Molybdofornacite

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Molybdofornacite
Molybdofornacite-283006.jpg
Molybdofornacite from the type locality, the Tsumeb Mine in Tsumeb, Oshikoto Region, Namibia (field of view: 1 mm)
General and classification
other names

IMA 1982-062

chemical formula
  • Pb 2 Cu [OH | (Mo, Cr) O 4 | (As, P) O 4 ]
  • Pb 2 Cu (AsO 4 , PO 4 ) (MoO 4 , CrO 4 ) (OH)
Mineral class
(and possibly department) 		Sulphates, Selenates, Tellurates, Chromates, Molybdates and Wolframates
System no. to Strunz
and to Dana 		7.FC.10 ( 8th edition : VI / F.02)
04/43/03/03
Similar minerals Fornacite
Crystallographic Data
Crystal system monoclinic
Crystal class ; symbol monoclinic prismatic; 2 / m
Space group P 2 1 / c (No. 14)Template: room group / 14
Lattice parameters a  = 8.10  Å ; b  = 5.95 Å; c  = 17.65 Å
β  = 109.2 °
Formula units Z  = 4
Twinning rarely "butterfly" contact twins at an angle of 120 °
Physical Properties
Mohs hardness 3 to 4
Density (g / cm 3 ) 6.6 (calculated)
Cleavage no
Break ; Tenacity mussely; very brittle
colour bright light green to olive green
Line color yellow white
transparency transparent
shine Diamond luster
Crystal optics
Refractive indices n α  = 2.05 (2)
n β  = not defined
n γ  = 2.15 (2)
Birefringence δ = 0.12 (1) (measured)
Optical character biaxial, positive / negative cannot be determined
Axis angle 2V = cannot be determined
Pleochroism clearly from X = Y = pale yellow to Z = green yellow
Other properties
Chemical behavior Dissolve in cold dilute HCl to give a white residue

Molybdofornacite is a very rarely occurring mineral from the mineral class of " sulfates (including selenates , tellurates , chromates and tungstates )" with the chemical composition Pb 2 Cu [OH | (Mo, Cr) O 4 | (As, P) O 4 ] with Mo > Cr and As > P . This makes the mineral a lead - copper - molybdate - arsenate with additional hydroxide ions (OH - ).

Molybdofornacite crystallizes in the monoclinic crystal system and develops up to a millimeter long, prismatic to strip-shaped, b-axis stretched crystals with high refraction and a clear diamond luster.

Etymology and history

The mineral collector A. Deininger from Windhoek ( Namibia ) is considered the discoverer of molybdofornacite , who provided the research material for the type publication and first drew attention to the unidentified phase, which later turned out to be molybdofornacite. The mineral was examined by a team of researchers at the University of Bochum led by Olaf Medenbach , K. Abraham and W. Gebert. After it was recognized by the International Mineralogical Association (IMA) in 1982, it was first officially described in 1983. The authors named the mineral because of its chemical composition and structural relationship with fornacite .

The holotype stage ( type material ) is in the collection of the Institute for Mineralogy, Geology and Geophysics of the Ruhr University Bochum .

classification

In the outdated, but partly still in use, 8th edition of the mineral classification according to Strunz , molybdofornacite belonged to the mineral class of "sulfates, chromates, molybdates, tungstates" (as well as some selenates and tellurates) and there to the department of "chromates", where it belongs together Deaesmithite , edoylerite , fornacite , phoenicochroit , santanaite , vauquelinite and wattersite the "Phoenicochroit-vauquelinite group" with the system no. VI / F.02 formed.

The 9th edition of Strunz's mineral systematics, valid since 2001 and used by the International Mineralogical Association (IMA), assigns molybdofornacite to the class of "sulfates (including selenates, tellurates, chromates, molybdates and tungstates)" and there in the department of "Chromate". However, this is further subdivided according to the possible presence and type of additional anions, so that the mineral according to its composition can be found in the subsection "With PO 4 , AsO 4 , SiO 4 ", where together with fornacite it is the fornacite group with the system -No. 7.FC.10 forms.

The systematics of minerals according to Dana , which is mainly used in the English-speaking world , assigns molybdofornacite to the class of "phosphates, arsenates and vanadates" and there to the department of " phosphates ". Here he is in the " vauquelinite group " with the system no. 43.04.03 and the other members vauquelinit and fornacit can be found in the subsection " Compound phosphates etc., (anhydrous compound anions with hydroxyl or halogen)".

