Rokühnite
A valid IMA mineral species
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About Rokühnite
Formula:
FeCl2 · 2H2O
Colour:
Colourless (transmitted light), light green
Specific Gravity:
2.35
Crystal System:
Monoclinic
Name:
Named after Robert Kühn (1911–2000), German mineralogist, and professor at Kaliforschungs-Institut der Kali und Salz AG, Hannover, Germany.
The hydrated counterpart of lawrencite. Other approved ferrous-iron-bearing chlorides include droninoite and hibbingite. Compare the hydrated ferric-iron chloride minerals: 'hydromolysite', 'Unnamed (Fe Chloride Hydrate I)', and 'Unnamed (Fe Chloride Hydrate II)'.
Crystal structure (synthetic material) details/elements:
- polymeric chains of iron and chlorine ions, near‐square planar configuration
- 2 iron ions share single Cl ion
- water groups fill the remaining octahedral positions about the iron ion
- hydrogen bonds involving water groups link adjacent chains, in direction || c.
Crystal structure (synthetic material) details/elements:
- polymeric chains of iron and chlorine ions, near‐square planar configuration
- 2 iron ions share single Cl ion
- water groups fill the remaining octahedral positions about the iron ion
- hydrogen bonds involving water groups link adjacent chains, in direction || c.
Unique Identifiers
Mindat ID:
3440
Long-form identifier:
mindat:1:1:3440:7
GUID
(UUID V4):
(UUID V4):
a776c116-8bd0-48b8-9910-97807e19ec57
IMA Classification of Rokühnite
Approved
First published:
1980
Classification of Rokühnite
3.BB.10
3 : HALIDES
B : Simple halides, with H2O
B : M:X = 1:2
3 : HALIDES
B : Simple halides, with H2O
B : M:X = 1:2
9.2.4.1
9 : NORMAL HALIDES
2 : AX2
9 : NORMAL HALIDES
2 : AX2
8.11.3
8 : Halides - Fluorides, Chlorides, Bromides and Iodides; also Fluoborates and Fluosilicates
11 : Halides of Fe and Ni
8 : Halides - Fluorides, Chlorides, Bromides and Iodides; also Fluoborates and Fluosilicates
11 : Halides of Fe and Ni
Mineral Symbols
As of 2021 there are now IMA–CNMNC approved mineral symbols (abbreviations) for each mineral species, useful for tables and diagrams.
| Symbol | Source | Reference |
|---|---|---|
| Rok | IMA–CNMNC | Warr, L.N. (2021). IMA–CNMNC approved mineral symbols. Mineralogical Magazine, 85(3), 291-320. doi:10.1180/mgm.2021.43 |
Physical Properties of Rokühnite
Transparency:
Transparent
Colour:
Colourless (transmitted light), light green
Comment:
Brown when oxidized.
Comment:
not determined
Tenacity:
Flexible
Cleavage:
Very Good
{100}; also good on {010}
{100}; also good on {010}
Density:
2.35 g/cm3 (Measured) 2.35 g/cm3 (Calculated)
Optical Data of Rokühnite
Type:
Biaxial (+)
RI values:
nα = 1.605 nβ = 1.633 nγ = 1.702
2V:
Measured: 64° , Calculated: 68°
Max Birefringence:
δ = 0.097
Image shows birefringence interference colour range (at 30µm thickness)
and does not take into account mineral colouration.
and does not take into account mineral colouration.
Surface Relief:
Moderate
Dispersion:
strong (r ≤ v)
Chemistry of Rokühnite
Mindat Formula:
FeCl2 · 2H2O
Elements listed:
Common Impurities:
Mg
Crystallography of Rokühnite
Crystal System:
Monoclinic
Cell Parameters:
a = 7.396(1) Å, b = 8.458(2) Å, c = 3.638(1) Å
β = 97.68(2)°
β = 97.68(2)°
Ratio:
a:b:c = 0.874 : 1 : 0.43
Unit Cell V:
225.54 ų (Calculated from Unit Cell)
Z:
2
Twinning:
Contact, on {201}.
