Almarudit

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Almarudit
General and classification
chemical formula K □ 2 (Mn, Fe, Mg) 2 Be 2 Al 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.18
Crystallographic Data
Crystal system hexagonal 
Crystal class ; symbol dihexagonal-dipyramidal; 6 / m  2 / m  2 / m
Space group P 6 / mcc (No. 192)Template: room group / 192 
Lattice parameters a  = 9.997  Å ; c  = 14.090 Å
Formula units Z  = 2
Frequent crystal faces {001}, {100}, {102}, {110} 
Physical Properties
Mohs hardness Please complete!
Density (g / cm 3 ) 2,720 (calculated) 
Cleavage not observed 
Break ; Tenacity irregular 
colour yellow - orange 
Line color pale orange 
transparency transparent 
shine Glass gloss 
Crystal optics
Refractive indices n ω  = 1.559 
n ε  = 1.560 
Birefringence δ = 0.001 
Optical character uniaxial negative 
Pleochroism strong:
n ω = orange
n ε = colorless

The mineral almarudite is a very rarely occurring ring silicate from the milarite group and has the chemical composition K □ 2 (Mn 2+ , Fe 2+ , Mg 2+ ) 2 Be 2 Al Si 12 O 30 . It crystallizes in the hexagonal crystal system and develops yellow to orange, thick tabular six-sided crystals . 

Etymology and history

Almarudit was discovered by Alice and Eugen Rondorf in 1982 in silicate-rich xenolites from Ettringer Bellerberg in the Eifel , Germany . A precise characterization in 2002 by Mihajlovic et al. revealed that it is a new mineral from the milarite group. They named it in honor of their research institution, the University of Vienna , after its Latin name Al ma ma ter Rud olphina Vindobonensis: Almarudit.

classification

In the outdated but still partially in use 8th edition of the mineral classification by Strunz of Almarudit belongs to the general ward of the " Ring silicates (cyclo silicates)" where he collaborated with Armenit , Berezanskit , Brannockit , Chayesit , Darapiosit , Dusmatovit , Eifelit , Emeleusit , Faizievit , Merrihueit , Oftedalit , Osumilith , Osumilith- (Mg) , Poudretteit , Roedderite , Shibkovite , Sogdianite , Sugilite , Trattnerite and Yagiite the " Milarite-Osumilite-Group " with the system no. VIII / E.22 forms.

The 9th edition of Strunz's mineral systematics, which has been in force since 2001 and is used by the International Mineralogical Association (IMA), also classifies the Almarudit 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 . With armenite, Berezanskit, Brannockit, Chayesit, Darapiosit, Dusmatovit, Eifelit, Friedrichbeckeit , Klöchit , Merrihueit, Milarit, Oftedalit, Osumilith, Osumilith- (Mg), Poudretteite, Roedderit, Shibkovite, Sogdianite, Sugilite and, Trianite, belongs to the Sugilite " Milaritgruppe " with the system no. 9.CM.05 .

The systematics of minerals according to Dana , which is mainly used in the English-speaking world , assigns the Almarudit to the class of "silicates and Germanates", but there in the more finely subdivided section 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

Almarudit is the Mn 2+ analog of Milarit. The measured composition from the type locality is [12] K 0.86 [9] (□ 1.79 Na 0.21 ) [6] (Mn 1.03 Fe 0.62 Mg 0.37 Zn 0.03 Ca 0, 02 ) [4] (Be 2.09 Al 0.79 ) Si 12 O 30 , the coordination number of the respective position in the crystal structure being indicated in square brackets .

The Almarudite crystals are zoned and vary mainly in their Mn-Fe, Mg-contents, i.e. the occupation of the 6-fold octahedrally coordinated A-position.

Crystal structure

Almarudite crystallizes in the hexagonal crystal system in the space group P 6 / mcc (space group  no.192 ) with the lattice parameters a = 9.997  Å and c  = 14.090 Å as well as two formula units per unit cell .  Template: room group / 192

Almarudite is isotypic to milarite , i.e. i.e., it crystallizes with the same structure as milarite. The 12-fold coordinated C position is almost fully occupied with potassium (K + ). The 9-fold coordinated B position can contain small amounts of sodium (Na + ), but is largely unoccupied and, in contrast to milarite, does not contain any water (H 2 O). Manganese (Mn 2+ ) is incorporated along with variable amounts of iron (Fe 2+ ), magnesium (Mg 2+ ) and calcium (Ca 2+ ) on the 6-coordinate A position, beryllium (Be 2+ ) and Aluminum (Al 3+ ) on the terahedrally coordinated T2 position. The T1 position, which builds up the 6 double rings, only contains silicon (Si 4+ ).

Education and Locations

Almarudit is so far only known from its type locality , a quarry of the A. Casper company on Ettringer Bellerberg , 2 km north of Mayen , in the Laacher See region of the Eifel , Rhineland-Palatinate , Germany . It is the first finding of a beryllium- rich mineral in the Eifel region.

There the mineral was found in a silicate-rich rock inclusion ( xenolite ) in leucite - tephrite - lava . It occurs together with tridymite , sanidine , clinopyroxene , amphibole , sillimanite , quartz , hematite and braunite . This occurrence corresponds to those of the other minerals from the milarite group that were previously found in the Vulkaneifel : Eifelite , Roedderite , Osumilite and Mg-Osumilite .

Almarutite forms contact metamorphic at high temperatures around 900 ° C and low pressure during the conversion of silicate-rich rock inclusions into alkali-rich melts.

See also

literature

Web links

Individual evidence

  1. ↑ 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 T. Mihajlovic, CL Lengauer, T. Ntaflos, U. Kolitsch, E. Tillmanns (2004): Two new minerals, rondorfite, Ca8Mg [SiO4] 4Cl2, and almarudite, K (□, Na) 2 (Mn, Fe, Mg) 2 (Be, Al) 3 [Si12O30], and a study of iron-rich wadalite, Ca12 [( Al8Si4Fe2) O32] C16, from the Bellerberg (Bellberg) volcano, Eifel, Germany , In: Neues Jahrbuch für Mineralogie, Abhandlungen , 149, pp. 265–294 ( PDF 4.7 MB )
  2. ↑ Webmineral - Almarudite (English)