Nontronite

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Nontronite
Nontronite-132873.jpg
Nontronite from Iron Knob , South Australia (field of view: 6 mm)
General and classification
other names

Chloropal

chemical formula
  • Na 0.3 Fe 3+ 2 (Si, Al) 4 O 10 (OH) 2 · nH 2 O
  • Fe 3+ 2 [(OH) 2 | (Si, Al) 4 O 10 ]. Na 0.3 (H 2 O) 4
  • (Ca 0.5 , Na) 0.3 Fe 3+ 2 (Si, Al) 4 O 10 (OH) 2 * nH 2 O
Mineral class
(and possibly department) 		Silicates and germanates - layered silicates (phyllosilicates)
System no. to Strunz
and to Dana 		9.EC.40 ( 8th edition : VIII / H.19)
71.03.01a.03
Similar minerals Beidellite , montmorillonite
Crystallographic Data
Crystal system monoclinic
Crystal class ; symbol monoclinic prismatic; 2 / m
Space group C 2 / m (No. 12)Template: room group / 12
Lattice parameters a  = 5.28  Å ; b  = 9.14 Å; c  = 9.78 Å
β  = 101.0 °
Formula units Z  = 2
Frequent crystal faces {001}, {110}
Physical Properties
Mohs hardness 1 to 2
Density (g / cm 3 ) measured: 2.2 to 2.3
Cleavage completely after {001}; {110} clearly
Break ; Tenacity broken, splintery; clayey
colour yellow, olive green to green, orange, brown
Line color greenish white
transparency translucent to almost opaque
shine earthy, resinous, waxy, dull
Crystal optics
Refractive indices n α  = 1.530 to 1.580
n β  = 1.555 to 1.612
n γ  = 1.560 to 1.615
Birefringence δ = 0.030 to 0.035
Optical character biaxial negative
Axis angle 2V = 25 to 68 ° (here: 2V x )
Pleochroism X: yellowish; Y: yellow-green (to dark brown); Z: olive green (to light brown); X <Y <Z or X <Z <Y
Other properties
Chemical behavior soluble in sodium hydroxide solution; Hydrochloric acid dissolves cations and leaves behind a silicate skeleton; adsorbs aniline dyes

Nontronite is a mineral from the mineral class of " silicates and germanates ", which can sometimes be abundant in various locations, but is generally not very common.

The mineral crystallizes in the monoclinic crystal system with the chemical composition Na 0.3 (Fe 3+ ) 2 (Si, Al) 4 O 10 (OH) 2 · nH 2 O, so it is a complex layered silicate from the group belonging to the clay minerals the smectite .

Nontronite is usually cryptocrystalline and rarely developed under the microscope recognizable crystals with pseudohexagonalem scaly habit . It is mostly found in the form of worm-shaped, radial to spherulitic, reticulated or tomentose mineral aggregates . Its color varies depending on foreign admixtures and impurities between light yellow and olive green to green. However, it also occurs in orange and brown colors. Its line color , on the other hand, is always greenish-white.

Etymology and history

The mineral nontronite was named after Nontron , a small town in the north of the Dordogne department . The actual type locality is at Le Mandereau in the municipality of Saint-Pardoux-la-Rivière . Nontronite was discovered here during the breakdown of manganese (manganese dioxide).

Nontronite is also known as chloropal because of its color and appearance . Another synonym is hydoferripyrophyllite .

The mineral was scientifically described for the first time in 1827 by Pierre Berthier . His type handpiece is in the mineralogy museum of the École Nationale Supérieure des Mines in Paris .

classification

In the 8th edition of the mineral systematics according to Strunz , which is now outdated but still in use , nontronite belonged to the mineral class of " silicates and germanates " and there to the department of " phyllosilicates ", where together with beidellite , brinrobertsite , montmorillonite , swinefordite , Volkonskoit and Yakhontovit formed the unnamed group VIII / H.19 .

