Mineral Information and Data:

Acuminite
SrAlF4(OH) • H2O [CNMNC approved formula]

Named from the Latin acuminis, sharp point, for spear head, the characteristic shape of the crystals.

IMA status : Approved Species 1987 (IMA1986-038)
CLASSIFICATION 
Dana
(8th edition) :
11.06.17.01  
  (11) Halide Complexes; Alumino-fluorides
  (11.06) with miscellaneous formulae
  (11.06.17)
  
Nickel-Strunz
(10th edition) :
03.CC.10
  (03) Halides
  (03.C) Complex Halides
  (03.CC) Soro-aluminofluorides
  
Crystal system: Monoclinic System
Point group (H-M): 2/m — prismatic
Unit cell: a = 13.223 Å, b = 5.175 Å, c = 14.251 Å, b = 111.61°
Crystal Habit: Spear point-shaped crystals 1 mm long, in aggregates
Twinning: On {100} as contact twins
  
Color: Colorless
Diaphaneity: Transparent
Luster: Vitreous
Hardness (Mohs): 3.5
Measured Density: 3.29 g/cm3
Cleavage: 1; (001) perfect
Streak: White
  
Polymorphism and Series: Dimorphous with tikhonenkovite.
Geologic Setting: In a small cavity in a specimen from a strontium-rich portion of a cryolite deposit.
Mineral Association: Fluorite, jarlite, thomsenolite, pachnolite, ralstonite, gearksutite, celestine
  

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