Mineral aggregate

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As a mineral aggregate , and mineral aggregate or short unit is an arbitrary, spatially narrowly limited, natural association ( association ) of the same or of different mineral species , respectively. In contrast to crystal twins , the crystals of an aggregate develop randomly in different directions.

The intergrown crystals , crystallites or grains sometimes form typical aggregate forms .

Appearance

The appearance of the aggregates depends, among other things, on the size, the formation, the structure as well as the type and number of minerals. There are a total of ten types of aggregate forms, the subspecies of which, depending on the growth form, have special names describing the crystal form, which often also correspond to their crystal habit :

  1. Bulky, coarse, amorphous, earthy : this type includes many ores such as chalcopyrite , but also gemstones such as opal or turquoise .
  2. Granular to crystalline aggregates : The range of this type of aggregate extends from cryptocrystalline and microcrystalline (still visible with the microscope) crystallites or grains to well-formed crystals. The following subspecies are:
    1. Fibrous, hairy or tomentose like chrysotile , natrolite or okenite
    2. Needle to stem-like such as kakoxen , rutile or stibnite
    3. Column-shaped like the quartz and many others
    4. Mica-like, scaly, flaky or lamellar such as muscovite or biotite, among others
    5. Radial and rosette-shaped such as aurichalcite , natrolite , hematite , ilmenite and molybdenite .
  3. Steps and drusen : A mineral aggregate with free-standing crystals is called a crystal step, or step for short , in the old miner's language . Many stages are parts of Druze . Aggregates of this type can be divided into comb-like or brush-like (more or less parallel to one another) and tabular (unhindered growth in two directions), depending on their ability to develop crystals.
  4. Secretions : If cavities are filled with colloidal or crystal-forming mineral substances, concentric or shell-like aggregates often arise. The best examples are from here silica formed Agate - almonds , but also Rhodochrosit and Schalenblende .
  5. Concretions : This type of aggregate forms in the form of irregular spheres or coarse-leaved to rose-shaped caking mainly in sediments . Sand roses (variety of gypsum or barite ), often of impressive size, are formed in this way.
  6. Oolites or spherulites : The term oolite or spherulite includes all aggregates in spherical, grape-like (botryoidal), bulbous, roe-like, kidney, opal-like or vesicular form. Also included are mineral aggregates with a shell structure, as well as the so-called "glass head" as a larger spherical to grape shape.
  7. Flow forms : The flow forms mainly include collomorphic aggregates such as stalactites ( stalactites , stalagmites and similar), but also the well-known curl forms, such as those found in plaster of paris or psilomelan .
  8. Tubes, capillaries and coral-like aggregates : Among other things, Kylindrite and aragonite in the iron blossom variety form these forms.
  9. Skeletons and dendrites : The vast majority of metallic minerals such as gold , silver , copper and others grow in this form and, in addition to skeletons and dendrites (tree-like aggregates), also form wires, webs and braids.
  10. Crusts, efflorescence ( efflorescence ) approaches and Forged : Many secondary minerals cover primary minerals that make them such. B. caused by weathering, with thin, crusty coatings or efflorescence. Other terms referring to this type of aggregate are approaches, coverings and fittings. The azurite - malachite efflorescence, which often merges into one another, and sulfur crusts on fumaroles are known.

literature

  • Paul Kukuk: geology, mineralogy and theory of deposits . Springer, Berlin, Göttingen, Heidelberg 1951, pp. 144 ff . ( limited preview in Google Book search).
  • Walter Schumann: Precious stones and gemstones. All kinds and varieties. 1900 unique pieces . 16th revised edition. BLV Verlag, Munich 2014, ISBN 978-3-8354-1171-5 , pp. 15 .

Web links

Individual evidence

  1. ↑ Martin Okrusch, Siegfried Matthes: Mineralogie. An introduction to special mineralogy, petrology and geology . 7th, completely revised and updated edition. Springer, Berlin [a. a.] 2005, ISBN 3-540-23812-3 , pp. 12 .
  2. ↑ Rudolf Graubner: Lexicon of Geology, Minerals and Rocks . Emil Vollmer Verlag, Munich 1980, ISBN 3-87876-327-1 .