Comment:
space group C2/m, C2 or Cm
X-Ray Powder Diffraction
Powder Diffraction Data:
| d-spacing | Intensity |
|---|---|
| 5.536 Å | (100) |
| 2.762 Å | (74) |
| 4.228 Å | (62) |
| 2.415 Å | (55) |
| 2.892 Å | (54) |
| 2.744 Å | (30) |
| 2.096 Å | (30) |
Geological Environment
Paragenetic Mode(s):
| Paragenetic Mode | Earliest Age (Ga) |
|---|---|
| Stage 7: Great Oxidation Event | <2.4 |
| 47a : [Near-surface hydration of prior minerals] | |
| 47g : [Halogen-bearing surface weathering minerals] |
Type Occurrence of Rokühnite
Co-Type Localities:
General Appearance of Type Material:
Rare thin plates, commonly comprising fibers, with dominating {110} form and a number of other forms present, up to 300 µm.
Place of Conservation of Type Material:
University of Hannover, Hannover, Germany (cotype).
Mineralogisch-Petrographisches Institut, Heidelberg, Germany (cotype).
Mineralogische Sammlungen, Mineralogisch-Petrographisches Institut, Universität Göttingen, Göttingen, Germany, number GZG.MIN.3.1.31.1 (type).
Mineralogisch-Petrographisches Institut, Heidelberg, Germany (cotype).
Mineralogische Sammlungen, Mineralogisch-Petrographisches Institut, Universität Göttingen, Göttingen, Germany, number GZG.MIN.3.1.31.1 (type).
Geological Setting of Type Material:
Secondary, in fissures within earlier K-rich salt beds.
Associated Minerals at Type Locality:
Reference:
von Hodenberg, R., von Struensee, G. (1980) Rokühnite, FeCl2•2H2O, a new mineral. Neues Jahrb. Mineral. Monatsh.: 125-130.
Synonyms of Rokühnite
Other Language Names for Rokühnite
Common Associates
Associated Minerals Based on Photo Data:
| 5 photos of Rokühnite associated with Carnallite | KMgCl3 · 6H2O |
| 1 photo of Rokühnite associated with Magnesiocopiapite | MgFe3+4(SO4)6(OH)2 · 20H2O |
Related Minerals - Strunz-mindat Grouping
| 3.BB.05 | Eriochalcite | CuCl2 · 2H2O |
| 3.BB.15 | Bischofite | MgCl2 · 6H2O |
| 3.BB.20 | Nickelbischofite | NiCl2 · 6H2O |
| 3.BB.25 | Sinjarite | CaCl2 · 2H2O |
| 3.BB.30 | Antarcticite | CaCl2 · 6H2O |
| 3.BB.35 | Aravaipaite | Pb3AlF9 · H2O |
| 3.BB.35 | Tachyhydrite | CaMg2Cl6 · 12H2O |
| 3.BB.40 | Ghiaraite | CaCl2 · 4H2O |
Other Information
Notes:
Water-soluble; topotactical hydration and rapid oxidation in air.
Health Risks:
No information on health risks for this material has been entered into the database. You should always treat mineral specimens with care.
Internet Links for Rokühnite
mindat.org URL:
https://www.mindat.org/min-3440.html
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Please feel free to link to this page.
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References for Rokühnite
Reference List:
Localities for Rokühnite
symbol to view information about a locality.
The Locality List
- This locality has map coordinates listed.
- This locality has estimated coordinates.
ⓘ - Click for references and further information on this occurrence.
? - Indicates mineral may be doubtful at this locality.
- Good crystals or important locality for species.
- World class for species or very significant.
(TL) - Type Locality for a valid mineral species.
(FRL) - First Recorded Locality for everything else (eg varieties).
All localities listed without proper references should be considered as questionable.
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Salzdetfurth Potash Works, Bad Salzdetfurth, Hildesheim District, Lower Saxony, Germany