Route iron diagram
A route iron diagram , also called a QAPF diagram , is a schematic diagram for classifying igneous rocks . It is named after the Bernese petrographer Albert Fahrtisen (1901–1998).
presentation
The point in the diagram is determined on the basis of the modal stock of the felsic minerals and their percentage of the rock. Only the mineral phases quartz ( Q ), alkali feldspars ( A ), plagioclase ( P ) and foide ( F ) are taken into account in the route iron diagram . Albite is included in the alkali feldspar. The diagram is made up of two concentration triangles. The two triangles touch on the feldspar baseline, since free quartz and foids in a magma cannot crystallize at the same time, but would react to form feldspar . The corner points of the route iron diagram are formed by rocks in which the respective minerals form the only rockic part, which does not, however, exclude other mafic minerals . Basically, there is a subdivision into plutonites and volcanic rocks , whereby a separate double classification triangle is used for each type of rock.
The composition of a rock results from the distance between the respective point in the iron line diagram and the corner points, i.e. the rocks with the respective pure basic mineral groups, and is usually determined by counting crystals in thin sections . A rock with 30% quartz, 50% alkali feldspar and 20% plagioclase would be classified as granite in the route iron diagram for plutonites , while a volcanite with the same mineral composition would be called rhyolite .
Rock types
The rocks of the route iron diagram for plutonites are: granite , quartzolite , diorite , gabbro , tonalite , monzonite , syenite , anorthosite , foidolite and their various mixed forms. The represented volcanic rocks are rhyolite , dacite , trachyte , latite , basalt , andesite , phonolite , tephrite , foidite and transitional forms. The rule that the proportion of mafic minerals increases from corner points A (alkali feldspar) to P (plagioclase) and from Q (quartz) to F (foid) can serve as a rough guide.
Applicability and Limitations
The route iron diagram may only be used to classify the magmatites if:
- the rock is less than 10% melilite leads
- the proportion of dark (mafic) minerals is less than 90% (otherwise the ultramafit triangle olivine - orthopyroxene - clinopyroxene is used)
- the rock no more than 10% glass leads
- the rock is neither a pyroclastite nor a carbonatite (> 50% carbonate minerals ), a charnockite , a kimberlite , a lamprophyre or a lamproitic .
Since volcanic rocks are often extremely finely crystalline and contain rock glasses, the proportions of individual felsic mineral phases can usually not be determined using microscopic methods. Therefore, other classification systems based on the chemical analysis of rock samples are often used for volcanic rocks, such as the differentiation according to the TAS diagram . Alternatively, a normative mineral inventory can be calculated from a chemical rock analysis, for example using the CIPW standard , which is used for classification in the route iron diagram.
swell
- John D. Winter: An introduction to igneous and metamorphic petrology. Prentice Hall, Upper Saddle River NJ 2002, ISBN 0-1324-0342-0 .
- RW LeMaitre (Ed.): Igneous Rocks. A Classification and Glossary of Terms. 2nd edition, Cambridge University Press, Cambridge 2004, ISBN 0-521-61948-3 .
