Heavy mineral

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Close-up of heavy mineral sands, mainly in this case of monazite consisting

Heavy mineral is a term from mineralogy and petrography , which in the broadest sense includes all minerals that have a density higher than 2.9 g / cm³. In a narrower sense, the term is restricted to those minerals that are present as detritic grains with mostly a very small proportion in clastic sediments and sedimentary rocks . In this sense, enriched in soap deposits , as ore minerals they are of geological importance. The so-called heavy mineral analysis , which is used in the reconstruction of former delivery areas (provenance analysis ) , is scientifically relevant .

Among the better-known heavy minerals mainly include many of the gemstone -Fabrication used minerals (gems) as tourmalines (2.82 to 3.32 g / cc), diamond (3.52 g / cc), shell (3.5 to 4 , 3 g / cm³) and zircon (4.6 to 4.7 g / cm³), but also precious metals such as gold (19.32 g / cm³) and platinum (21.45 g / cm³).

Sedimentology

The primary content of heavy minerals in sediments and sedimentary rocks fluctuates greatly and is primarily dependent on the rocks that are or have been in the delivery area or have been eroded there. In general, the delivery area must be a deeply eroded rump mountain dominated by crystalline rocks so that the corresponding detritus contains significant proportions of heavy minerals. Igneous rocks of the earth's mantle ( peridotite ) and the deep earth's crust ( gabbro ) as well as (relatively) highly metamorphic rocks (e.g. amphibolite , granulite , eclogite ) generally have higher heavy mineral contents (“heavy mineral fertility”) than, for example, granitoids or sedimentary ones Rocks .

However, compared to other minerals such as the almost ubiquitous quartz , heavy minerals can be depleted or enriched through secondary processes in sediments. Hydraulic sorting is one of these processes, which is subject to the principle of sedimentation speed in the surrounding medium (e.g. in water): Grains that were deposited together as sediment usually had the same sinking speed and thus a similar hydraulic rate Behavior in the flowing medium. The sinking speed depends primarily on the specific density of the material, but also on the shape and size of the sinking particles. This means that in the same sediment, the grains from the denser material (heavy minerals) are often finer-grained than those from the less dense material (e.g. quartz or feldspar ). Extreme sorting processes can lead to the formation of heavy mineral soaps , in which the sediment body locally mainly consists of heavy minerals.

Gold washer when the light particles were washed out of the washing pan

This principle is also used in the artificial enrichment of heavy minerals, for example when panning for gold with a washing pan. The denser particles accumulate at the bottom of the pan as they move back and forth, while the less dense particles are displaced upwards. By carefully flushing out over the edge of the washing pan, the less dense particles are then separated off and the dense fraction remains.

In research, heavy minerals are z. B. enriched for provenance analyzes in the laboratory. This is accomplished , among other things, with the help of so-called heavy liquids with densities between 2.8 and 3.3 g / cm³. In the past, these were bromoform and diiodomethane , which are now only used sporadically due to their toxicity. A popular non-toxic alternative is the water-soluble sodium poly tungstate . The pre-sieved sediment sample is placed in a funnel filled with sodium polytungstate and equipped with a stopcock at the outlet and suspended by stirring . While the heavy minerals sediment relatively quickly, the lighter minerals float up or remain in suspension. The heavy mineral fraction is withdrawn by briefly opening the stopcock. It is then filtered (the expensive filtrate is reused) and any residues of the heavy fluid are removed by rinsing with water.

Heavy minerals are chemically stable to different degrees. The most stable minerals include zircon, tourmaline and rutile, while heavy minerals such as olivine, pyroxene and amphibole are rather unstable. Intensive chemical weathering, long transport cycles or diagenesis can attack and dissolve chemically unstable minerals.

ID

Example of a heavy mineral sample with staurolite, kyanite, tourmaline and plenty of biotite under the polarizing microscope (single polarized light)

In traditional heavy mineral analysis, the heavy mineral fraction enriched in the laboratory is analyzed with an embedding medium such as B. Canada balsam covered slide applied and then examined under a polarizing microscope . The mineral grains are identified on the basis of their optical properties such as birefringence , pleochroism and extinction as well as other characteristics such as shape and color. In recent years, the use of alternative techniques such as Raman spectroscopy or X-ray spectroscopy has also increased.

Web links

literature

  • Rudolf Graubner: Lexicon of geology, minerals and rocks . Emil Vollmer Verlag, Munich 1980, ISBN 3-87876-327-1 , p. 348 .
  • Eduardo Garzanti, Sergio Andò: Heavy Minerals for Junior Woodchucks. Minerals. Vol. 9, No. 3, 2019, Item No. 148, doi: 10.3390 / min9030148 .

Individual evidence

  1. ^ Hans Murawski, Wilhelm Meyer: Geological dictionary . 12th edition. Spektrum Akademischer Verlag, Heidelberg 2010, ISBN 978-3-8274-1810-4 , pp. 150 .
  2. ↑ Heavy minerals. In: Spectrum online encyclopedia of geosciences. Spektrum Akademischer Verlag, accessed December 8, 2016 .
  3. ^ Eduardo Garzanti, Sergio Andò: Heavy mineral concentration in modern sands: Implications for provenance interpretation . In: Maria A. Mange, David T. Wright (eds.): Heavy Minerals in Use (=  Developments in Sedimentology . Volume 58 ). Elsevier, Amsterdam 2007, p. 517-545 , doi : 10.1016 / S0070-4571 (07) 58020-9 .
  4. ^ Eduardo Garzanti, Sergio Andò, Giovanni Vezzoli: Settling equivalence of detrital minerals and grain-size dependence of sediment composition . In: Earth and Planetary Science Letters . tape 273 , no. 1–2 , 2008, pp. 138–151 , doi : 10.1016 / j.epsl.2008.06.020 (alternative full text access : academia.edu ).
  5. ↑ a b c Wolfgang Boenigk: Heavy mineral analysis . Enke, Stuttgart 1983, ISBN 978-3-432-92931-6 , pp. 158 .
  6. ^ Mary Roden-Tice: Mineral separation and provenance lab exercise. Exercise script, State University of New York at Plattsburgh, 1997 ( Word file 60 kB).
  7. ^ A b MA Mange, FW Maurer: Heavy minerals in color . Chapman & Hall, 1992, pp. 147 .
  8. ^ S Andò, D Bersani, P Vignola, E Garzanti: Raman spectroscopy as an effective tool for high-resolution heavy-mineral analysis: Examples from major Himalayan and Alpine fluvio-deltaic systems . In: Spectrochimica Acta Part A . tape 73 , 2009, pp. 450-455 .