plane of symmetry or rotation about an axis. So, to imagine the simplest double, attach the plate of gypsum crystals narrow edge to the mirror - get a classic double fusion, called "swallow tail." Legitimate aggregates of a few individuals are called the tees chetvernikami, shesternikami.
TYPES simular
Depending on the form of the unit and the law twinning distinguish different types of twins. In case with "swallow tail", which is a double fusion, forming its crystals are reversed with respect to each other. Typical examples of this type of twins are also Japanese counterpart quartz and frequent doubles spinel and diamond. In the case of twins germination impression that they form crystals penetrate each other. In fact, individuals have grown at the same time, equally "share" a common part of the crystal together. These twins are very characteristic of staurolite, pyrite, calcite, dolomite.
It happens that the crystals simulate a higher symmetry than the one that is inherent in this mineral in reality. They are called mimetic twins. For example, T-form hexagonal crystals of aragonite, although aragonite belongs to the orthorhombic system. Called polysynthetic twins, the addition of many individual crystals, fused on the same plane. They are very typical of albite. Whereas in the cyclic and crank some individuals twins fused with constant mutual twinned orientation, like rutile, chrysoberyl, cerussite.
Why is there a simular
The emergence of a variety of factors contribute to the twins in the first place particular relative position of seed crystals during crystallization from solution or melt. If the crystallographic directions of nascent crystals are the same, formed generally parallel to the machine if they are focused on one of the twin laws - grows double. In addition, the twins in the mineral may occur during the transition of one polymorph to another. For example, high-temperature hexagonal quartz into a low temperature to form a trigonal dofineyskih twins. Also may cause doubles mechanical effect on the crystals. In particular, in the calcite marble observed polysynthetic twins, because these rocks formed at high pressures.
DIVERSITY simular
The crystal structure of each mineral favors the formation of only certain types of twins, typical for him. For example, twins and other kalinatrievyh orthoclase feldspar (anorthoclase, sanidine and microcline) obey the three laws of twinning: Carlsbad, bavenskomu and manebahskomu. In Carlsbad twin individuals, fused along one plane are rotated relative to each other by 180 °. For cassiterite, tin is an important mineral, as well as rutile typical articulated counterparts, like the open mouth chicks. Pyrite forms a counterpart of the "Iron Cross", presented two pentagondodekaedrov germination. For staurolite characterized counterparts in the form of a cross with the right angles and Saltire (with an angle of 60 °).
WHAT minerals form a twinning
Susceptible to the formation of twins, many mineral species. We list the most famous cases. Diamond, representative of the class of native elements, forms a counterpart to spinel law. In the class of sulfides and sulfosalts twins are very characteristic of arsenopyrite (in the form of a cross), bornite (shesterniki), chalcocite, chalcopyrite, cinnabar, galena, sphalerite and tetrahedrite. Halides differ doubles sprouting on fluorite law. In a class of oxides and hydroxides have been mentioned twins cassiterite, rutile, spinel and chrysoberyl. Among the most famous stand carbonates pseudohexagonal aragonite and cerussite tees, different twins of calcite and dolomite. In sulphate famous twins of gypsum, and among the phosphates - gerderita and monazite. In the class of silicate minerals, a lot of doubles. In particular, kalinatrievye feldspar and albite polysynthetic twins mentioned above. They can add doubles germination chabazite, cross chetverniki harmotome, doubles fusion titanite and tees sprouting micas.
HOW TO DISTINGUISH DOUBLE
Some units that are considered twins, in fact they are not. For example, aggregates of cubic crystals of pyrite deposits Navahun (province La Rioja, Spain) at times very much like twins, but in reality only approximately imitate their symmetry. The same thing happens with some units needles that intersect with each other and create the illusion of the twin law. Some splices two quartz crystals are sometimes mistaken for Japanese counterparts. To determine the double in front of you or not, you need to explore the geometry of the sample. Twins of the same type have fixed angles between faces. In addition, the twins are distinguished by twin incoming angles or joints which separate individuals are fused together. On the faces of polysynthetic twins manifested twin hatch.