Rockbridge

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Rockbridge
Phosphosiderite-Rockbridgeite-k330a.jpg
Rockbridge on phosphosiderite from Indian Mountain, Alabama , USA
General and classification
other names

Kobokobit

chemical formula Fe 2+ Fe 3+ 4 [(OH) 5 | (PO 4 ) 3 ]
Mineral class
(and possibly department) 		Phosphates, arsenates and vanadates
System no. to Strunz
and to Dana 		8.BC.10 ( 8th edition : VII / B.10)
09.41.02.01
Crystallographic Data
Crystal system orthorhombic
Crystal class ; symbol orthorhombic-dipyramidal; 2 / m  2 / m  2 / m
Space group Bbmm (No. 63, position 5)Template: room group / 63.5
Lattice parameters a  = 13.78  Å ; b  = 16.81 Å; c  = 5.17 Å
Formula units Z  = 4
Physical Properties
Mohs hardness 3.5 to 4.5
Density (g / cm 3 ) measured: 3.45; calculated: 3.60
Cleavage perfect after {100}, good after {010}, indistinct after {001}
Break ; Tenacity uneven; brittle
colour black, dark green, olive green, red brown (if oxidized)
Line color greenish-gray
transparency partially translucent
shine Glass gloss to matt
Crystal optics
Refractive indices n α  = 1.873 to 1.875
n β  = 1.880
n γ  = 1.895 to 1.897
Birefringence δ = 0.022
Axis angle 2V = 45 ° (measured)
Pleochroism in the X-direction pale brown, in the Y-direction bluish-green, in the Z-direction dark blue-green
Other properties
Chemical behavior easily soluble in hydrochloric acid

Rockbridge is a rather seldom occurring mineral from the mineral class of phosphates , arsenates and vanadates . It crystallizes in the orthorhombic crystal system with the chemical composition Fe 2+ Fe 3+ 4 [(OH) 5 | (PO 4 ) 3 ] and is therefore chemically an iron (II, III) phosphate with additional hydroxide ions .

Rockbridge is translucent and develops mostly fibrous crystals stretched along the a-axis , which can grow up to 3 cm in size. Depending on the characteristics, the crystal surfaces have a glass-like gloss or are matt.

Etymology and history

Rockbridgeit was first discovered in the Midvale Mine, an open pit mine on South Mountain near the town of the same name in Midvale in Rockbridge County in the US state of Virginia .

The mineral was described in 1949 by Clifford Frondel , who named it after the county in which it was first found ( type locality ).

classification

Already in the outdated, but partly still in use 8th edition of the mineral systematics according to Strunz , the rock bridge belonged to the mineral class of "phosphates, arsenates and vanadates" and there to the department of "anhydrous phosphates, with foreign anions F, Cl, O, OH", where he and Frondelit formed the unnamed group VII / B.10 .

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 the Rockbridge to the division of “Phosphates, etc. with additional anions; without H 2 O “. However, this is further subdivided according to the relative size of the cations involved and the molar ratio of the additional anions to the phosphate, arsenate or vanadate complex, so that the mineral according to its composition is classified in the subsection “With only medium-sized cations; (OH etc.): RO 4  > 1: 1 and <2: 1 "can be found where, together with Frondelit and Plimerit, the" Rockbridge group "now named after him with the system no. 8.BC.10 forms.

The systematics of minerals according to Dana , which is mainly used in the English-speaking world , assigns Rockbridge to the class of "phosphates, arsenates and vanadates" and there in the category of "anhydrous phosphates etc., with hydroxyl or halogen". Here he is in the " Rockbridge group " named after him with the system no. 41.09.02 within the subsection "Anhydrous phosphates etc., with hydroxyl or halogen with (AB) 5 (XO 4 ) 3 Z q ".

Chemism

Regarding the chemical composition, it is not clear whether Rockbridge may contain manganese . While the Mineralogical Tables of Hugo Strunz formula (Fe 2+ , Mn 2+ ) Fe 3+ 4 [(OH) 5 | (PO 4 ) 3 indicates], as well as the data sheet of the Mineralogical Society of America , the International announces Mineralogical Association the formula Fe 2+ Fe 3+ 4 [(OH) 5 | (PO 4 ) 3 ] (without manganese). However, Rockbridge forms a mixed crystal series with its manganese analogue Frondelit (Mn 2+ Fe 3+ 4 [(OH) 5 | (PO 4 ) 3 ]). Since it also crystallizes isotypically , it should not be clearly definable in reality.

