Hydroboracite
Hydroboracite is a rarely occurring mineral from the mineral class of " borates ". It crystallizes in the monoclinic crystal system with the chemical composition CaMg [ B3O4 (OH ) 3] 2.3 H2O, that is a hydrous calcium magnesium borate.
Hydroboracite developed acicular or tabular to prismatic crystals, usually fan-shaped or radialstrahligen mineral aggregates are arranged. But it also occurs in the form of fibrous or fine-grained aggregates. In its pure form Hydroboracite is colorless and transparent. But it may also appear white and also assume a yellow color rarely by foreign admixtures by multiple refraction due to lattice defects or polycrystalline training, the transparency decreases accordingly.
Special Features
Hydroboracite is only partially in boiling water but readily soluble in warm hydrochloric acid and nitric acid. Before the blowpipe it melts easily to a clear, colorless bead, the Lötrohrflamme discolored slightly greenish during the melting process.
Etymology and history
First analyzed and described the mineral in 1833 (published 1834) by Germain Henri Hess, who is based on its water content ( ancient Greek hydor, "water" ) and its relationship with Boracite ( Mg 3 [Cl | BO 3 | B6O10 ] ), referred to as Hydroboracite. The name is misleading, because Hess was referring not to the chemical composition, but only on the then adopted, equal molar ratio of cations ( bases).
Hydroboracite was first discovered in a mineral collection, from the Caucasus, where he was mistaken for plaster. As a type locality applies, however, the boron deposit on Indersee ( Indian Lake, Indian salt dome ) in the region Atyrau in Kazakhstan.
Classification
In the now outdated but still in use 8th edition of the mineral classification by Strunz the Hydroboracite belonged to common mineral class of " carbonates, nitrates and borates " and then to the Department of " chain borates [ B2O4 ] 2 - to [ B6O10 ] 2 - ", where he along with the colemanite " colemanite Hydroboracite group " with the system no. V/J.03 and the other Member Jarandolith formed.
The 9th edition used since 2001 and valid by the International Mineralogical Association (IMA ) of the Strunz'schen Mineral classification assigns the Hydroboracite in the now independent class of " borates " and there in the department of " Triborate " one. This is also further divided according to the crystal structure, so that the mineral according to its construction in the subsection " chains and belt ( Ino ) Triborate " is to find where the only member is the unnamed group 6.CB.15.
The mainly common in English-speaking classification of minerals according to Dana assigns the Hydroboracite as the outdated Strunz'sche systematics in the common class of " carbonates, nitrates and borates " and there in the department and the same subdivision of the " water -containing borates with hydroxyl or halogen " a. Here he can be found as the only member of the unnamed group 26.03.06.
Education and Locations
Hydroboracite is a typical secondary mineral that forms by weathering of Colemannit under the influence of groundwater. As an accompanying mineral, among others still Tunellit occur near this.
A rare mineral formation Hydroboracite could be detected only in a few localities, where so far (as of 2013) are about 40 localities known as. Apart from its type locality Indersee the mineral in Kazakhstan occurred previously to only in the salt dome Chelkar in Aktobe region.
In Germany the mineral previously could only be found in the shaft " Brefeld " at Tarthun in Saxony -Anhalt and in a quarry on Kohnstein and Himmelberg near Niedersachswerfen in the Thuringian district of Nordhausen.
Other localities lie among others in Argentina, Azerbaijan, China, Iran, Canada, Turkey and the U.S. state of California.
Crystal structure
Hydroboracite crystallizes in the monoclinic space group P2 / c ( Raumgruppen-Nr. 13) with the lattice parameters a = 11.77 Å; b = 6.68 Å; c = 8.24 Å and 102.6 ° β = and two formula units per unit cell.