Boracite
| Boracite | |
|---|---|
 |
|
| General and classification | |
| chemical formula |
|
|
Mineral class (and possibly department) |
Borates (formerly carbonates, nitrates and borates) |
|
System no. to Strunz and to Dana |
6.GA.05 ( 8th edition : V / L.04) 06/25/01/01 |
| Crystallographic Data | |
| Crystal system | orthorhombic |
| Crystal class ; symbol | orthorhombic-pyramidal; mm 2 |
| Space group | Pca 2 1 (No. 29) |
| Lattice parameters | a = 8.55 Å ; b = 8.55 Å; c = 12.09 Å |
| Formula units | Z = 4 |
| Physical Properties | |
| Mohs hardness | 7 to 7.5 |
| Density (g / cm 3 ) | measured: 2.91 to 3.10; calculated: 2.97 |
| Cleavage | no |
| Break ; Tenacity | shell-like to uneven |
| colour | colorless, white, light gray, yellowish, light green to dark green |
| Line color | White |
| transparency | transparent to translucent |
| shine | Glass gloss to diamond gloss |
| Crystal optics | |
| Refractive indices |
n α = 1.658 to 1.662 n β = 1.662 to 1.667 n γ = 1.668 to 1.673 |
| Birefringence | δ = 0.010 to 0.011 |
| Optical character | biaxial positive |
| Axis angle | 2V = 82 ° |
| Other properties | |
| Chemical behavior | Difficult but completely soluble in hydrochloric acid (HCl), very slowly soluble in H 2 O |
| Special features | strongly pyroelectric and piezoelectric |
Boracite ( boracite ), dated as cubical quartz or Lüneburg diamond known, is a rarely occurring mineral from the mineral class of " borates " ( formerly carbonates, nitrates and borates ). It crystallizes in the orthorhombic crystal system with the chemical composition Mg 3 [Cl | BO 3 | B 6 O 10 ] and develops mostly isometric crystals , but also fibrous or granular to massive aggregates that are either colorless or white, gray, yellowish or light due to foreign admixtures - can be colored to dark green.
Etymology and history
Boracite was first discovered in the Lüneburg Kalkberg in Lower Saxony and described by G. Lasius in 1787 as cubic quartz crystals from Lüneburg . Because of its location and its sheen, the mineral was also given the addition of Lüneburg Diamond . Its name, which is still valid today, boracite, was given in 1789 by Abraham Gottlob Werner , who named it after its main component boron .
Other synonyms used by various researchers include calcareous borax , boraxspath , cube stone (around Lüneburg), calcareous quartz , sedative spath (according to H. Westrumb) and stassfurtite .
classification
In the 8th edition of the mineral systematics according to Strunz , which is outdated but still in use , boracite still belonged to the common mineral class of "carbonates, nitrates and borates " and there to the department of "framework borates", where it was named after the "boracite group" the system no. V / L.04 and the other members Chambersite , Congolith , Ericait and Trembathit 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 boracite to the now independent class of "borates" and there in the department of "heptaborates and other megaborates". This is further subdivided according to the type of crystal structure , so that the mineral can be classified in the sub-section “Framework Heptaborate (.05); Layered Nanoborates (.10 to .20); Scaffolding dodecaborates (.25); Mega-Scaffold-Borate (.30 to .35) "can be found, where the" Boracite group "with the system no. 6.GA.05 and the other members chambersite , ericaite and high boracite .
The systematics of minerals according to Dana , which is mainly used in the English-speaking world , assigns boracite to the common class of "carbonates, nitrates and borates", like the outdated Strunz system, but there it belongs to the category of "anhydrous borates with hydroxyl or halogen". Here he is the namesake of the "Boracite group (Orthorhombic: Pca 2 1 )" with the system no. 06/25/01 and the other members Chambersit and Ericait can be found in the subsection " Anhydrous borates with hydroxyl or halogen ".
Crystal structure
| Crystal system | orthorhombic |
| Space group | Pca 2 1 (No. 29) |
|
Lattice parameter (unit cell ) |
a = 8.55 Å ; b = 8.55 Å c = 12.09 Å |
| Number (Z) of the formula units |
Z = 4 |
Boracite initially crystallizes in the cubic crystal system in the space group F 4 3 c (space group no. 219) with the lattice parameter a = 12.10 Å and 8 formula units per unit cell . At the transition temperature of 268 ° C the crystal lattice folds into the orthorhombic-pyramidal crystal class of Pca 2 1 (No. 29) with the lattice parameters a = 8.55 Å, b = 8.55 Å and c = 12.09 Å as well 4 formula units per unit cell. This special process of structural change in the same aggregate state is also called polymorphism .
This property of boracite is also the reason why paramorphoses of boracite according to cubic shapes are often found.
properties
Boracite is polymorphic and changes its crystal system at a temperature of 268 ° C (see also crystal structure ). The low-temperature form is strongly pyroelectric and forms the anti-analog ( negative ) pole at (111) and the analog ( positive ) pole at (1 1 1) .
Furthermore, boracite is difficult to melt in front of the soldering tube and colors the outer flame green. It is difficult to dissolve in hydrochloric acid , but completely soluble, and in water it dissolves only very slowly.
