Neptunite

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Neptunite
Neptunite-172377.jpg
Black neptunite crystals on white natrolite from the "Dallas Gem Mine" on the San Benito River in the county of the same name , California, USA
(size: 5.6 × 3.4 × 3.4 cm)
General and classification
chemical formula KNa 2 Li (Fe 2+ ) 2 Ti 2 Si 8 O 24
Mineral class
(and possibly department) 		Silicates and Germanates
System no. to Strunz
and to Dana 		09.EH.05 ( 8th edition : VIII / F.37)
70.04.01.01
Crystallographic Data
Crystal system monoclinic
Crystal class ; symbol monoclinic m
Room group (no.) Cc (No. 9)
Lattice parameters a  = 16.48  Å ; b  = 12.49 Å; c  = 10.00 Å
β  = 115.4 °
Formula units Z  = 4
Twinning {301}
Physical Properties
Mohs hardness 5 to 6
Density (g / cm 3 ) measured: 3.19 to 3.23; calculated: [3.24]
Cleavage completely after {110}
Break ; Tenacity shell-like
colour dark brown to black
Line color red brown to cinnamon brown
transparency translucent to opaque
shine Glass gloss
Crystal optics
Refractive indices n α  = 1.690 to 1.691
n β  = 1.693 to 1.700
n γ  = 1.719 to 1.736
Birefringence δ = 0.029 to 0.045
Optical character biaxial positive
Axis angle 2V = 40 °
Pleochroism visible: X = light yellow; Y = yellow-orange; Z = red-orange to red-brown
Other properties
Special features piezoelectric

The mineral neptunite is a rarely occurring chain silicate from the neptunite group. It crystallizes in a monoclinic crystal system with the idealized chemical composition KNa 2 Li (Fe 2+ ) 2 Ti 2 Si 8 O 24 and, from a chemical point of view, belongs to the complex layered silicates with transition structures to other silicates and the metal cations potassium , sodium , lithium , Iron and titanium .

Neptunite usually develops long prismatic, occasionally bent or twisted crystals up to about eight centimeters in length with usually square cross-sections. The predominantly opaque and dark brown to black crystals show a glass-like sheen on their surfaces . In thin layers, however, neptunite is blood-red translucent and leaves a red-brown to cinnamon-brown line on the marking board .

With a Mohs hardness of 5 to 6, neptunite is one of the medium-hard minerals that, like the reference minerals fluorite (5) and apatite (6), can be scratched well or barely with a knife.

Etymology and history

Neptunite was first discovered in a pegmatite near Narsarsuaq (Narssârssuk) in West Greenland and described in 1893 by Gust Flink , who named the mineral after the god Neptune from Roman mythology .

classification

In the now outdated, but still in use 8th edition of the mineral classification according to Strunz , the neptunite belonged to the mineral class of "silicates and germanates" and there to the department of "chain silicates and band silicates (inosilicates)", where together with manganese neptunite and watatsumiite it was one formed an independent group.

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 assigns neptunite to the class of "silicates and germanates", but there in the department of "phyllosilicates". This department is further subdivided according to the structural structure of the layers, so that the mineral can be found in the sub-department "Transition structures between layered silicate and other silicate units" according to its structure, where it, together with manganese neptunite and watatsumiite, is named after it Neptunite group "with the system no. 9.EH.05 forms.

The systematics of minerals according to Dana , which is mainly used in the English-speaking world , assigns the neptunite to the class of “silicates and Germanates”, but there in the department of “chain silicates: columnar or tube structures”. Here he is again named after the "Neptunit Group" with the system no. 70.04.01 and the other members Mangan-Neptunit, Watatsumiit and Magnesioneptunit can be found in the subsection " Chain silicates: column or tube structures with linked chains in cage form ".

Chemism

Neptunite (black) with benitoite (blue) and joaquinite (Ce) (reddish) from the "Dallas Gem Mine", California, USA
(size: 6 × 5.3 × 2.8 cm)
Blood-red translucent neptunite from the "Dallas Gem Mine" ( total size of the sample : 10.4 × 6.3 × 4.8 cm)

In nature, neptunite mostly occurs with proportions of manganese , whereby iron and manganese represent each other in the formula ( Diadochie ), but are always in the same proportion to the other components of the mineral. The chemical composition is given in various sources as KNa 2 Li (Fe 2+ , Mn) 2 Ti 2 [O 2 | Si 8 O 22 ].

As the proportion of manganese increases, neptunite finally changes into the mineral manganese neptunite (also manganese septunite , KNa 2 Li (Mn 2+ ) 2 Ti 2 Si 8 O 24 ). Neptunite and manganese neptunite thus form a complete mixed crystal row .

In the also related mineral watatsumiite , the titanium contained in the formula of neptunite has been replaced by vanadium .

Crystal structure

Neptunite crystallizes monoclinically in the space group Cc (space group no. 9) with the lattice parameters a  = 16.48  Å ; b  = 12.49 Å; c  = 10.00 Å and β = 115.4 ° and 4 formula units per unit cell .

properties

Neptunite crystals show a piezoelectric effect, which means that they build up an electrical voltage , similar to the well-known quartz, with changing elastic deformation .

Education and Locations

Neptunit formed in Natrolith- veins that in Serpentinitkörpern included layers of Glaukophan - shale cutting. Besides natrolite, accompanying minerals include aegirine , arfvedsonite , benitoite , eudialyte , joaquinite (Ce) , lomonosovite , nordite (La) , sodalite and ussingite .

As a rare mineral formation, neptunite could so far (status: 2011) only be proven at a few sites, with around 40 sites being known. In addition to its type locality Narsaarsuk, the mineral occurred in Greenland at several locations of the Ilimaussaq intrusion in the vicinity of Narsaq in the administrative district of Kitaa .

The "Dallas Gem Mine" on the San Benito River in the county of the same name in the US state of California, where beautifully developed crystals up to eight centimeters long were found, is particularly worth mentioning due to its extraordinary neptunite finds.

Other locations include Australia (New South Wales), Brazil (Minas Gerais, Santa Catarina), Ireland (County Louth), Canada (Newfoundland and Labrador, Québec), Mongolia (Gobi Desert), Russia (Eastern Siberia, northwestern Russia ) ), Tajikistan (Tian Shan) and other states in the USA (Montana, New Mexico, North Carolina).

See also

Individual evidence

  1. ↑ a b c IMA / CNMNC List of Mineralnames (English, PDF 1.8; Neptunite: p. 202; 1.9 MB)
  2. ↑ Webmineral - Neptunite (English)
  3. ↑ a b c d Hugo Strunz , Ernest H. Nickel: Strunz Mineralogical Tables . 9th edition. E. Schweizerbart'sche Verlagbuchhandlung (Nägele and Obermiller), Stuttgart 2001, ISBN 3-510-65188-X , p. 690 .
  4. ↑ a b c d e Handbook of Mineralogy - Neptunite (English, PDF 77.3 kB)
  5. ↑ a b c Petr Korbel, Milan Novák: Mineral Encyclopedia . Nebel Verlag GmbH, Eggolsheim 2002, ISBN 3-89555-076-0 , p. 246 ( Dörfler Natur ).
  6. ↑ a b c d Neptunite at mindat.org (engl.)
  7. ^ Paul Ramdohr , Hugo Strunz : Klockmanns textbook of mineralogy . 16th edition. Ferdinand Enke Verlag, Stuttgart 1978, ISBN 3-432-82986-8 , p. 736 .
  8. ↑ Mindat - Number of known sites of Neptunite

literature

Web links

Commons : Neptunite  - collection of images, videos and audio files