Coffee hydrocyanite

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Coffee hydrocyanite
General and classification
chemical formula K 4 [Fe 2+ (CN) 6 ] · 3 (H 2 O)
Mineral class
(and possibly department) 		Organic compounds
System no. to Strunz
and to Dana 		10.AD.10 ( 8th edition : IX / A.02)
02.50.05.01
Similar minerals no
Crystallographic Data
Crystal system tetragonal
Crystal class ; symbol tetragonal-dipyramidal (4 / m)
Space group I 4 1 / a
Lattice parameters a  = 9.394  Å ; c  = 33.72 Å Please complete the source as an individual reference!
Formula units Z  = 8 Please complete the source as an individual reference!
Physical Properties
Mohs hardness 2 to 2.5
Density (g / cm 3 ) 1.98 (1.89 calculated)
Cleavage completely after {100}
colour white, pale yellow-green
Line color White
transparency Please complete!
shine Please complete!
Crystal optics
Refractive indices n ω  = 1.577
n ε  = 1.584
Birefringence δ = 0.007
Optical character uniaxial positive
Other properties
Chemical behavior Easily soluble in water, blue color from the addition of Fe (III) ions

Cafehydrocyanite is an extremely rare mineral from the class of organic compounds and the only known mineral from the group of cyanides . From a chemical point of view, cafehydrocyanite is potassium hexacyanidoferrate (II) . Only three sites of this mineral are known to have been found in Siberia and the Urals, although it is still disputed today whether it is a purely naturally formed mineral or whether anthropogenic influences have contributed to its formation. Coffee hydrocyanite forms white stalactites in old mine shafts.

Etymology and history

The name is derived from the chemical name Ka lium-hexa Cyani do fer rat- hydr at starting. Cafehydrocyanite was first described by AS Povarennykh and LD Rusakova in 1973. The IMA published the mineral as early as 1974 , although the CNMNC has not yet officially confirmed it.

classification

In the meanwhile outdated, but still in use 8th edition of the mineral classification according to Strunz , cafehydrocyanite belonged to the mineral class of "organic compounds" and there to the department of "salts of organic acids", where together with abelsonite , calclacite , dashkovaite , earlandite , formicaite , hoganite , Julienite , mellite , paceite forms the so-called mellit-julienite group.

The 9th edition of Strunz's mineral systematics, which has been in effect since 2001 and is used by the International Mineralogical Association (IMA), also assigns cafehydrocyanite to the class of "organic compounds" and there to the department of "salts of organic acids". Here it can be found in the “Cyanate” sub-section, under which cyanides can also be found. Here, cafehydrocyanite is the only mineral from group 10.AD.10 .

The systematics of minerals according to Dana also assigns cafehydrocyanite to the class of "organic minerals" and there in the section "salts of organic acids (mellitates, citrates, cyanates and acetates)". Here he is the only representative of the 50.02.05 group.

Cafehydrocyanite is the only known cyanide mineral. The first person to describe cafehydrocyanite suggested that there might be other representatives from the group of ferrocyanides and ferricyanides in addition to cafehydrocyanite. Officially recognized finds have not yet been made (as of 2011).

Even if cafehydrocyanite is classified as a salt of an organic acid according to the classifications of Strunz and Dana, the formation of cyanides is not necessarily linked to biological processes. In chemistry, the salts of hydrogen cyanide are not classified as organic , but as inorganic substances.

Crystal structure

Cafehydrocyanite crystallizes in the tetragonal crystal system in the space group I4 1 / a with the lattice parameters a = 9.394  Å and c = 33.72 Å as well as eight formula units per unit cell . The content of water of crystallization is not uniform, although it is stated with three water molecules per formula unit. IR spectroscopic measurements suggest that the anhydrous variant and the monohydrate are present at the same time as the trihydrate.

properties

From a chemical point of view, cafehydrocyanite is potassium hexacyanidoferrate (II) (K 4 [Fe 2+ (CN) 6 ] · 3 (H 2 O)). It is easily soluble in water. A characteristic, clear evidence of this mineral is the formation of Prussian blue by the addition of iron (III) ions to the aqueous solution.

Although cafehydrocyanite can be regarded as a derivative of the toxic hydrogen cyanide, the hexacyanidoferrate (II) complex is very stable and therefore non-toxic. The yellow blood liquor salt, which is analogous to cafehydrocyanite, is approved as a food additive. Only at temperatures above 400 ° C does the mineral decompose with the formation of potassium cyanide . Various sources indicate that cafehydrocyanite releases toxic hydrogen cyanide in contact with hot, inorganic acids. These statements can be doubted without exception, since the cyanidoferrate complex is very stable and does not decompose even in strong inorganic acids .

Education and Locations

So far, two sites in Siberia and one in the Urals have become known:

It occurs there in mine shafts in the form of pale yellow-green stalactites. Here the mineral forms aggregates with small crystals from 0.2 to 1.5 mm. Associated minerals are melanterite and gypsum on pyrite and pyrrhotite veins.

So far it has been disputed where the cyanide-containing waters come from, which led to the formation of cafehydrocyanite. The first descriptors AS Povarennykh and LD Rusakova state organically polluted water from the uppermost soil layers as the source. In particular, the first descriptors insist that the sources for the organic pollution are not of anthropogenic origin. It is also disputed that cyanide as waste water from gold extraction could have led to the formation of cafehydrocyanite, as allegedly no cyanide leaching is carried out in the area of ​​the site (gold deposits at Medvezhii Log).

These claims are being challenged today. For this reason, cafehydrocyanite is listed as a mineral published by the IMA, but not generally recognized.

use

Due to the fact that cafehydrocyanite has so far only been found at one site, it has no use as a raw material. Since cafehydrocyanite is yellow blood liquor salt , the same uses are theoretically conceivable.

See also

Individual evidence

  1. ↑ AS Povarennykh, LD Rusakova; The new mineral cafehydrocyanite (in Russian): Geol. Zhurn. (Ukraine), 33, 24-30.
  2. ↑ Michael Fleischer (1974): New Mineral Names , in: American Mineralogist , Volume 59, pp. 208-212, ( English, PDF 559.7 kB ).
  3. ↑ Mindat - Cafehydrocyanite .
  4. ↑ Jander, Blasius; Textbook of analytical and preparative inorganic chemistry; Hirzel publishing house.

literature

Web links