Modifications and varieties
The compound α- (Mg, Fe) 3 [Cl | B 7 O 13 ] is dimorphic and, in addition to the orthorhombic boracite, occurs in nature as a trigonal crystallizing trembathite .
Huyssenite is an iron-containing variety of boracite. As Eisenboracit other hand, a Boracit-variety with a mole fraction of up to 36% is wustite (FeO), respectively.
A cloudy variant of boracite is known as the parasite .
Education and Locations
Boracite formed by sedimentation or metamorphosis in evaporite - deposits . Accompanying minerals include anhydrite , carnallite , gypsum , halite , hilgardite , kainite and sylvin . It is very often found in small, ingrown or grown crystals and in drusen . By decomposition, the boracite crystals transform into aggregates of fibrous individuals, without losing their external form, which contain a few percent water and represent a new mineral, parisite .
As a rare mineral formation, boracite could only be detected in a few places, whereby so far (as of 2016) around 60 sites are known. In addition to its type locality, Lüneburg Kalkberg, the mineral was also found in Germany in several places in Lower Saxony (Celle, Elze, Göttingen, Hanover, Helmstedt, Hildesheim, Lüneburg), Saxony-Anhalt (Börde, Staßfurt) and Thuringia (Bleicherode, Sondershausen, Bad Salzungen ) as well as on the Segeberger Kalkberg in Schleswig-Holstein.
The province of Chapare and Cochabamba in Bolivia, where crystals up to 1.5 cm in size have been found, are worth mentioning due to the extraordinary boracite finds .
Other locations include Tasmania in Australia; Jiangcheng (Pu'er) in China; Cleveland and North Yorkshire in England (Great Britain); Lorraine in France; New Brunswick in Canada; Kazakhstan ; Muzo in Colombia; Boyacá in Colombia ; Inowrocław , Lubin and Kłodawa in Poland; Irkutsk Oblast in Russia; and Clarke County, Alabama, San Bernardino County, and several locations in the state of Louisiana in the United States of America (USA).
use
Boracite is of no further significance for museums and collectors other than as a mineral sample. Sometimes well-trained and clear pieces are ground into faceted gemstones by hobby cutters and offered for exchange and / or purchase.
See also
literature
- Hermann Ludwig: About the Boracite and Stassfurthite . In: Archives of Pharmacy . tape 148 , no. 2 , 1859, p. 129–141 , doi : 10.1002 / ardp.18591480202 ( digitized version [accessed October 2, 2017]).
- Karl Friedrich Rammelsberg : Handbook of mineral chemistry . 2nd Edition. Wilhelm Engelmann Verlag, Leipzig 1875, p. 210 ( available online at archive.org - Internet Archive [accessed October 2, 2017]).
Web links
- Mineral Atlas: Boracite (Wiki)
- RRUFF Database-of-Raman-spectroscopy - Boracite (English)
- American-Mineralogist-Crystal-Structure-Database - Boracite (English)
Individual evidence
- ↑ a b Stefan Weiß: The large Lapis mineral directory. All minerals from A - Z and their properties . 6th completely revised and supplemented edition. Weise, Munich 2014, ISBN 978-3-921656-80-8 .
- ^ IMA / CNMNC List of Mineral Names; July 2017 (PDF 1.66 MB)
- ↑ a b c d Hugo Strunz , Ernest H. Nickel : Strunz Mineralogical Tables. Chemical-structural Mineral Classification System . 9th edition. E. Schweizerbart'sche Verlagbuchhandlung (Nägele and Obermiller), Stuttgart 2001, ISBN 3-510-65188-X , p. 358 .
- ↑ Webmineral - Boracite (English)
- ↑ a b Boracite . 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; 66 kB ; accessed on October 2, 2017]).
- ↑ a b c d e Mindat - Boracite (English)
- ↑ underground / above ground - salt minerals from all over the world . kunstundkultur.de; Special exhibition in the German Salt Museum
- ^ A b Helmut Schrätze , Karl-Ludwig Weiner : Mineralogie. A textbook on a systematic basis . de Gruyter, Berlin; New York 1981, ISBN 3-11-006823-0 , pp. 561-562 .
- ↑ Boracit in Economic Natural History for the German Farmer and Youth in Middle Schools , Volume 5 in the Google Book Search
- ↑ a b Friedrich Klockmann : Klockmanns textbook of mineralogy . Ed .: Paul Ramdohr , Hugo Strunz . 16th edition. Enke, Stuttgart 1978, ISBN 3-432-82986-8 , pp. 593 (first edition: 1891).
- ↑ a b Old mineral names and synonyms (PDF; 2.65 MB) database collection by Indra Günther
- ↑ Mindat - Number of localities for Boracite
- ↑ Petr Korbel, Milan Novák: Mineral Encyclopedia (= Dörfler Natur ). Nebel Verlag, Eggolsheim 2002, ISBN 978-3-89555-076-8 , p. 135 .
- ↑ List of localities for boracite in the Mineralienatlas and Mindat
- ^ 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. 230 .