Astrophyllite
- K2Na ( Fe2 , Mn2 ) 7Ti2 [ O2 | F | (OH ) 4 | Si8O24 ]
- K2NaFe2 7 Ti2Si8O26 (OH ) 4F
Astrophyllite is a rarely occurring mineral from the mineral class of " silicates and Germanates ". It crystallizes in the triclinic crystal system with the idealized chemical composition K2NaFe2 7 Ti2Si8O26 (OH ) 4F.
The iron contained in the formula is, however, often replaced with natural Astrophyllite by small amounts of manganese. In addition, the Astrophyllite belongs structurally to the chain and belt silicates with additional anions of oxygen, fluorine, and hydroxides, which developed according to the Strunz crystal chemical structural formula in the form K2Na ( Fe2 , Mn2 ) 7Ti2 [ O2 | F | (OH) 4 | Si8O24 ] can be expressed.
Astrophyllite developed needle- leaved to crystals radialstrahliger, star-shaped mineral aggregates are located predominantly in the form. Occasionally he finds himself but also grown in quartz. The mineral is generally opaque and transparent in thin layers. The color of the glass to shiny metallic crystals usually varies between bronze yellow to gold, but can also be brown to reddish brown.
Special Features
The mineral is strongly pleochroistisch, that is depending on the direction from which the light passes through the crystal, it is absorbed to different degrees and leads in case of Astrophyllite a color change to dark red orange along the x- axis, orange yellow, along the y -axis and lemon yellow along the z- axis.
The type material of the mineral is in the Mineralogical Collection of the University of Freiberg ( Register-Nr. 26246 ) was preserved.
Etymology and history
Was first discovered on the Astrophyllite lying in Langesundsfjord Låven island in the Norwegian Vestfold. This has been described mineral in 1854 by Theodor Scheerer ( 1813-1875 ), who for "leaf" named it ἄστρον [ astron ] for " star" and φύλλον [ phyllon ] due to its leafy crystal formation and cleavage, as well as its striking star-shaped aggregate forms from the Greek words.
Classification
In the now outdated but still in use 8th edition of the mineral classification by Strunz the Astrophyllite belonged to the department of "transitional structures between chain and layer silicates ", where he was named. , The " Astrophyllite series" with the System No. Formed VIII/G.12 and the other members Cäsiumkupletskit, Hydroastrophyllit, Kupletskit, Kupletskit - (Cs), Magnesiumastrophyllit, Nalivkinit, Niobokupletskit, Niobophyllit and Zirkophyllit.
The 9th edition valid since 2001 and of the International Mineralogical Association (IMA ) used the Strunz'schen Mineral classification assigns the Astrophyllite other hand, in the department of " chains and chain silicates ( inosilicates ) ". This is also further divided according to the type of chain formation, so that the mineral according to its construction in the subsection " chain and chain silicates with branched 2 -periodic single chains, Si2O6 2SiO3 → Si4O12 " is to find where it together with Hydroastrophyllit, Nalivkinit, Kupletskit - (Cs), Kupletskit, Magnesiumastrophyllit, Niobokupletskit, Niobophyllit and Zirkophyllit forms the unnamed group 9.DC.05.
The mainly common in English-speaking classification of minerals according to Dana assigns the Astrophyllite in the class of " silicates and Germanates " there, however, in the fine already divided the department " chain silicates: chains with side branches or loops " one. Here he is as the namesake of the " Astrophyllitgruppe with the system no. 69.01.01 and the other members Kupletskit, Kupletskit - (Cs), Hydroastrophyllit, Magnesiumastrophyllit, Nalivkinit, Niobokupletskit, Niobophyllit and Zirkophyllit within the subdivision " chain silicates: chains with side branches or loops (P = 2 and N = 4, 2 side branches ) " to find.
Education and Locations
Astrophyllite forms in basic intrusive rocks such as nepheline syenites and alkaline granites and their pegmatites. Accompanying minerals nepheline occur in addition to, among others, still albite, aegirine, Arfvedsonite, biotite, Catapleiit, Delhayelit, eudialyte, Kupletskit, Leukophan, Natrolitn, Rasvumit and zircon.
A rare mineral formation Astrophyllite has so far (as of 2013) are detected only at a few localities, of which approximately 150 localities as have been established. Apart from its type locality Låven the mineral still occurred at several locations in Langesundsfjorden such as Aroya and Barkevik in the Vestfold as well as a few places in the provinces of Buskerud and Telemark.
The most massive Astrophyllite crystals and radialstrahligen aggregates of up to 10 centimeters in diameter were found on Eweslogtschorr in the Chibinen on the Russian Kola Peninsula. Similarly, large Astrophyllite is also known from the "St. Peters Dome" in El Paso County ( Colorado).
Other localities lie among others in Brazil, Chile, China, Greenland, Guinea, Canada, Kazakhstan, Kyrgyzstan, Malawi, Morocco, Mongolia, Namibia, Nigeria, Pakistan, Portugal, the East Siberian and northern regions of Russia, Slovakia, South Africa, Tajikistan, Ukraine and in several states of the USA.
Crystal structure
Astrophyllite crystallized in the triclinic space group A1 ( Raumgruppen-Nr. 2) with the lattice parameters a = 5.36 Å; b = 11.76 Å; c = 21.08 Å; α = 85.1 °; β = 90.0 ° and γ = 103.2 ° and two formula units per unit cell.