Crystal structure

Molybdofornacite crystallizes in the monoclinic crystal system in the space group P 2 1 / c (space group no. 14) with the lattice parameters a  = 8.100  Å ; b  = 5.946 Å; c  = 17.65 Å and β = 109.17 ° as well as four formula units per unit cell . Template: room group / 14

Molybdofornacite is isotypic (isostructural) to fornacite and vauquelinite, which means that it crystallizes with the same crystal structure as fornacite. In the fornacite, the two non-equivalent Pb ions are surrounded by nine O atoms. The Cu 2+ ion has the coordination 4 + 2. (As, P) and Cr are tetrahedrally surrounded by O atoms. The structure of the Fornacits can be considered as made up of thick layers of edge-connected Pb [9] - polyhedron , parallel to (001) by z ≈ 0 and C / 2, and of zig-zag chains of edge-related CuO 4 (OH) 2 -Pseudo- octahedra describe parallel [010] (along the screw axis). The AsO 4 - and CrO 4 - tetrahedron link the layers of the Pb-polyhedra and the Cu-chains into a three dimensional network. The lack of preferred directions also explains the poor cleavage of the mineral.

Chemism

Molybdofornacite is the molybdenum-dominant analogue of the chromium-dominated fornacite and also the arsenic-dominant analogue of an unnamed phosphorus-dominated analogue. It has the measured composition Pb 1.97 Cu 0.98 (As 0.86 P 0.23 ) Σ = 1.09 O 4 (Mo 0.77 Cr 0.17 ) Σ = 0.94 O 4 (OH) , which corresponds to mixed crystals of molybdofornacite with fornacite. For Tsumeb it applies that the crystals originating from there contain a content of 60–80 mol% molybdofornacite end member, which is due to an intensive chemical zonal structure inside the crystals.

properties

morphology

Molybdofornacite forms up to 1 mm long, prismatic to strip-shaped, elongated according to [010] crystals, which are slightly flattened and also striped in the direction of the b-axis. Due to their small size, no surface shapes could be identified on them. They grew up individually or aggregated in loose clusters. V-shaped contact twins , fused together at an angle of 120 ° , which resemble the “butterfly twins” of calcite, are rare .

physical and chemical properties

The crystals of molybdofornacite are bright light green to olive green, the line color of the mineral is described as yellow-white. The transparent crystals have a pronounced diamond-like sheen , which is also reflected in the comparatively high light refraction of 2.08 to 2.12.

The mineral is not cleavable , but due to its brittleness, similar to glass or quartz , it breaks under low mechanical stress, with the fracture surfaces having a shell-like design. With a Mohs hardness between 3 and 4, molybdofornacite is one of the medium-hard minerals and is thus between the reference minerals calcite and fluorite . The calculated density is 6.6 g / cm³.

Molybdofornacite dissolves in cold dilute HCl to form a white residue.

Education and Locations

Crusts of yellowish green molybdofornacite crystals together with malachite from the "Miniera San Pablo" near Inca de Oro, Atacama region, Chile (field of view: 4 mm)

Molybdofornacite forms secondary and occurred in the lower oxidation zone of the hydrothermal polymetallic ore deposit Tsumeb, which is located in dolomite stones. The exact place of origin of the approx. 2 × 3 cm measuring type stage molybdofornacite within the Tsumeb deposit is not known. Accompanying minerals of molybdofornacite are dioptase crystals up to 1 mm in size and light green fragrance in finely crystalline aggregates . The stage consists of a fine-grained, loose mixture of quartz, calcite and powdery Fe hydroxides, as is characteristic of many Dioptas stages from the deep oxidation zone of Tsumeb. In addition to the very typical association of molybdofornacite with dioptase and iron oxides, wulfenite is often added at other levels .

Molybdofornacite has only been described as a very rare mineral formation from a few sites. So far (as of 2016) only about 15 sites are known. The type locality is the world-famous Cu-Pb-Zn-Ag-Ge-Cd deposit of the "Tsumeb Mine" (Tsumcorp Mine) in Tsumeb , Oshikoto region , Namibia . In Germany , molybdofornacite appeared from a silicified barite dike at point 5.0 on the Borstein cliff near Reichenbach , district of Lautertal (Odenwald) in the Odenwald ( Hesse ), as well as at the robbery lock (Alte Burg pit) near Gräfenroda in the Thuringian Forest , Thuringia . In Switzerland , the mineral was found in the former Cu-Ag-Pb mine in Gosan, Saint-Luc , Val d'Anniviers , Canton of Valais . Locations in Austria are unknown.

