Oftedalit
| Oftedalit | |
|---|---|
| General and classification | |
| chemical formula | (Sc, Ca, Mn 2+ ) 2 K (Be, Al) 3 Si 12 O 30 |
|
Mineral class (and possibly department) |
Silicates and germanates - ring silicates |
|
System no. to Strunz and to Dana |
9.CM.05 ( 8th edition : VIII / E.22) 63.02.01a.19 |
| Similar minerals | Milarite , Agakhanovite- (Y) |
| Crystallographic Data | |
| Crystal system | hexagonal |
| Crystal class ; symbol | dihexagonal-dipyramidal; 6 / m 2 / m 2 / m |
| Space group | P 6 / mcc (No. 192) |
| Lattice parameters | a = 10.097 Å ; c = 13.991 Å |
| Formula units | Z = 2 |
| Frequent crystal faces | {001}, {100} |
| Twinning | - |
| Physical Properties | |
| Mohs hardness | 6th |
| Density (g / cm 3 ) | calculated: 2.614 |
| Cleavage | bad |
| Break ; Tenacity | shell-like |
| colour | gray-white |
| Line color | White |
| transparency | transparent |
| shine | Glass gloss |
| Crystal optics | |
| Refractive indices |
n ω = 1.556 n ε = 1.553 |
| Birefringence | δ = 0.003 |
| Optical character | uniaxial negative |
| Pleochroism | - |
The mineral oftedalite is an extremely rare ring silicate from the milarite group with the idealized chemical composition (Sc, Ca, Mn 2+ ) 2 K (Be, Al) 3 Si 12 O 30 . It crystallizes in the hexagonal crystal system and develops six-sided, short prismatic crystals of less than one millimeter in size and gray-white in color.
Oftedalite is found as a late deposit from hydrothermal solutions in small druses in potassium feldspars , granitic pegmatites rich in scandium and beryllium . The only documented site is the type locality , the cleavelandite - amazonite pegmatite near Heftetjern in Tørdal , southern Norway .
Etymology and history
Scandium is a common trace element , but it is rarely enriched locally. As a result, very few (13) designated scandium minerals exist and the first ( thortveitite ) was discovered in Norway in 1911. Kristiansen and Černý began speculating about the possible occurrence of previously unknown Sc-containing milarite in Heftetjern pegmatite at the beginning of 1998 and in the spring of the same year, F. Bernhard from the University of Graz was able to detect Sc-milarite from this location.
The first attempt to have oftedalite recognized as a new mineral by the CNMMN of the IMA initially failed because of the 50% rule. For a mineral with a new composition, this requires that one element dominates at a position in the crystal lattice, i.e. that it makes up more than 50% of the population. The proposed oftedalite end link contains a maximum of as much scandium as calcium, so it cannot exceed this mark and would therefore always only be a scandium milarite, but not a new mineral.
It was not until 2005 that new samples in which Ca was partially replaced by Mn 2+ and Sc 3+ is the dominant cation in the A position, led to the recognition of the new mineral oftedalite by the IMA.
The mineral was named after Ivar Oftedal (1894–1976), professor of mineralogy at the Institute of Geology at the University of Oslo . Oftedal did a lot of work on the geochemistry of scandium and published the first publications on the mineralogy of the Tørdal pegmatites.
classification
After the outdated, but still partially common 8th edition of the mineral classification by Strunz would Oftedalit with Agakhanovit- (Y) , Almarudit , Armenit , Berezanskit , Brannockit , Darapiosit , Dusmatovit , Emeleusit , Faizievit , Merrihueit , Oftedalit, Osumilith , Osumilith- (Mg ) , Poudretteit , roedderite , sogdianite , sugilite and yagiite to the general division of " ring silicates (cyclosilicates)" in the " milarite-osumilite group " with system no. VIII / E.22 .
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 classifies oftedalite in the “ring silicates” section. This is, however, further subdivided according to the structure of the rings, so that the mineral can be found in the sub-section "[Si 6 O 18 ] 12− -six double rings" according to its structure . Therein it is part of Agakhanovit- (Y), Almarudit, Armenit, Berezanskit, Brannockit, Eifelit , Darapiosit, Dusmatovit, Friedrichbeckeit , Klöchit , Merrihueit, Milarit, Oftedalit, Osumilith, Osumilith- (Mg), Poudretteit, Roedderit, Trattnerit , Shibkovit , Sogdianite, sugilite and yagiite to the " milarite group " with the system no. 9.CM.05 .
