Aluminosity
The Aluminosität is after the SiO 2 content of the second most important parameter for characterizing petrological igneous rocks . It is mainly used for granitoids .
definition
The aluminosity was determined in 1972 by M. Barrière as the molecular quotient A '/ F. The two quantities A 'and F are in turn defined as follows:
- A '= [Al 2 O 3 ] + [Fe 2 O 3 ] - [Na 2 O] - [K 2 O] - [CaO]
- F = [FeO] + [MnO] + [MgO]
The values determined from geochemical rock analyzes are inserted into the two equations, the square brackets denoting molecular quotients, ie the respective weight percentages of the analysis are divided by the molar mass of the oxide concerned .
The calculated values are then categorized as follows:
- Negative values, ie A '/ F <0, are hypaluminous rocks.
- A '/ F values between 0 and 0.33 result in normally aluminous rocks.
- If A '/ F> 0.33, the rocks are hyperaluminous .
Examples
The following rock analyzes may serve as examples for the individual categories:
| Oxide wt.% |
Delos granodiorite | Mykonos granite | Andalusite granite Mont Pilat (F) |
|---|---|---|---|
| SiO 2 | 68.00 | 73.40 | 72.00 |
| TiO 2 | 0.57 | 0.27 | 0.10 |
| Al 2 O 3 | 15.20 | 13.60 | 14.75 |
| Fe 2 O 3 | 0.61 | 0.32 | 1.05 |
| FeO | 2.20 | 1.01 | 0.70 |
| MnO | 0.05 | 0.02 | 0.10 |
| MgO | 1.24 | 0.34 | 0.10 |
| CaO | 3.62 | 1.85 | 0.70 |
| Na 2 O | 3.09 | 2.63 | 3.55 |
| K 2 O | 4.11 | 5.35 | 5.00 |
| P 2 O 5 | 0.14 | 0.07 | 0.40 |
| H 2 O | 0.65 | 0.55 | 0.90 |
| A '/ F | - 0.08 | 0.24 | 2.19 |
The granodiorite from Delos is therefore hypaluminos, the granite from Mykonos normal aluminos and the andalusite - muscovite granite from Mont Pilat is hyperaluminos.
meaning
In ordinary igneous rocks (the carbonatites are an exception ), aluminum (given in analyzes as Al 2 O 3 ) is the second most common element after silicon . Analogous to the silicon saturation of a rock, the principle of aluminum saturation , which is expressed in the aluminum saturation index, can also be used to characterize magmas . This index takes into account the ratio of aluminum to the alkalis ( sodium and potassium ) plus calcium , which is 1 in feldspars and foids , ie feldspars and feldspar representatives are saturated with aluminum.
However, since most granitoids are oversaturated with aluminum and contain more Al 2 O 3 than is necessary for the formation of feldspars and foids, additional minerals such as muscovite , biotite (rich in aluminum), the aluminum silicates andalusite and sillimanite , but also cordierite , Garnet ( almandine or Spessartine ), corundum , tourmaline and topaz are formed. The above-mentioned hyperaluminous rock from Mont Pilat illustrates this very nicely, it contains andalusite and muscovite. In their standard , all of these rocks are identified by the component corundum (c).
If granitic magmas should nevertheless be undersaturated with aluminum, which is less often the case, the formation of muscovite does not occur, biotite that occurs is no longer aluminum-rich and the growth of hornblende can begin, as is realized, for example, in granodiorites (the Delos granodiorite is hypaluminous ). These rocks are also known as metaluminos .
In the case of even more extreme aluminum undersaturation , peralkaline rocks are formed, such as alkali granites which, in addition to feldspars, contain alkaline amphiboles or pyroxenes . An example of this is the granite from Gouré in Niger , which contains Aegirin and Arfvedsonite . In their norm, these rocks show the component Akmit (ac).
literature
- Wolfhard Wimmenauer: Petrography of igneous and metamorphic rocks . Ferdinand Enke Verlag, Stuttgart 1985, ISBN 3-432-94671-6 .
- Myron G. Best and Eric H. Christiansen: Igneous Petrology . Blackwell Science, 2001, ISBN 0-86542-541-8 .
Individual evidence
- ↑ Barrière, M .: Sur la distinction des granites hypalumineux, alumineux et hyperalumineux . In: Comptes Rendues de l'Académie Scientifique de Paris . tape 274 , 1972, pp. 2416-2418 .
- ↑ Ravier, J. and Chenevoy, M .: Les granites à muscovite du Mont Pilat (Massif Central) . In: Bull. Soc. Géol. France, Ser. 7 . tape 8 , 1966, pp. 133-148 .