Jun 13 2014
The term acuminite was coined from the Latin word acuminis, meaning sharp point, or spear head, to denote the characteristic shape of the crystals in the mineral. It is an aluminum complex salt.
Properties of Acuminite
The following are the key properties of acuminite:
- Cell Data
- Space group: C2/c or Cc
- a = 13.223(1)
- b = 5.175(1)
- c = 14.251(1)
- β = 111.61(2)°
- Z = 8
- Crystal Data
- Monoclinic
- Point group: 2/m or m
- As acute di-pyramidal crystals, showing {110}, {111}, and rare {112}, similar to spearheads, to 1mm
- Twinning: On {100}, as contact twins
- X-ray powder pattern: 4.767 (10), 4.706 (10), 3.505 (10), 3.353 (10), 2.075 (9), 3.310 (8), 3.286 (8)
- Chemical Composition
| Elements |
Content 1 |
Content 2 |
| Sr |
37.04 |
38.84 |
| F |
33.52 |
33.68 |
| Al |
11.86 |
11.96 |
| H2O |
[7.80] |
7.98 |
| OH |
[6.82] |
7.54 |
| Ca |
0.02 |
- |
| Total |
[97.06] |
100.00 |
- Optical Properties
- Transparent
- Optical Class: Biaxial (+)
- Orientation: X = b; Y^c = 15(1)°
- Dispersion: r > v, strong
- α = 1.4507(3)
- β = 1.4528(3)
- γ = 1.4624(3)- 1.4627(3)
- 2V(meas.) = 46°-57°
- Estimated Properties
| Electron density |
Bulk density (electron density) = 3.11 g/cm3
note: Specific gravity of acuminite = 3.31 g/cm3. |
| Photoelectric |
PEAcuminite = 44.23 barns/electron
U = PEAcuminite x ρ electron density = 137.56 barns/cm3 |
| Fermion index |
Fermion index = 0.02
Boson index = 0.98 |
Radioactivity
|
Acuminite is not radioactive
|
How to Identify Acuminite
Acuminite can be identified in the field by its colorless structure, with white streak, and vitreous luster.
The density of acuminite is 3.295 g/cm3, with a hardness of 3.5 – approximate to a copper penny.
Global Distribution
Acuminite is distributed chiefly in the Ivigtut cryolite deposit, south-western Greenland.
Occurrence of Acuminite and Useful Mineral Association
Acuminite occurs in a small cavity in a specimen from a strontium-rich portion of a cryolite deposit.
It is often associated with minerals such as pachnolite, fluorite, ralstonite, gearksutite, celestine, jarlite, and thomsenolite.
References