Fahleit

from Wikipedia, the free encyclopedia
Fahleit
General and classification
other names
  • IMA 1982-061
  • GS1
chemical formula CaFe 3+ 2 Zn 5 [AsO 4 ] 6 · 14H 2 O
Mineral class
(and possibly department) 		Phosphates, arsenates and vanadates
System no. to Strunz
and to Dana 		8.CH.55 ( 8th edition : VII / C.11)
40.05.08.02
Crystallographic Data
Crystal system orthorhombic
Crystal class ; symbol not defined
Space group not defined
Lattice parameters a  = 6.60  Å ; b  = 11.6 Å; c  = 22.0 Å
Formula units Z  = 2
Physical Properties
Mohs hardness not definable; 2
Density (g / cm 3 ) not definable; 3.08
Cleavage very perfectly perpendicular to the grain
Break ; Tenacity not specified; inelastic (mildly) pliable, cuttable
colour straw yellow, gray, gray green, light green
Line color no information, probably gray-white with a light yellow or light green cast
transparency no information, probably translucent to transparent
shine Silk to mother-of-pearl gloss
Crystal optics
Refractive indices n α  = 1.628
n β  = 1.631
n γ  = 1.656
Birefringence δ = 0.028
Optical character biaxial positive
Axis angle 2V = 39 ° (calculated)
Other properties
Chemical behavior Easily soluble in cold acids

Fahleit is a very rarely occurring mineral from the mineral class of " phosphates , arsenates and vanadates ". It crystallizes in the orthorhombic crystal system with the chemical formula CaFe 3 + 2 Zn 5 [AsO 4 ] 6 · 14H 2 O, and is thus chemically seen a crystal water-containing zinc - calcium - iron arsenate.

Fahleit forms inelastic, flexible, fibrous crystals about 1.5 cm long, although the individual crystals only have a diameter of a few micrometers. They are often bent and twisted and form felt-like adhesions. There are also tiny spherical aggregates . The mineral was - along with Lavendulan , Cuproadamin , Konichalcit , Tsumcorit , quartz , calcite and gypsum - on Tennantit - chalcocite - ore in the Tsumeb Mine , Namibia, found. Fahleit is the youngest mineral in paragenesis .

Etymology and history

The German mineral dealer Rolf Fahle (* 1943), who noticed the mineral among other specimens from Tsumeb, is considered to be the discoverer of Fahleit . Corresponding investigations led to the determination of the presence of a new mineral, which was recognized by the International Mineralogical Association (IMA) in 1982 and described as Fahleit in 1988 by a German research team with Olaf Medenbach , Kurt Abraham and Karl Schmetzer in the science magazine "New Yearbook for Mineralogy, MONTHS" has been. The mineral was named after the finder Rolf Fahle, a mineral dealer from Munich who specializes in the minerals of Tsumeb.

The type material of the mineral (holotype) is kept at the Institute for Mineralogy, Geology and Geophysics of the Ruhr University Bochum .

classification

Already in the outdated, but partly still in use 8th edition of the mineral classification according to Strunz , the Fahleit belonged to the mineral class of "phosphates, arsenates and vanadates" and there to the department of "water-containing phosphates without foreign anions ", where together with barahonaite (Al ) , Barahonaite (Fe) , hopeite , parahopeit , phosphophyllite , radovanite and smolianinovite the "hopeit-parahopeit group" with the system no. VII / C.11 formed.

The 9th edition of Strunz's mineral systematics , which has been in effect since 2001 and is used by the IMA, also classifies the Fahleit in the category of “water-containing phosphates without foreign anions”. However, this is further subdivided according to the size of the cations involved and the ratio of the anion complex RO 4 to H 2 O. The mineral is therefore classified according to its composition in the sub-section “With large and medium-sized cations; RO 4  : H 2 O <1: "to find where it only with one Smolianinovit the" Fahleitgruppe "with the system no. 8.CH.55 forms.

The systematics of minerals according to Dana , which is mainly used in the English-speaking world , assigns the Fahleit to the class of "phosphates, arsenates and vanadates" and there in the department of "water-containing phosphates etc.". Here it is together with Smolianinovite, Barahonait- (Fe) and Barahonait- (Al) in the "Smolianinovite group" with the system no. May 40, 2008 to be found in the subsection of " Water-containing phosphates etc., with various formulas ".

Chemism

Mean values ​​from five microprobe analyzes on Fahleit resulted in 26.2% ZnO; 4.0% CaO; 10.2% Fe 2 O 3 ; 0.5% MnO; 43.6% As 2 O 5 and 15.5% H 2 O. On the basis of 24 oxygen atoms per formula unit, the empirical formula Zn 5.02 Ca 1.11 Mn 0.11 Fe 1.99 (AsO 4 ) 5.9 1 · 13.42H 2 O. This was idealized to Zn 5 CaFe 2 (AsO 4 ) 6 · 14H 2 O, which contains 26.01% ZnO; 3.58% CaO; 10.21% Fe 2 O 3 ; 44.08% As 2 O 5 and 16.12% H 2 O are required.

