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Acuminite

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Formula:
Sr[AlF
 
4
(OH)(H
 
2
O)]
System:MonoclinicColour:Colourless, white, ...
Hardness:3½
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

IMA status:Approved 1986
Strunz 8th edition ID:3/C.01-20
Strunz 9th edition ID:3.CC.10
Nickel-Strunz 10th (pending) edition ID:3.CC.10

3 : HALIDES
C : Complex halides
C : Soro-aluminofluorides
Dana 8th edition ID:11.6.17.1

11 : HALIDE COMPLEXES
6 : Aluminofluorides - Isolated Octahedra
Hey's CIM Ref.: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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Type Occurrence of Acuminite

Type Locality:Ivigtut Cryolite deposit, Ivittuut (Ivigtut), Arsuk Firth, Arsuk, Kitaa (West Greenland) Province, Greenland
General Appearance of Type Material:acute dipyramidal crystals, resembling spear points, to 1 mm
Place of Conservation of Type Material:University of Copenhagen, Copenhagen, Denmark
Geological Setting of type material:Strontium-rich portion of a cryolite deposit
Associated Minerals at type locality:
ThomsenoliteRalstonitePachnoliteJarliteGearksutite
FluoriteCelestine

Physical Properties of Acuminite

Lustre:Vitreous
Diaphaneity (Transparency):Transparent
Colour:Colourless, white, becomes yellow on exposure to x-rays
Streak:white
Hardness (Mohs):3½
Cleavage:Perfect
on {001}
Density (measured):3.295 g/cm3
Density (calculated):3.305 g/cm3

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:
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°
Maximum Birefringence:δ = 0.012

Chart 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[AlF
 
4
(OH)(H
 
2
O)]
Essential elements:Al, F, H, O, Sr
All elements listed in formula:Al, F, H, O, Sr
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:
Sr
 
0.98
Al
 
1.02
F
 
4.07
(OH)
 
0.93
 · 1.00H
 
2
O
Common Impurities:Ca

Relationship of Acuminite to other Species

Related Minerals - Nickel-Strunz Grouping):

- +
3.CC.05Gearksutite
Ca[Al(F,OH)
 
5
(H
 
2
O)]
3.CC.10Tikhonenkovite
Sr[AlF
 
4
(OH)(H
 
2
O)]
3.CC.15Artroeite
Pb[AlF
 
3
(OH)
 
2
]
3.CC.20Calcjarlite
Na(Ca,Sr)
 
3
Al
 
3
(OH)
 
2
F
 
14
3.CC.20Jarlite
Na(Sr,Na)
 
7
MgAl
 
6
F
 
32
(OH,H
 
2
O)
 
2
3.CC.20Jørgensenite
Na
 
2
(Sr,Ba)
 
14
Na
 
2
Al
 
12
F
 
64
(OH,F)
 
4
Related Minerals - Hey's Index Grouping:

- +
8.4.1Sellaite
MgF
 
2
8.4.2Neighborite
NaMgF
 
3
8.4.3Bischofite
MgCl
 
2
 · 6H
 
2
O
8.4.4Korshunovskite
Mg
 
2
Cl(OH)
 
3
 · 4H
 
2
O
8.4.5Carnallite
KMgCl
 
3
 · 6H
 
2
O
8.4.6Tachyhydrite
CaMg
 
2
Cl
 
6
 · 12H
 
2
O
8.4.7Fluorite
CaF
 
2
8.4.8Sinjarite
CaCl
 
2
 · 2H
 
2
O
8.4.9Antarcticite
CaCl
 
2
 · 6H
 
2
O
8.4.10Chlorocalcite
KCaCl
 
3
8.4.11Rorisite
CaFCl
8.4.12Jarlite
Na(Sr,Na)
 
7
MgAl
 
6
F
 
32
(OH,H
 
2
O)
 
2
8.4.13Calcjarlite
Na(Ca,Sr)
 
3
Al
 
3
(OH)
 
2
F
 
14
8.4.14Tikhonenkovite
Sr[AlF
 
4
(OH)(H
 
2
O)]
8.4.16Frankdicksonite
BaF
 
2
8.4.17Usovite
Ba
 
2
CaMgAl
 
2
F
 
14
8.4.18Bøgvadite
Na
 
2
SrBa
 
2
Al
 
4
F
 
20

Other Names for Acuminite

Synonyms:
IMA1986-038
Other Languages:
Dutch:Acuminiet
German:Acuminit
Italian:Acuminite
Latvian:Akuminīts
Portuguese:Acuminite
Russian:Акуминит
Spanish:Acuminita

Other Information

Health Warning: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

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    • Localities for Acuminite

    The 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.
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    (TL) indicates type locality. ? 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
     
    • Kitaa (West Greenland) Province
      • Arsuk
        • Arsuk Firth
          • 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
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