Crystal structure

Rockbridge crystallizes orthorhombically in the space group Bbmm (space group no. 63, position 5) with the lattice parameters a  = 13.78  Å , b  = 16.81 Å and c  = 5.17 Å as well as four formula units per unit cell . Template: room group / 63.5

The phosphorus atoms are present in the rock bridge as phosphate tetrahedra [PO 4 ] 3− .

The crystal structure of rockbridge and dufrenite, but also beraunite, is very similar. The iron atoms form octahedral so-called h - clusters , some of which touch each other at corners and sides. When two clusters touch, the O-O'-bond length decreases. The h -cluster, which is also known from some other minerals, forms here together with the phosphate tetrahedra dense plates that are cubically oriented. This explains the cubic fissile nature of rockbridge. In Rockbridge the distances between the h -clusters are longer than in Dufrenit and Beraunit, which can be explained by the fact that there is more iron in Rockbridge. The unit cell has an orthorhombic-primitive lattice of iron atoms, the rest of the iron atoms are inside the unit cell. The iron atoms are six-coordinate, the phosphorus atoms forms [PO 4 ] 3- - tetrahedron .

Education and Locations

Whitmoreit , Beraunit and Rockbridgeit in Paragenesis ­

Rockbridge forms in deposits of limonite (brown iron ore) as an alternative product to primary iron phosphates. It is often communicated with triphyline , hureaulite , barbosalite , roscherite and limonite.

Rockbridge has 203 sites.

In Germany , the mineral can be found at some sites. In Baden-Württemberg there is a site in the Black Forest , more precisely in Oberwolfach . In Bavaria , there are thirteen branches, one in Lower Bavaria , the rest in the Upper Palatinate . There are two sites in Hesse , both near Wetzlar . One of them is in Waldgirmes , the other in Weilburg . In North Rhine-Westphalia , there are three sites in Arnsberg , Warstein (both Sauerland ) and Neunkirchen ( Siegerland ). In Rhineland-Palatinate there are other sites in Siegerland, near Betzdorf and Herdorf , plus two sites in the Westerwald , in Linz on the Rhine and Altenkirchen .

There are two sites in Austria . One is in Carinthia in Spittal an der Drau , the other in Styria in the Koralpe mountain range , more precisely in Herzogberg (municipality of Kindberg ).

In Switzerland there is a site in the canton of Ticino , more precisely in Brissago .

The other sites are spread across Argentina , Australia , Belgium , Brazil , Chile , Finland , France , Italy , Japan , Kazakhstan , the Democratic Republic of the Congo , Libya , Madagascar , Morocco , Namibia , Norway , Portugal , Rwanda , South Africa , Spain , Sweden , the Czech Republic , the United Kingdom, the United States of America .

See also

literature

  • Clifford Frondel: The dufrenite problem . In: American Mineralogist . tape 34 . Mineralogical Society of America, 1949, pp. 513-540 ( PDF 1.8 MB ).
  • GJ Redhammer, G. Roth, G. Tippelt, M. Bernroider, W. Lottermoser, G. Amthauer, R. Hochleitner: Manganoan rockbridgeit Fe 4.32 Mn 0.62 Zn 0.06 (PO 4 ) 3 (OH) 5 : structure analysis and 57 Fe Mössbauer spectroscopy . In: Acta Crystallographica . 2006, p. 24-28 .

Web links

Commons : Rockbridgeite  - Collection of images, videos and audio files

Individual evidence

  1. ↑ a b c d The New IMA List of Minerals (as of December 2014)
  2. ^ A b 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.  447 .
  3. ↑ a b c d e f g Rockbridge. In: John W. Anthony, Richard A. Bideaux, Kenneth W. Bladh, Monte C. Nichols (eds.): Handbook of Mineralogy, Mineralogical Society of America, 2001 ( PDF 66.9 kB ).
  4. ↑ a b Mineralienatlas: Rockbridgeit
  5. ↑ a b c d Mindat - Rockbridge (English)
  6. ^ Description of the type locality of Rockbridgeit in the Mineralienatlas and Mindat
  7. ↑ Clifford Frondel: The dufrenite trouble . In: American Mineralogist . tape 34 . Mineralogical Society of America, 1949, pp. 523 ( PDF 1.8 MB ).
  8. ↑ Marie Louise Lindberg: Frondelite and the frondelite-rockbridge series . In: American Mineralogist . Mineralogical Society of America, 1949, pp. 541-549 ( PDF ).
  9. ^ Paul B. Moore: Crystal Cheministry of the Basic Iron Phosphates . In: American Mineralogist . tape 55 . Mineralogical Society of America, 1970, pp. 135-169 ( PDF ).
  10. ↑ Mindat - Rockbridgeite localities (English)