Acuminite

• IMA 1986-038

Acuminit is a very rarely occurring mineral from the mineral class of halides. It crystallizes in the monoclinic crystal system with the chemical composition Sr [ AlF4 (OH)] · H2O and developed only millimeter-sized, spitzpyramidale and colorless crystals.

Etymology and history

Acuminit was first described in 1987 by H. Pauly and OV Petersen, who named it because of its characteristic habit after the Latin word acumen ( spearhead ).

Classification

In the now outdated but still in use 8th edition of the mineral classification by Strunz the Acuminit belonged to the mineral class of " halides " and then to the Department of " Doppelhalogenide, hydrated ," where he along with Artroeit, Chukhrovit - (Ce), Chukhrovit ( Y), Creedit, Gearksutite and Tikhonenkovit the unnamed group III/C.01 formed.

The 9th edition valid since 2001 and of the International Mineralogical Association (IMA ) used the Strunz'schen Mineral classification assigns the Acuminit also in the class of " halides " and there in the department of " complexes halides " one. This division, however, is further subdivided according to the crystal structure, so that the mineral is found in accordance with its construction in the subsection " group Aluminofluoride ( Soro - Aluminofluoride ) ", where it together with Tikhonenkovit the " Tikhonenkovit group " with the System No.. 3.CC.10 forms.

The mainly common in English-speaking classification of minerals according to Dana assigns the Acuminit in the class of " halides " and then in the Department of - a "Complex halides of aluminum fluorides ." Here he is the only member of the unnamed group 11:06:17 within the sub-division of " complexes halides - aluminum fluorides with different formulas " to find.

Modifications and varieties

The compound Sr [ AlF4 (OH)] · H2O occurs in nature on dimorphic and can next Acuminit also in the structure of likewise monoclinic prismatic Tikhonenkovit, but with a different space group and lattice parameters, crystallize.

Education and Locations

Acuminit has only been found in one place, the same time is also the type locality of this mineral: In a small cavity of a strontium- rich cryolite deposit at Ivittuut Greenland. The mineral appeared there in paragenesis with fluorite, Jarlit, Thomsenolith, Pachnolith, Ralstonite, Gearksutite and Celestine.

Crystal structure

Acuminit crystallizes in the monoclinic crystal system, space group C2 / c ( Raumgruppen-Nr. 15) with the lattice parameters a = 13.223 Å; b = 5.175 Å, c = 14.25 Å and β = 111.6 ° and 8 formula units per unit cell ..