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Acuminite

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Formula:
Sr[AlF4(OH)(H2O)]
System:
Monoclinic
Colour:
Colourless, white, ...
Hardness:
Name:
From Latin "acumen", "spearhead", in allusion to the shape of its crystals.
An aluminium complex salt. Only found in two cavities in a single specimen 10,5 cm across.

Classification of Acuminite

Approved 1986
3.CC.10

3 : HALIDES
C : Complex halides
C : Soro-aluminofluorides
11.6.17.1

11 : HALIDE COMPLEXES
6 : Aluminofluorides - Isolated Octahedra
8.4.15

8 : Halides - Fluorides, Chlorides, Bromides and Iodides; also Fluoborates and Fluosilicates
4 : Halides of the alkaline earths and Mg
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First Recorded Occurrence of Acuminite

General Appearance of First Recorded Material:
acute dipyramidal crystals, resembling spear points, to 1 mm
Place of Conservation of First Recorded Material:
University of Copenhagen, Copenhagen, Denmark
Geological Setting of First Recorded Material:
Strontium-rich portion of a cryolite deposit
Associated Minerals at First Recorded Locality:

Physical Properties of Acuminite

Vitreous
Diaphaneity (Transparency):
Transparent
Colour:
Colourless, white, becomes yellow on exposure to x-rays
Streak:
white
Hardness (Mohs):
Cleavage:
Perfect
on {001}
Density:
3.295 g/cm3 (Measured)    3.305 g/cm3 (Calculated)

Crystallography of Acuminite

Crystal System:
Monoclinic
Cell Parameters:
a = 13.223Å, b = 5.175Å, c = 14.251Å
β = 11.61°
Ratio:
a:b:c = 2.555 : 1 : 2.754
Unit Cell Volume:
V 196.25 ų (Calculated from Unit Cell)
Z:
8
Morphology:
{110}, {_111}, rarely {_112}
Twinning:
on {100}, as contact twins
X-Ray Powder Diffraction Data:
d-spacingIntensity
2.075 (9)
3.286 (8)
3.310 (8)
3.353 (10)
3.505 (10)
4.706 (10)
4.767 (10)
Comments:
Recorded on type material

Optical Data of Acuminite

Type:
Biaxial (+)
RI values:
nα = 1.451 nβ = 1.453 nγ = 1.462 - 1.463
2V:
Measured: 46° to 57°, Calculated: 50° to 52°
Max Birefringence:
δ = 0.012
Image shows birefringence interference colour range (at 30µm thickness) and does not take into account mineral colouration.
Surface Relief:
Moderate
Dispersion:
r > v, strong

Chemical Properties of Acuminite

Formula:
Sr[AlF4(OH)(H2O)]
Essential elements:
All elements listed in formula:
Analytical Data:
Ca and Sr determined by atomic absorption spectroscopy (AA), Al by CDTA, F electrochemically; OH calculated from the charge balance, H2O from theory
Ca (0.02)
Sr (37.04)
Al (11.86)
F (33.52)
OH (6.82)
H2O (7.80)

sum 97.06 wt.-%
Empirical Formula:
Sr0.98Al1.02F4.07(OH)0.93 · 1.00H2O
Common Impurities:
Ca

Relationship of Acuminite to other Species

3.CC.05GearksutiteCa[Al(F,OH)5(H2O)]
3.CC.10TikhonenkoviteSr[AlF4(OH)(H2O)]
3.CC.15ArtroeitePb[AlF3(OH)2]
3.CC.20CalcjarliteNa(Ca,Sr)3Al3(OH)2F14
3.CC.20JarliteNa(Sr,Na)7MgAl6F32(OH,H2O)2
3.CC.20JørgenseniteNa2(Sr,Ba)14Na2Al12F64(OH,F)4
8.4.1SellaiteMgF2
8.4.2NeighboriteNaMgF3
8.4.3BischofiteMgCl2 · 6H2O
8.4.4KorshunovskiteMg2Cl(OH)3 · 4H2O
8.4.5CarnalliteKMgCl3 · 6H2O
8.4.6TachyhydriteCaMg2Cl6 · 12H2O
8.4.7FluoriteCaF2
8.4.8SinjariteCaCl2 · 2H2O
8.4.9AntarcticiteCaCl2 · 6H2O
8.4.10ChlorocalciteKCaCl3
8.4.11RorisiteCaFCl
8.4.12JarliteNa(Sr,Na)7MgAl6F32(OH,H2O)2
8.4.13CalcjarliteNa(Ca,Sr)3Al3(OH)2F14
8.4.14TikhonenkoviteSr[AlF4(OH)(H2O)]
8.4.16FrankdicksoniteBaF2
8.4.17UsoviteBa2CaMgAl2F14
8.4.18BøgvaditeNa2SrBa2Al4F20

Other Names for Acuminite

Name in Other Languages:
Dutch:Acuminiet
German:Acuminit
Italian:Acuminite
Latvian:Akuminīts
Portuguese:Acuminite
Spanish:Acuminita

Other Information

Health Risks:
No information on health risks for this material has been entered into the database. You should always treat mineral specimens with care.

References for Acuminite

Reference List:
Pauly, H. and Petersen, O. V. (1987) Acuminite, a new Sr fluoride from Ivigtut, South Greenland. Neues Jahrbuch für Mineralogie, Monatshefte, 502-514. - American Mineralogist (1988), 73, 1492 (abstract).

Krogh Andersen, E., Ploug-Sorensen, G. and Leonardsen, E. (1991) The structure of acuminite, a strontium aluminium fluoride mineral. Zeitschrift für Kristallographie 194, 221-227.

Anthony, J. W. et al. (1997) Handbook of Mineralogy, Vol. 3, 2.

Internet Links for Acuminite

Localities for Acuminite

map shows a selection of localities that have latitude and longitude coordinates recorded. Click on the symbol to view information about a locality. The symbol next to localities in the list can be used to jump to that position on the map.
(TL) indicates type locality for a valid mineral species. (FRL) indicates first recorded locality for everything else. ? indicates mineral may be doubtful at this locality. All other localities listed without reference should be considered as uncertain and unproven until references can be found.
Greenland
 
  • Sermersooq
    • Arsuk Fjord
      • Ivittuut (Ivigtut)
[Neues Jahrb.Min.,Mh.(1987) 502-514; Nickel & Nichols, 1991]; Petersen, O. V and K Secher (1993) The Minerals of Greenland. Mineralogical Record 24:36
Mineral and/or Locality  
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