Acrochordite

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Acrochordite
Eveite, Akrochordite-661878 (cropped) .jpg
Brownish acrochordite grains with greenish Eveit from Långban , Sweden ( overall size : 1.5 cm × 1.0 cm × 1.0 cm)
General and classification
chemical formula
  • Mn 2+ 4 Mg [(OH) 2 | AsO 4 ] 2 • 4H 2 O
  • (Mn, Mg) 5 [(OH) 2 | AsO 4 ] 2 • 4H 2 O
Mineral class
(and possibly department) 		Phosphates, arsenates, vanadates
System no. to Strunz
and to Dana 		8.DD.10 ( 8th edition : VII / D.16)
04.42.01.01
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  = 8.68  Å ; b  = 17.63 Å; c  = 6.83 Å
β  = 99.5 °
Formula units Z  = 2
Physical Properties
Mohs hardness 3.5
Density (g / cm 3 ) measured: 3.19 to 3.35; calculated: 3.26 to 3.29
Cleavage completely after {010}
colour colorless, yellowish to reddish brown, light pink
Line color not defined
transparency translucent
shine frosted
Crystal optics
Refractive indices n α  = 1.672
n β  = 1.676
n γ  = 1.683
Birefringence δ = 0.011
Optical character biaxial positive

Akrochordite is a very rare mineral from the mineral class of " phosphates , arsenates and vanadates ". It crystallizes in the monoclinic crystal system with the chemical composition (Mn, Mg) 5 [(OH) 2 | AsO 4 ] 2 · 4H 2 O and develops mostly spherical, wart-shaped (name!), But also prismatic crystals up to about 2 mm in size or granular to massive mineral aggregates .

Etymology and history

Akrochordite was first found in 1922 in the Swedish mining community Långban and described by Gustav Flink (actually Gustaf Flink , 1849–1931), who named the mineral after the Greek word ἀκροχορδών [akrochordon] for wart due to its characteristic wart-shaped crystal formation .

classification

In the outdated 8th edition of the mineral classification according to Strunz , the acrochordite belongs to the division of “water-containing phosphates with foreign anions ”, where together with Chenevixite and Luetheit it forms the unnamed group VII / D.16 .

In the 9th edition of Strunz's mineral systematics, which has been in effect since 2001 , the acrochordite is in the “Phosphates etc.” section with additional anions; with H 2 O ”and the subsection“ With only medium-sized cations; (OH etc.): RO 4  = 2: 1 “sorted. There it forms the "acrochordite group" with the system no. 8.DD.10 with the other member Guanacoit .

The systematics of minerals according to Dana , which is common in the English-speaking world , assigns the acrochordite to the section of "hydrated phosphates etc., with hydroxyl or halogen" and there in the sub-section of " water-containing phosphates etc., with hydroxyl or halogen with (AB) 5 ( XO 4 ) 2 Z q × x (H 2 O) “, where it also forms the unnamed group 42.04.01 together with guanacoite .

Crystal structure

Acrochordite crystallizes isotypically with guanacoite in the monoclinic crystal system in the space group P 2 1 / c (space group no. 14) with the lattice parameters a  = 8.68  Å ; b  = 17.63 Å; c  = 6.83 Å and β = 99.5 ° and 2 formula units per unit cell . Template: room group / 14

Education and Locations

Akrochordit forms as a rare mineral in Hausmannit - ore in a metamorphic formed Iron - manganese - ore bodies or in stratiform zinc - deposits . In addition to hausmannite, accompanying minerals include pyrochroite , barite , eveit , brandtite , sarkinite , chlorophoenicite and various carbonates .

So far only three sites are known for the acrochordite (status: 2010), in Sweden in addition to its type locality Långban also the "Moss Mine" near Nordmark in the Filipstad municipality and in the USA the "Sterling Mine" on Sterling Hill near Ogdensburg in New Jersey .

See also

literature

  • Paul Brian Moore, Pradip K. Sen Gupta, Elmer O. Schlemper: Akrochordite, (Mn, Mg) 5 (OH) 4 (H 2 O) 4 (AsO 4 ) 2 : A sheet structure with amphibole walls . In: American Mineralogist . tape 74 , 1989, pp. 256–262 ( minsocam.org [PDF; 833 kB ; accessed on March 26, 2018]).
  • Friedrich Klockmann : Klockmann's textbook of mineralogy . Ed .: Paul Ramdohr , Hugo Strunz . 16th edition. Enke, Stuttgart 1978, ISBN 3-432-82986-8 , pp. 650 (first edition: 1891).

Web links

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

Individual evidence

  1. ↑ Stefan Weiß: The large Lapis mineral directory. All minerals from A - Z and their properties . 6th completely revised and supplemented edition. Weise, Munich 2014, ISBN 978-3-921656-80-8 .
  2. ↑ 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.  503 .
  3. ↑ Webmineral - Akrochordite (English)
  4. ↑ a b c acrochordite . 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 March 26, 2018]).
  5. ↑ a b c d e acrochordite at mindat.org (English)
  6. ^ Project Runeberg: Nordisk familjebok - Flinder's river - Flintporslin