The systematics of minerals according to Dana , which is mainly used in the English-speaking world , assigns oftedalite to the class of “silicates and Germanates”, but there it is in the more finely subdivided division of “ring silicates: condensed rings”. Here it is in the " Milarite Osumilith Group (Milarite Osumilith Subgroup) " with the system no. 63.02.01a can be found in the subsection “ Ring Silicates : Condensed, 6-membered Rings”.
Chemism
Oftedalit is the scandium analog of Agakhanovite (Y) and forms a mixed crystal row with milarite according to the coupled exchange reaction :
- [A] Ca 2+ + [T2] Al 3+ = [A] Sc 3+ + [T2] Be 3+
The composition measured from the type locality is [C] K 0.98 [B] □ 2 [A] (Sc 0.96 Y 0.03 Ca 0.79 Mn 2+ 0.18 Fe 2+ 0.18 ) ∑2.0 [T2 ] (Be 2.91 Al 0.09 ) ∑3.00 [T1] Si 11.98 O 30 , where the position in the crystal structure is indicated in square brackets.
Crystal structure
Oftedalite crystallizes in the hexagonal crystal system in the space group P 6 / mcc (space group no. 192) with the lattice parameters a = 10.097 Å and c = 13.991 Å as well as two formula units per unit cell .
Oftedalite is isotypic to milarite , i.e. H. it crystallizes with the same structure as milarite.
The 12-fold coordinated C-position is fully occupied with potassium and the 9-fold coordinated B-position is empty. The octahedrally coordinated A position is fully occupied by 1 Sc 3+ and 1 (Ca 2+ , Mn 2+ , Fe 2+ ). In addition to beryllium, the T2 position contains only small amounts of Al 3+ and the T1 position, which builds up the double rings, contains only silicon (Si 4+ ).
Education and Locations
Oftedalite forms from residual hydrothermal solutions of scandium- and beryllium-rich pegmatites at low temperatures and pressures and is found there in small drusen in potassium feldspar.
Type locality is the Heftetjern pegmatite, a Cleavelandit- Amazonit pegmatite in Tordal , Norway , where it in small drusen either together with green tourmaline , yttrium Milarit , Bazzit and an unidentified micaceous mineral or Thortveitit , Bazzit, Kristiansenit and Bertrandit to is found. With falling temperatures, Sc-milarite (oftedalite) + bazzite was initially formed during the reaction of thortveitite with a solution rich in Be, K and Ca. Thortveitite was later degraded to Kristiansenite by Ca- and Sn-containing solutions.
See also
Web links
- Mineral Atlas: Oftedalit (Wiki)
Individual evidence
- ↑ a b c d e f g h i j k l m n o p q r s t u v w x y z aa MA Cooper, FC Hawthorne, NA Ball, P. Černý: Oftedalite, (Sc, Ca, Mn 2 +) 2 K (Be, Al) 3 Si12 O30, A New Member Of The Milarite Group From The Heftetjern Pegmatite, Tørdal, Norway: Description And Crystal Structure . In: The Canadian Mineralogist . tape 44 , 2006, pp. 943-949 ( PDF (347 kB) ).
- ^ A b c G. Raade, F. Bernhard and L. Ottoloini: Replacement textures involving four scandium silicate minerals in the Heftetjern granitic pegmatite, Norway . In: European Journal of Mineralogie . tape 16 , 2004, pp. 945-950 ( PDF (3.8 MB) ).
- ↑ a b c R. Kristiansen: A unique assemblage of Scandium-bearing minerals from the Heftetjern-pegmatite, Tørdal, south Norway. In: Norsk Bergverksmuseum Skrift . tape 41 , 2009, p. 75-104 ( PDF (17.3 MB) ).
- ↑ a b Heftetjern pegmatite, Tørdal, Drangedal, Telemark, Norway
- ↑ Webmineral - New Dana Classification of Cyclosilicates cyclosilicates Condensed ring