Fahleit is the cobalt - and nickel - free zinc - dominant analogue of the cobalt - dominated smolianinovite, Co 3 Fe 3+ 2 (AsO 4 ) 4 · 11H 2 O.

Crystal structure

Fahleit crystallizes orthorhombically with the lattice parameters a  = 6.60  Å ; b  = 11.6 Å and c  = 22.0 Å as well as two formula units per unit cell . The crystal class and the space group of Fahleits could not yet be determined due to the lack of suitable material.

The structure of the Fahleit could not be described until today due to the lack of single crystals.

properties

morphology

Fahleit is found in the form of bundles and tufts of fibrous crystals 1.5 cm in length and only a few micrometers in diameter, which are fused together like a felt. Individual fibers are often bent and twisted. There are also tiny spherical aggregates. At least part of the indefinite phase GS1 from the Tsumeb Mine has been shown to be Fahleit.

physical and chemical properties

Fahleit crystals are straw yellow, gray, gray green or light green in color. The color of their streak is not specified, but the powder color of the pale color should be gray-white with light yellow or greenish tones, depending on the color of the crystals. The surfaces of the crystals, which are transparent to translucent depending on the color, show a silky to pearlescent sheen . Fahleit is colorless and transparent in the translucent light as well as in the thin section. Fahleit has medium-high light and high birefringence (δ = 0.028). Under parallel as well as crossed polars, the mineral resembles tufts of Amiant or flax .

The fiber-like crystals of Fahleit show a very perfect cleavage perpendicular to the longitudinal direction of the fibers, they are characteristically cuttable and inelastic (mildly) flexible. The mineral is very soft and has a Mohs hardness of 2, making it one of the soft minerals that, like the reference mineral plaster, can be easily scratched with the fingernail. Measured values ​​for the density of the fahleite do not exist, the calculated density for the mineral is given as 3.08 g / cm³

Fahleit is readily soluble in cold acids .

Education and Locations

Fahleit occurs as a typical secondary formation in the heavily corroded chalcosine-tennantite ore of a complex Cu-Pb-Zn deposit in carbonate rocks. Zinc, iron and arsenic originate from the decomposition of primary sulphidic ore minerals such as sphalerite and tennantite, the calcium from the carbonatic host rock . Accompanying minerals include blue, spherical lavender aggregates, light green cuproadamine, yellow-green konichalcite, tsumcorite, quartz, calcite and gypsum, which is very characteristic in the socialization. Fahleit is the youngest mineral in paragenesis.

As of 2016, Fahleit could only be described as a very rare mineral formation from one source. Its type locality is the second oxidation zone of the world-famous Cu-Pb-Zn-Ag-Ge-Cd deposit of the "Tsumeb Mine" (Tsumcorp Mine) in Tsumeb , Oshikoto Region , Namibia . The exact location within the Tsumeb Mine, however, remains unknown.

use

Due to its rarity, Fahleit is only of interest to mineral collectors.

See also

literature

  • Olaf Medenbach, Kurt Abraham, Karl Schmetzer: Fahleite from Tsumeb / Namibia, a new mineral belonging to the smolianinovite group . In: New yearbook for mineralogy, monthly books . 1988 (issue 4), 1988, p. 167-171 .
  • Fahleit , In: John W. Anthony, Richard A. Bideaux, Kenneth W. Bladh, Monte C. Nichols (Eds.): Handbook of Mineralogy, Mineralogical Society of America , 2001 ( PDF, 66 kB )

Web links

Individual evidence

  1. ↑ a b c d e 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.  486 .
  2. ↑ a b c d e f g h i j k l m n o p q Olaf Medenbach, Kurt Abraham, Karl Schmetzer: Fahleite from Tsumeb / Namibia, a new mineral belonging to the smolianinovite group . In: New yearbook for mineralogy, monthly books . 1988 (issue 4), 1988, p. 167-171 .
  3. ↑ a b Mindat - Description of Fahleit
  4. ↑ a b c d e Fahleit , In: John W. Anthony, Richard A. Bideaux, Kenneth W. Bladh, Monte C. Nichols (Eds.): Handbook of Mineralogy, Mineralogical Society of America , 2001 ( PDF, 66 kB )
  5. ↑ Type mineral catalog Germany - storage of the holotype stage Fahleit
  6. ↑ Catalog of Type Mineral Specimens - F. (PDF 73 kB) In: docs.wixstatic.com. Commission on Museums (IMA), December 12, 2018, accessed August 29, 2019 .
  7. ↑ Georg Gebhard: Tsumeb . 1st edition. GG Publishing, Grossenseifen 1999, p. 254, 257 + 323 .
  8. ↑ Mindat - characterization of the indefinite phase GS1 by Tsumeb
  9. ↑ Mindat - Number of locations for Fahleit
  10. ↑ Find location list for Fahleit at the Mineralienatlas and at Mindat