Furthermore, the mineral is from the "Filon Ste Barbe" in the "Mine des Montmins", Échassières , Ébreuil , Allier , Auvergne , France and from the "Miniera San Pablo", Inca de Oro, Province of Chañaral , the "Miniera Dulcinea de Llampos ”, Cachiyuyo de Llampos District, Copiapó Province , both Región de Atacama , Chile , and a larger number of sites in the United States . In Arizona, these include the Charleston Lead Mine, the Empire Mine and Tombstone , all in the Tombstone District, Tombstone Hills, Cochise County , the Moon Anchor Mine near Hummingbird Spring and the Tonopah-Belmont Mine near Belmont Mountain, Tonopah , both in Osborn District, Big Horn Mts, Maricopa County , as well as the "Rawhide Mine", Buckskin Mts, Mohave County , and also in Nevada the "Alice Mine" near Goodsprings in the district of the same name, Spring Mts, Clark County (Nevada ) , the “Silver Coin Mine” near Valmy, Iron Point District, Humboldt Co. , and the “Belmont Mine” near Belmont in the district of the same name, Toquima Range , Nye County , and finally in New Mexico the “Comfort Claim” in Mahoney Mining Area, Tres Hermanas District, Luna County .

Occasionally, molybdofornacite is found in the "Bullfrog No. 3 km northwest of Magdalena near Silver Hill". 2 Mine ”, North Magdalena District, Socorro County , New Mexico, but the mineral found here has been shown to be fornacite.

use

With a PbO content of around 56% by weight, molybdofornacite would be a rich lead ore. Due to its extreme rarity, however, the mineral is only of interest to collectors.

See also

literature

  • Olaf Medenbach, K. Abraham, W. Gebert: Molybdofornacit, a new lead-copper-arsenate-molybdate hydroxide from Tsumeb, Namibia . In: New Yearbook for Mineralogy. Monthly books . tape 10 , 1983, p. 289-295 .
  • Pete J. Dunn, George Y. Chao, Joel D. Grice, James A. Ferraiolo, Michael Fleischer , Adolf Pabst , Janet A. Zilczer: New mineral names. Molybdofornacite . In: American Mineralogist . tape 69 , 1984, pp. 565-569 ( minsocam.org [PDF; 642 kB ; accessed on January 12, 2017]).

Web links

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

Individual evidence

  1. ↑ a b c d Hugo Strunz , Ernest H. Nickel : Strunz Mineralogical Tables. Chemical-structural Mineral Classification System . 9th edition. E. Schweizerbart'sche Verlagbuchhandlung (Nägele and Obermiller), Stuttgart 2001, ISBN 3-510-65188-X , p.  417 .
  2. ↑ a b c d e f Molybdofornacite . In: John W. Anthony, Richard A. Bideaux, Kenneth W. Bladh, Monte C. Nichols (Eds.): Handbook of Mineralogy, Mineralogical Society of America . 2001 ( handbookofmineralogy.org [PDF; 64  kB ; accessed on January 12, 2017]).
  3. ↑ a b c d e f g h i j k l m n o p q r Olaf Medenbach, K. Abraham, W. Gebert: Molybdofornacit, a new lead-copper-arsenate-molybdate hydroxide from Tsumeb, Namibia . In: New Yearbook for Mineralogy. Monthly books . tape 10 , 1983, p. 289-295 .
  4. ^ G. Cocco, L. Fanfani, PF Zanazzi: The crystal structure of fornacite . In: Journal of Crystallography . tape 124 , 1967, pp. 385–397 ( rruff.info [PDF; 667 kB ; accessed on January 12, 2017]).
  5. ^ Ernest H. Nickel , GJ Hitchen: The phosphate analog of molybdofornacite from Whim Creek, Western Australia . In: Mineralogical Record . tape  25 , no. 3 , 1994, p. 203-204 .
  6. ↑ Georg Gebhard: Tsumeb . a German-African story. 1st edition. Gebhard-Giesen, Reichshof 1991, ISBN 978-3-925322-02-0 , p. 259 .
  7. ↑ Mindat - Number of localities for molybdofornacite
  8. ↑ a b List of sites for molybdofornacite in the Mineralienatlas and Mindat
  9. ↑ Mindat - Minerals of Bullfrog No. 2 Mine, New Mexico