Scapolite
The Skapolithgruppe ( scapolite, Wernerite, Gabbronit, Dipyr, Mizzonit, Fuscit ) is a group of tetragonal Gerüstalumosilikate with the general composition:
D4 [ T4O8 ] 3 (X, Z), 2 / v; v = 1, 2 In this formula:
- D: Large cations are surrounded by nine or more anions: Na , Ca2 , K , Sr2 , Ba2 , Fe2
- T: Small cations which are surrounded by 4 anions tetrahedrally: Si4 , Al3
- O: oxygen
- X: Monatomic anions: Cl -, Br -
- Z: Polyatomic anions: CO3, SO4, HSO4, H CO3, OH, H2O
- V: valence ( charge) of the anions X and Z
The Skapolithgruppe includes the minerals
- Marialite: Na4Al3Si9O24Cl,
- Mejonit: Ca4Al6Si6O24CO3 and
- Silvialith: (Ca, Na) 4Al6Si6O24 (SO4, CO3 )
Which form a complete solid solution series.
The sometimes over 1m large crystals are prismatic, elongated along the crystallographic c axis. Their shape is dominated by the prism surfaces {100 } and { 110}. The prisms are mainly limited by the pyramidal faces { 101 }. Often a Flächenstreifung in longitudinal direction of the prism surfaces.
Special Features
Scapolite is colorless in itself, but can coloring by trace elements pink, purple, blue, yellow, or brown in color. Inclusions of graphite lead to a gray to black color. The stroke color is white. The crystals are transparent to opaque cloudy with vitreous luster. The density is 2.50 to 2.80 g/cm3. The hardness of scapolite is comparable to that of feldspar ( Mohs hardness 5-6).
- Color variations of scapolite
Yellow ( Tanzania)
Violet (Afghanistan)
Golden brown (Tanzania )
Gray ( Canada )
Scapolite fluoresce in UV light orange to bright yellow and more rarely red.
Etymology and history
The name Skapolith derived from the Greek of skapos ( rod) and lithos (stone).
Introduced the term scapolite in 1800 by José Bonifácio de Andrada e Silva. In the same paper also Wernerite is described. For decades, both names were used in parallel, alternately as a group and Varietätenbezeichnung until the CNMMN (Commission on New Minerals and Mineral Names) in 1997 rejected the name Wernerite and scapolite stipulated as the group name.
Classification
In the scheme of Strunz ( 9th edition ) the Skapolithgruppe ( 9.FB.15 ) belongs to the mineral class 9 ( silicates ), Department of framework silicates without zeolitic H 2 O ( F) with additional anions ( B).
In the scheme of the Dana Skapolithgruppe ( 76.03.01 ) belongs to the class of framework silicates with SI - Al framework (76 ) with other Be / Al / Si framework structures ( 03).
Education and Locations
Scapolite is found worldwide in kontaktmetamorphen Kalksilikatgesteinen ( skarn ) and metamorphic basic rocks ( Metagabbros, Metadiorite ) and gneisses.
In gneisses and metabasites to scapolite forms in the reaction of feldspars with NaCl - rich solutions and will be accompanied by plagioclase, hornblende, clinopyroxene.
In scapolite skarn deposits are found associated with calcite, diopside, epidote, phlogopite, tremolite, garnet, vesuvianite, wollastonite, sphene, feldspar, fluorite, pyrite.
Sulfate rich Scapolite can be found in garnet granulites of the lower crust and upper mantle. There they occur together with plagioclase, Ca - amphibole, clinopyroxene, and spinel pyropreichen grenades. The mineral structure suggest that Scapolite these deposits were not secondarily formed by conversion of feldspars, but are directly crystallized from water-containing Alkalibasaltmagmen.
Crystal structure
Scapolite crystallize in the tetragonal space group I4 / m and P42 / n with two formula units per unit cell. Marialite and Meionite - rich Scapolite crystallize in the space group I4 / m whereas the mixed crystal structure has the space group P42 / n. Accordingly, the Skapolithgruppe can be divided into three isomprphe series:
- 9.0 > Si > 8.4: Marialite rich Scapolite
- 8.4 > Si > 7.3: intermediere Scapolite
- 7.3 > Si > 6.0: Meionite rich Scapolite
Structurally, this series mainly differ in the distribution of Al and Si on the different lattice positions in Alumosilikatgerüst.
Si and Al are surrounded by four oxygens so that the oxygen atoms are at the corners of a tetrahedron, the center of which the Si or Al cation is ( tetrahedral coordination ). These (Si, Al) O4 tetrahedrons are four corners together to form a three-dimensional scaffold linked ( tectosilicate ). This framework consists of 4 's and 5-series surround rings of (Si, Al) O4 tetrahedra, which, similar to zeolites, larger voids. In each of these cavities there is a (X, Z)- anion ( Cl, CO3 ) and 4 D- cations (Na, Ca).
The large D- cations in the Alumosilikathohlräumen are surrounded by seven oxygens and a (X, Z ) anion.