Cahnit

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Cahnit
Cahnite-Rhodonite-245680.jpg
Cahnite (white) on rhodonite (pink) from the Franklin Mine, Franklin (New Jersey) , USA ( overall size : 8.1 cm × 5.3 cm × 4.4 cm)
General and classification
chemical formula Ca [AsO 4 | B (OH) 4 ]
Mineral class
(and possibly department) 		Borates (formerly carbonates, nitrates and borates)
System no. to Strunz
and to Dana 		6.AC.70 ( 8th edition : V / G.08)
43.04.04.01
Crystallographic Data
Crystal system tetragonal
Crystal class ; symbol tetragonal-disphenoidic; 4th
Room group (no.) I 4 (No. 82)
Lattice parameters a  = 7.11  Å ; c  = 6.20 Å
Formula units Z  = 2
Frequent crystal faces {100}, {110}, {111}, {1 1 1}, {311}
Twinning according to {110}, cross-shaped penetrating twins
Physical Properties
Mohs hardness ≈ 3
Density (g / cm 3 ) measured: 3.156; calculated: [3.22]
Cleavage completely after {110}
Break ; Tenacity brittle
colour colorless, white, rarely light yellow or green
Line color White
transparency transparent to translucent
shine Glass gloss
Crystal optics
Refractive indices n ω  = 1.655 to 1.662
n ε  = 1.656 to 1.663
Birefringence δ = 0.001
Optical character uniaxial positive
Other properties
Special features light yellow fluorescence

Cahnite is a very rarely occurring mineral from the mineral class of " borates ". It crystallizes in the tetragonal crystal system with the chemical composition Ca [AsO 4 | B (OH) 4 ], so from a chemical point of view it is a calcium boroarsenate.

Cahnit usually develops disphenoidic ( wedge-shaped ) or pseudo- octahedral crystals and cross-shaped penetrating twins with a glass-like sheen on the surfaces. In its pure form, cahnite is colorless and transparent. However, due to multiple refraction due to lattice construction defects or polycrystalline formation, it can also appear white and in rare cases take on a light yellow or green color due to foreign admixtures, whereby the transparency decreases accordingly.

Special properties

Under UV light , some cahnites show a light yellow fluorescence , similar to that of neon-colored highlighters .

Etymology and history

The mineral was first discovered by Lazard Cahn (1865-1940) in the "Franklin Mine" near Franklin in Sussex County in the US state of New Jersey. Cahn handed the samples over to Charles Palache , but the amount of material was insufficient for a precise analysis. Due to the characteristic shape and angle of the small, white crystal twins, however, Palache suspected a close relationship with the barium zeolite edingtonite . In his first description of 1921, he therefore saw the mineral as "Calcium Edingtonite" and named it Cahnite after its discoverer. The name of the new mineral was published in 1921 in the magazine "American Mineralogist" of the Mineralogical Society of America , but the first description, entitled "Holdenite and Cahnite, two new minerals from Franklin Furnace, New Jersey", was neither read nor printed.

Five years later, George Stanton from Franklin was able to rediscover the mineral in somewhat larger quantities in the northern part of the Franklin Mine. The identity of the new material was initially hidden because, in contrast to the first find, it was poorly crystallized. LH Bauer had in the meantime managed to uncover the special chemical composition of the first mineral. After a spectroscopic examination of one of the original crystals and a comparison with the new sample, the identity of the two minerals could be confirmed. The complete mineral description by Palache and Bauer was published in 1927 in the American Mineralogist.

Type material of the mineral is stored at Harvard University in Cambridge, Massachusetts (Catalog No. 90010, 90015, 90016, 90019) and the National Museum of Natural History in Washington, DC (Catalog No. 95568) in the United States.

classification

In the outdated, but partly still in use 8th edition of the mineral classification according to Strunz , the cahnite belonged to the common mineral class of "carbonates, nitrates and borates" and there to the department of "island borates", where together with bandylith it formed the "bandylite-cahnite group" “With the system no. V / G.08 and the other member Teepleit 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 Cahnite to the newly defined class of "borates" and there in the department of "monoborates". This is further subdivided according to the structure of the borate complex, the possible presence of further anions and the number of certain borate building groups, so that the mineral according to its structure in the subdivision “B (O, OH) 4 , with and without additional anions; 1 ( T ), 1 (T) + OH etc. ”can be found, where it is the only member of the unnamed group 6.AC.70 .

The systematics of minerals according to Dana , which is mainly used in the English-speaking world , assigns the Cahnit to the class of "phosphates, arsenates and vanadates" and there in the department of " phosphates ". Here he is the only member of the unnamed group 43.04.04 within the sub-section "Compound phosphates etc., (anhydrous compound anions with hydroxyl or halogen)".

Education and Locations

Almost colorless Cahnite crystal twin from Franklin Mine, Sussex County, New Jersey (size: 1 cm × 0.8 cm × 0.5 cm)

At its type locality "Franklin Mine" (New Jersey, USA), cahnite formed in pegmatites that had cut through stratiform and metamorphic zinc ore bodies . As Begleitminerale came here Axinite , barite , Datolith , Flinkit , Franklinite , Friedelite , garnet , Groutit , Hausmannite , Hetaerolith , Hedyphan , Hodgkinsonit , Jarosewichit , Kentrolith , Pyrochroit , Rhodonit , Samfowlerit and Willemit on.

Cahnite is a very rare mineral formation and has so far only been discovered at around 10 sites worldwide (as of 2014). Its type locality "Franklin Mine" is the only known site in the United States of America.

Other previously known sites include a carbonatite complex near Catalão in the Brazilian state of Goiás, the Vallerano quarry near Valleranello and Capo di Bove in the Alban Mountains in the Italian metropolitan city of Rome , the "Fuka Mine" near the city of Takahashi (Okayama prefecture) on the Japanese island of Honshū, pit no. 3 in the Huanggang-Fe-Sn deposit in the Hexigte banner in the Inner Mongolia Autonomous Region (China), the " Kombat Mine" (Otjozondjupa region) in Namibia, Klodeborg (Klodeberg) near Øyestad (commune Arendal , Aust-Agder) in Norway, the “Solongo B” deposit on the Witim Plateau in the Russian Republic of Buryatia (Siberia) and the “Hisarcik” and “Killik” mines near Emet in the Turkish province of Kütahya.


Crystal structure

Cahnite crystallizes tetragonally in space group I 4 (space group no. 82) with the lattice parameters a  = 7.11  Å and c  = 6.20 Å as well as two formula units per unit cell .

See also

literature

Web links

Commons : Cahnite  - 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.  337 .
  2. ↑ a b c d e Cahnite , In: John W. Anthony, Richard A. Bideaux, Kenneth W. Bladh, Monte C. Nichols (Eds.): Handbook of Mineralogy, Mineralogical Society of America , 2001 ( PDF 62.3 kB )
  3. ↑ a b Mindat - Cahnite
  4. ↑ Mindat - Number of localities for Cahnit
  5. ↑ Find location list for Cahnit in the Mineralienatlas and Mindat