Hydroboracite

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Hydroboracite
Hydroboracite (aka) .jpg
White hydroboracite from Niedersachswerfen , Thuringia
General and classification
chemical formula CaMg [B 3 O 4 (OH) 3 ] 2 • 3H 2 O
Mineral class
(and possibly department) 		Borates (formerly carbonates, nitrates and borates)
System no. to Strunz
and to Dana 		6.CB.15 ( 8th edition : V / J.03)
03/26/06/01
Crystallographic Data
Crystal system monoclinic
Crystal class ; symbol monoclinic prismatic; 2 / m
Room group (no.) P 2 / c (No. 13)
Lattice parameters a  = 11.77  Å ; b  = 6.68 Å; c  = 8.24 Å
β  = 102.6 °
Formula units Z  = 2
Physical Properties
Mohs hardness 2 to 3
Density (g / cm 3 ) measured: 2.15 to 2.17; calculated: 2.170
Cleavage completely after {010}, clearly after {100}
Break ; Tenacity not defined
colour colorless to white, yellow
Line color White
transparency transparent to translucent
shine Glass gloss, silk gloss in fibrous aggregates
Crystal optics
Refractive indices n α  = 1.520 to 1.523
n β  = 1.534 to 1.535
n γ  = 1.569 to 1.571
Birefringence δ = 0.049
Optical character biaxial positive
Axis angle 2V = measured: 60 to 66 °; calculated: 62 to 66 °
Other properties
Chemical behavior partially soluble in boiling water

Hydroboracite is a rarely occurring mineral from the mineral class of " borates ". It crystallizes in the monoclinic crystal system with the chemical composition CaMg [B 3 O 4 (OH) 3 ] 2 · 3H 2 O, so it is a water-containing calcium - magnesium borate.

Hydroboracite develops needle-like or tabular to prismatic crystals , which are mostly arranged in fan-shaped or radial-radial mineral aggregates . But it also occurs in the form of fibrous or fine-grained aggregates. In its pure form, hydroboracite is colorless and transparent. However, due to multiple refraction due to lattice construction defects or polycrystalline formation, it can also appear white and rarely also take on a yellow color due to foreign admixtures, whereby the transparency decreases accordingly.

Special properties

Hydroboracite is only partially soluble in boiling water, but easily soluble in warm hydrochloric acid and nitric acid. In front of the soldering tube , it melts slightly to a clear, colorless pearl, whereby the soldering tube flame turns slightly green during the melting process.

Etymology and history

Lemon yellow hydroboracite on Görgeyite from the boron deposit near Atyrau , Kazakhstan (size: 6.5 × 6.3 × 3.8 cm)

The mineral was first analyzed and described in 1833 (published in 1834) by Germain Henri Hess , who based it on its water content ( ancient Greek hýdor , "water" ) and its relationship with boracite (Mg 3 [Cl | BO 3 | B 6 O 10 ]) called hydroboracite. The name is misleading, however, as Hess did not refer to the chemical composition, but only to the same molar ratio of the cations (bases) assumed at the time .

Hydroboracite was first discovered in a mineral collection from the Caucasus , in which it was mistakenly mistaken for gypsum . As type locality but the true boron - deposit on Indersee ( Indian Lake , Indian salt dome ) in the area of Atyrau in Kazakhstan .

classification

In the now outdated, but still in use 8th edition of the mineral classification according to Strunz , the hydroboracite belonged to the common mineral class of "carbonates, nitrates and borates" and there to the division of "chain borates [B 2 O 4 ] 2− to [B 6 O 10 ] 2− ", where together with colemanite he created the" colemanite hydroboracite group "with the system no. V / J.03 and the other member Jarandolith formed.

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 hydroboracite to the now independent class of "borates" and there in the department of "triborates". This is further subdivided according to the crystal structure, so that the mineral can be found according to its structure in the sub-section "Chain and Ribbon Triborates (Ino-Triborates)", where the unnamed group 6.CB.15 is the only member .

Also the systematics of minerals according to Dana , which is mainly used in the English-speaking world , assigns the hydroboracite to the common class of "carbonates, nitrates and borates" and there in the section and subdivision of the same name of "water-containing borates with hydroxyl or halogen " a. Here he can be found as the only member of the unnamed group 03/26/06 .

Education and Locations

Tufts of needle hydroboracite from the "Boraxo Mine" in Death Valley National Park , California, USA (size: 3.4 × 2.5 × 2.3 cm)

Hydroboracite is a typical secondary mineral that is formed from colemannite by weathering under the influence of groundwater . In addition to this, tunellite can also occur as an accompanying mineral.

As a rare mineral formation, hydroboracite could only be detected at a few sites, with around 40 sites being known to date (as of 2013). In addition to its type locality Indersee, the mineral has only appeared in Kazakhstan in the Chelkar salt dome in the Aqtöbe area .

In Germany, the mineral has so far only been found in the “Brefeld” shaft near Tarthun in Saxony-Anhalt and in a quarry on the Kohnstein and near Himmelsberg near Lower Saxony in the Nordhausen district of Thuringia .

Other locations are in Argentina, Azerbaijan, China, Iran, Canada, Turkey and in the US state of California.

Crystal structure

Hydroboracite crystallizes monoclinically in the space group P 2 / c (space group no. 13) with the lattice parameters a  = 11.77  Å ; b  = 6.68 Å; c  = 8.24 Å and β = 102.6 ° and two formula units per unit cell .

See also

literature

Web links

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

Individual evidence

  1. ↑ a b c d e 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.  344 .
  2. ↑ a b c Hydroboracite , In: John W. Anthony, Richard A. Bideaux, Kenneth W. Bladh, Monte C. Nichols (Eds.): Handbook of Mineralogy, Mineralogical Society of America , 2001 ( PDF 66.6 kB )
  3. ↑ a b c Mindat - Hydroboracite
  4. ↑ a b H. Hess : The hydroboracite, a new mineral species. In: Annals of Physics and Chemistry. Volume 31 (1834), pp. 49-53, doi: 10.1002 / andp.18341070402 ( PDF 265.2 kB ).
  5. ↑ Mineralienatlas - type locality Inder See (accessed on June 26, 2014)
  6. ↑ Mindat - Number of localities for hydroboracite
  7. ↑ List of locations for hydroboracite in the Mineralienatlas and Mindat