The 9th edition of Strunz's mineral systematics , which has been in effect since 2001 and is used by the International Mineralogical Association (IMA), assigns nontronite to the class of " silicates and germanates " and there in the department of "phyllosilicates". This section is, however, further subdivided according to the type of layer formation, so that the mineral, according to its structure, can be found in the sub-section “Layered silicates (phyllosilicates) with mica sheets , composed of tetrahedral and octahedral networks”, where it can be found together with Beidellite, Kurumsakite , Montmorillonite , Volkonskoit (redefined recognized by the CNMNC) and Yakhontovit the "montmorillonite group" with the system no. 9.EC.40 forms.

The systematics of minerals according to Dana , which is mainly used in the English-speaking world , assigns nontronite to the class of “silicates and Germanates” and there in the “layered silicate minerals” section. Here it is together with beidellite, montmorillonite, volkonskoite and swinefordite in the "smectite group (dioctahedral smectites)" with system no. 71.03.01a within the subsection of " Layered Silicates: Layers of six-membered rings with 2: 1 clay minerals ".

Crystal structure

Nontronite.png
Structure of nontronite. The unit cell is shown with a view of the [100] plane. To make the layers clearer, the cell was doubled to the left.

_ OH 1− _ O 2− _ Fe 3+
_ Well + , only 50% of the places are occupied
_ Al 3+ 13.4% and Si 4+ 86.6%

Nontronite crystallizes in the monoclinic crystal system in the space group C 2 / m (space group no. 12) with the lattice parameters a  = 5.28  Å ; b  = 9.14 Å; c  = 9.78 Å and β  = 101.0 ° as well as 2 formula units per unit cell .

The basic building blocks of the nontronite structure are:

The tetrahedra are linked by shared oxygen atoms to form pseudo-hexagonal layers. Two of these tetrahedral layers are each held together by a layer of octahedral coordinated Fe 3+ cations . The connection of the individual octahedra within the octahedron layer takes place via common edges. In nontronite, only two of the three central octahedral positions are occupied by the Fe 3+ cations, so the mineral is dioctahedral. The layers are stacked in the direction [001] (along the crystallographic c axis) in the sequence TOT (tetrahedron-octahedron-tetrahedron). The connection between the tetrahedra and octahedra takes place via common corners, so the tetrahedra are each oriented with a point in the direction of the octahedron. Due to the TOT sequence, nontronite is one of the 2: 1 or three-layer clay minerals .

Within the tetrahedral layers, the Si 4+ cations can be substituted by Fe 3+ and / or Al 3+ cations up to a ratio of 1: 8. In the octahedra Fe 3+ can be substituted by Al 3+ and Mg 2+ . The substitutions create a negative layer charge, more than 60% of which is concentrated in the tetrahedron layer. The charge balance takes place through the incorporation of cations such as Na + , Ca 2+ and occasionally Mg 2+ , which are located between the TOT layers and also link them to one another. Water molecules are also embedded in these intermediate layers . Nontronite is therefore a cation exchanger and swellable due to the incorporation of water.

Mixed series and varieties

Nontronit forms a limited mixed series with Beidellite . Beidellite has up to one atom of Fe 3+ per formula unit, nontronite more than two atoms per formula unit. Montmorillonite and nontronite can also be viewed as a mixed series, which comes about through the substitution of Al 3+ for Fe 3+ . In this case, too, the mixed series is not complete; there is a very large mixing gap between 25 and 75 mol% Fe 3+ .

A variety , the chrome nontronite represents.

Education and Locations

Druse with nontronite and calcite from Wolf Creek Pass, San Juan Mountains , USA (diameter: 35 mm)
Nontronite from the Kremnitzer Mountains in Slovakia

Nontronite is a weathering product of biotite , as well as of basalts , kimberlites and other ultramafic rocks in general (found as crevices and coatings in these rocks). It can also displace pseudomorphic horn apertures and pyroxenes . Occasionally it is also observed as impregnation in porous contact rocks of olivine basalts.

