Acuminite

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Acuminite
General and classification
other names

IMA 1986-038

chemical formula
  • SrAlF 4 (OH) • H 2 O
  • Sr [AlF 4 (OH)] • H 2 O
Mineral class
(and possibly department) 		Halides
System no. to Strunz
and to Dana 		3.CC.10
06/11/17/01
Crystallographic Data
Crystal system monoclinic
Crystal class ; symbol monoclinic prismatic; 2 / m
Space group C 2 / c (No. 15)Template: room group / 15
Lattice parameters a  = 13.223 (1)  Å ; b  = 5.175 (1) Å; c  = 14.251 (1) Å
β  = 111.61 (2) °
Formula units Z  = 8
Frequent crystal faces {110}, { 1 11}, rarely { 1 12}
Twinning Contact twins after {100}
Physical Properties
Mohs hardness 3.5
Density (g / cm 3 ) measured: 3.295; calculated: 3.305
Cleavage completely after {001}
colour colorless, white; turning yellow from exposure to X-rays
Line color White
transparency transparent
shine Glass gloss
Crystal optics
Refractive indices n α  = 1.451
n β  = 1.453
n γ  = 1.462 to 1.463
Birefringence δ = 0.012
Optical character biaxial positive
Axis angle 2V = 46 to 57 ° (measured); 50 to 52 ° (calculated)

Acuminite is a very seldom occurring mineral from the mineral class of " halides " with the chemical composition Sr [AlF 4 (OH)] · H 2 O and therefore, chemically speaking, a water-containing strontium- aluminofluoride.

Acuminite crystallizes in the monoclinic crystal system and develops only millimeter-sized, dipyramidal and spearhead-like crystals and crystal groups. The mineral is usually colorless and transparent. However, due to multiple light refraction due to lattice defects or polycrystalline formation, it can also appear white and take on a yellow color due to the action of X-rays .

Etymology and history

Acuminit was first discovered in the cryolite - deposit at Ivittuut on Greenland . After recognition by the International Mineralogical Association (IMA) in 1986 (internal entry number of the IMA: 1986-038), the first description was published by H. Pauly and OV Petersen one year later. Due to its characteristic habitus , the two named the mineral after the Latin word acumen (spearhead).

The type material of the mineral (holotype, HT) is in the Mineralogical Collection of the University of Copenhagen under catalog no. 1986.378 kept. It is a sample, a vial with fragments and crystals as well as various mounted crystals.

classification

Since the acuminite was only recognized as an independent mineral in 1986, it is not yet listed in the 8th edition of the mineral classification according to Strunz , which has been outdated since 1977 . Only in the Lapis mineral directory according to Stefan Weiß, which, out of consideration for private collectors and institutional collections, is still based on this old, but revised and updated form of the systematics by Karl Hugo Strunz , the mineral received the system and mineral number. III / C.01-20 . In the "Lapis system" this corresponds to the class of "halides" and there to the section "double halides (mostly with OH, H 2 O)", where acuminite together with Artroeit , Chukhrovit- (Ca) , Chukhrovit- (Ce) , Chukhrovit- (Nd) , Chukhrovit- (Y) , Creedit , Gearksutit , Jakobssonit , Leonardsenit , Meniaylovit and Tikhonenkovit form an independent but unnamed group (as of 2018).

The 9th edition of Strunz's mineral systematics, valid since 2001 and updated by the International Mineralogical Association (IMA) until 2009, also assigns acuminite to the class of "halides" and there in the department of "complex halides". This is, however, further subdivided according to the crystal structure, so that the mineral can be found according to its structure in the sub-section "Group aluminofluoride (Soro-aluminofluoride)", where together with Tikhonenkovite the "Tikhonenkovite group" with the system no. 3.CC.10 forms.

Also the systematics of minerals according to Dana , which is mainly used in the English-speaking world , assigns acuminite to the class of "halides" and there to the department of "complex halides - aluminum fluorides". Here he is the only member of the unnamed group 06/11/17 within the sub-section “Complex Halides - Aluminum Fluorides with Different Formulas”.

Chemism

According to Pauly and Petersen, the wet chemical analysis revealed an average composition of 37.04% Sr , 11.86% Al , 33.52% F , 6.82% OH (calculated from the anion deficit ) and 7.80% H 2 O ( calculated on the basis of a water molecule per formula unit) and 0.0026% Li and 0.0185% Ca .

This corresponds to the empirical formula Sr 0.98 Al 1.02 F 4.07 (OH) 0.93 · H 2 O, whereby the elements lithium and calcium contained in traces were disregarded as foreign additions. The idealized formula SrAlF 4 (OH) · H 2 O was confirmed once again when the crystal structure was clarified in 1991 by EK Andersen, G. Ploug-Sørensen and E. Leonardsen.

