Ã…kermanite
Akermanite is a rarely occurring mineral from the mineral class of " silicates and Germanates ". It crystallizes in the tetragonal crystal system with the composition Ca2Mg [ Si2O7 ], and is therefore chemically seen a calcium magnesium silicate group.
Akermanite usually develops short prismatic crystals up dünntafelige, but also granular to massive mineral aggregates and crossing twins. In its pure form it is colorless and transparent. But it may also appear white and accept by foreign admixtures a yellowish gray, green or brown color by multiple refraction due to lattice defects or polycrystalline training, the transparency decreases accordingly.
With gehlenite ( Ca2Al [ AlSiO7 ] ) akermanite forms a complete solid-solution series, between the two end members of a coupled substitution of Al3 Al3 by Mg2 Si4 takes place. In addition, shares of calcium by sodium and potassium, magnesium may be replaced by ferrous iron and aluminum by ferric iron.
Special Features
Akermanite strong tendency to decomposition.
Etymology and history
Was first described in 1884 by akermanite IHL Vogt, who named the mineral after the Swedish metallurgists and metallurgical expert Richard Anders Åkerman ( 1837-1922 ). This had the purpose of analysis provided some melting products available, in which the mineral or its synthetic analog was first discovered. Later akermanite also found as a natural mineral formation at different sites.
Classification
In the now outdated but still in use 8th edition of the mineral classification by Strunz the akermanite belonged to the mineral class of " silicates and Germanates " and then to the Department of " group silicates ( Sorosilikate ) " where he, along with Andrémeyerit, Barylith, gehlenite, Gugiait hardystonite, Jeffreyit, Kaliobarylith, melilite, and Meliphan Okayamalith the " melilite group " with the system no. VIII/C.02 formed.
The 9th edition used since 2001 and valid by the International Mineralogical Association (IMA ) of the Strunz'schen Mineral classification assigns the akermanite also included in the category of " silicates and Germanates " and there in the department of " group silicates ( Sorosilikate ) ". This division, however, is further subdivided according to the nature of the silicate group formation, the possible presence of other anions and the coordination of the cations, so that the mineral according to its composition and its construction in the subsection " Si2O7 groups without non- tetrahedral anions; Cations " is to find where it together with Barylith, Cebollit, gehlenite, Gugiait, hardystonite, Jeffreyit, melilite and Okayamalith the " in tetrahedral and higher coordination melilite group "with the system no. 9.BB.10 forms.
The mainly common in English-speaking classification of minerals according to Dana assigns the akermanite in the class of " silicates and Germanates " there, however, in the already fine divided department of " group silicates: Si2O7 groups, generally without additional anions " one. Here he is with gehlenite, melilite and Okayamalith in the " melilite group " with the system no. 55.04.01 within the sub-division of " group silicates: Si2O7 groups, generally without additional anions and cations in and with lower coordination " to find.
Education and Locations
Akermanite is found mainly as a crystalline component in iron and steel slags but also cement clinker may contain akermanite and melilite. In nature it is formed by contact metamorphism in siliceous limestone and dolomite, as well as in Sanidinit facies. Furthermore, akermanite also arise in alkaline and calcium-rich igneous rocks. Accompanying minerals occur among others diopside and its iron-and aluminum-containing variety Fassait, forsterite, grossular, Larnit, merwinite, monticellite, spurrite and wollastonite.
30 natural and anthropogenic ( slag heaps ) known localities for akermanite: Total worldwide so far (2012 version). In Germany, the mineral is found among others on the slag heap of the Herzog- Julius- hut at Astfeld in Lower Saxony and in the quarry " Caspar " on Ettringer Bellerberg at Ettringen in the Eifel region of Rhineland- Palatinate.
Other localities include the mountain Dupezeh at Qala Diza ( Qeladze, قلعة دزة ) in Iraq; in the Hatrurim formation of the Israeli Negev desert; Monte Somma and Monte Cavalluccio near Sacrofano in Italy; Hamada in Japan; Labrador, Oka and Saint -Joseph -du- Lac, Canada; on a slag heap near Tsumeb in Namibia; at Flekkeren in the Norwegian municipality Skien; in the basalt quarry near So - Rakos (sub Krebsbach ) in Harghita County in Romania; in Odikhincha massif in the Krasnoyarsk region and in the Chibinen and Kowdor on the Kola Peninsula in Russia; in Kilchoan on the Ardnamurchan Peninsula Scottish; at Söråker in Sweden; on the " Hendriksplaats Farm" at Mashishing (formerly Lydenburg ) and in the " Wessels Mine " at Hotazel in South Africa; at Poličany / Kutná Hora ( slag locality) and Želénky / Duchcov in the Czech Republic and at the Iron Hill in Gunnison County (Colorado), on the Cascade Mountains (New York) and in the " Marble Canyon Mine" in Culberson County ( Texas) in the United States of America.
Crystal structure
Akermanite crystallized tetragonal in the space group P421m ( Raumgruppen-Nr. 113) with the lattice parameters a = 7.84 Å and c = 5.01 Å and two formula units per unit cell.
The crystal structure comprises [ Si2O7 ] groups, the c-axis are arranged with their longitudinal axes extended in parallel. Between the silicate groups, the cations of calcium and magnesium are incorporated, with calcium and magnesium, of eight are each surrounded by four oxygen ions ( see coordination number ).