Nontronite also occurs in poorly drained volcanic ash soils . In some mineral deposits it was formed hydrothermally by solutions enriched in silicon and iron (for example in graphite and sulphide deposits ); it also forms hydrothermally at oceanic ridges and other exit points in the deep sea. In kontaktmetamorph altered limestones nontronite may also occur. It forms authentic in recent marine sediments. Microorganisms may also play an important role in its development.

Minerals accompanying nontronite include apophyllite , calcite , chabazite , hollandite , pyrite , quartz (with the varieties chalcedony and opal ), siderite and strictite .

Nontronite can displace the laumontite previously formed in volcanic gas bubbles .

Sites in France in addition to the type locality in the Dordogne are the departments Charente , Haute-Vienne , Rhône and Saône-et-Loire . Deposits in Germany are in Bavaria near Hagendorf , in Hesse on Vogelsberg , in Lower Saxony near Göttingen and near Sankt Andreasberg, and in Saxony near Geilsdorf and Wolkenstein . Nontronit also occurs in the following countries:

Occurrence on Mars

There is reason to believe that nontronite is also present on the planet Mars . The first indications were already provided by Mariner 9 in 1971, which were then confirmed by Viking 1 and Viking 2 in 1975 (the X-ray spectrometers of the two probes found up to 47% nontronite). These assumptions were again confirmed with the infrared spectrometer from Mars Express .

See also

literature

Web links

Commons : Nontronite  - collection of images, videos and audio files

Individual evidence

  1. ↑ a b IMA / CNMNC List of Mineral Names; Nontronite ( Memento from October 28, 2016 in the Internet Archive ) (PDF 1.6 MB; p. 205)
  2. ^ Hugo Strunz , Ernest H. Nickel: Strunz Mineralogical Tables . 9th edition. E. Schweizerbart'sche Verlagbuchhandlung (Nägele and Obermiller), Stuttgart 2001, ISBN 3-510-65188-X , p.  671 .
  3. ↑ a b c d Nontronite . In: John W. Anthony, Richard A. Bideaux, Kenneth W. Bladh, Monte C. Nichols (Eds.): Handbook of Mineralogy, Mineralogical Society of America . 2001 ( handbookofmineralogy.org [PDF; 86  kB ; accessed on April 28, 2018]).
  4. ↑ Webmineral - Nontronites
  5. ↑ a b c American Mineralogist Crystal Structure Database - Nontronite 2006 (English)
  6. ↑ a b c d e Mindat - Nontronite
  7. ↑ WE Tröger: Optical determination of the rock-forming minerals. 4th revised edition. E. Schweizerbart'sche publishing bookstore; 1971; ISBN 3-510-65011-5 , p. 109
  8. ↑ Stefan Weiß: The large Lapis mineral directory. All minerals from A - Z and their properties . 5th completely revised and supplemented edition. Weise, Munich 2008, ISBN 978-3-921656-70-9 .
  9. ↑ A. Manceau, D. Chat owner, WP Gates: Physics and Chemistry of Minerals. Volume 25, 1998, pp. 347-365, Polarized EXAFS, distance-valence least-squares modeling (DVLS) and quantitative texture analysis approaches to the structural refinement of Garfield nontronite. Amcsd dataset 0008146
  10. ^ Diffraction Data In: European Journal of Mineralogy. 18, 2006 (English)
  11. ^ Caillère, S. et al .: Minéralogie des argiles. Classification and nomenclature. Masson, 1982.
  12. ↑ Birgit Köhler, Arieh Singer, Peter Stoffers: Biogenic Nontronite from Marine White Smoker Chimneys . In: Clays and Clay Minerals . tape 42 , 1994, pp. 680-701 , doi : 10.1346 / CCMN.1994.0420605 .
  13. ↑ F. Poulet, et al .: Phyllosilicates on Mars and implication for early martian climate . In: Nature . tape 438 , 2005, pp. 623-627 .