Crystal structure

Layering of the acuminite structure

Acuminite crystallizes monoclinically in the space group C 2 / c (space group no. 15) with the lattice parameters a  = 13.223 (1)  Å ; b  = 5.175 (1) Å, c  = 14.251 (1) Å and β = 111.61 (2) ° and 8 formula units per unit cell . Template: room group / 15

The crystal structure of acuminite consists of nine- coordinate strontium polyhedra (Sr [9] ) and six-coordinate aluminum octahedra with shared corners and edges, which together form bumpy layers perpendicular to the b-axis.

Crystal structure of acuminite

  • looking parallel to the a-axis

  • looking parallel to the b-axis

  • looking parallel to the c-axis

  • spatial representation in the standard crystallographic orientation

Color table: __ Sr     __ Al     __ F     __ O

Modifications and varieties

The compound Sr [AlF 4 (OH)] · H 2 O is dimorphic and occurs naturally in addition to acuminite as monoclinic, but with a different space group and lattice parameters, crystallizing Tikhonenkovite .

Education and Locations

At its type locality near the Greenland city of Ivittuut, acuminite was found in a small cavity of a mineral sample from a strontium-rich part of the cryolite deposit. Here the mineral appeared in paragenesis with celestine , fluorite , gearksutite , jarlite , pachnolite , ralstonite and thomsenolite .

So far only two other locations for acuminite are known: A granite - pegmatite deposit near Papachacra in the Argentine Department of Belén and the Sarabau Mine (also Lucky Hill Mine ), a hydrothermal deposit of the Carlin type and Skarn mineralization with antimony (Sb), Arsenic (As) and Gold (Au), near the mining town of Bau in the Malay state of Sarawak .

See also

Web links

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

literature

  • H. Pauly, OV Petersen: Acuminite, a new Sr-fluoride from Ivigtut, South Greenland . In: New yearbook for mineralogy, monthly books . 1987, p. 502-514 (English).
  • John Leslie Jambor , Ernst AJ Burke, T. Scott Ercit, Joel D. Grice: New mineral names . In: American Mineralogist . tape 73 , 1988, pp. 1492–1499 (English, rruff.info [PDF; 867 kB ; accessed on January 5, 2020]).
  • EK Andersen, G. Ploug-Sørensen, E. Leonardsen: The structure of acuminite, a strontium aluminum fluoride mineral . In: Journal of Crystallography . tape 194 , 1991, pp. 221–227 (English, rruff.info [PDF; 271 kB ; accessed on January 5, 2020]).

Individual evidence

  1. ↑ a b Malcolm Back, William D. Birch, Michel Blondieau and others: The New IMA List of Minerals - A Work in Progress - Updated: November 2019. (PDF 1720 kB) In: cnmnc.main.jp. IMA / CNMNC, Marco Pasero, November 2019, accessed January 5, 2020 .
  2. ^ A b c 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.  163-164 (English).
  3. ↑ David Barthelmy: Aduminite MineralData. In: webmineral.com. Retrieved January 5, 2020 .
  4. ↑ a b c d e EK Andersen, G. Ploug-Sørensen, E. Leonardsen: The structure of acuminite, a strontium aluminum fluoride mineral . In: Journal of Crystallography . tape 194 , 1991, pp. 221–227 (English, rruff.info [PDF; 271 kB ; accessed on January 5, 2020]).
  5. ↑ a b c d e f g h Acuminite . In: John W. Anthony, Richard A. Bideaux, Kenneth W. Bladh, Monte C. Nichols (Eds.): Handbook of Mineralogy, Mineralogical Society of America . 2001 (English, handbookofmineralogy.org [PDF; 73  kB ; accessed on January 5, 2020]).
  6. ↑ a b c d e Acuminite. In: mindat.org. Hudson Institute of Mineralogy, accessed January 5, 2020 .
  7. ↑ Catalog of Type Mineral Specimens - A. (PDF 85 kB) In: docs.wixstatic.com. Commission on Museums (IMA), December 12, 2018, accessed January 5, 2020 .
  8. ↑ Stefan Weiß: The large Lapis mineral directory. All minerals from A - Z and their properties. Status 03/2018 . 7th, completely revised and supplemented edition. Weise, Munich 2018, ISBN 978-3-921656-83-9 .
  9. ↑ Ernest H. Nickel, Monte C. Nichols: IMA / CNMNC List of Minerals 2009. (PDF 1816 kB) In: cnmnc.main.jp. IMA / CNMNC, January 2009, accessed January 5, 2020 .
  10. ↑ John Leslie Jambor , Ernst AJ Burke, T. Scott Ercit, Joel D. Grice: New mineral names . In: American Mineralogist . tape  73 , 1988, pp. 1492–1499 (English, rruff.info [PDF; 867 kB ; accessed on January 5, 2020]).
  11. ↑ Find location list for Acuminite in the Mineralienatlas and Mindat , accessed on January 5, 2020.