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This application is primarily aimed at geoscientists. It's for calculating the CIPW standard from geochemical analyses and allows to classify the rock using the Streckeisen- (QAPF) and TAS- (Total Alkali vs. Silica) diagrams. For interested visitors who are unfamiliar with the CIPW standard and these rock classification diagrams, there are some explanations below.

The CIPW norm

The standard is named after four American petrologists: Cross, Iddings, Pirsson, Washington. The CIPW standard is a method of petrology. It is used to better compare the chemical properties of magmatic rocks. For example, with this standard, the saturation levels of SiO₂ of different rocks can be better assessed and classified. For this purpose, a chemical rock analysis is converted into a normative mineral population.
This normative mineral population consists of so-called standard minerals (see Table 1). By converting the chemical analysis of magmatic rocks into the CIPW standard, the problem of different degrees of crystallization is avoided and rocks with different glass and mineral content can be compared. An anhydrous melt is assumed. Therefore, water- (OH-) containing standard minerals are not provided.

A short list of references can be found under "Help and infos".


The CIPW standard minerals

Mineral name

Symbol

Molecules

Quartz

Q

SiO₂

Corundum

C

Al₂O₃

Orthoclase

or

K₂O ∙ Al₂O₃ ∙ 6 SiO₂

Albite

ab

Na₂O ∙ Al₂O₃ ∙ 6 SiO₂

Anorthite

an

CaO ∙ Al₂O₃ ∙ 2 SiO₂

Leucite

lc

K₂O ∙ Al₂O₃ ∙ 4 SiO₂

Nepheline

ne

Na₂O ∙ Al₂O₃ ∙ 2 SiO₂

Kaliophilite

kp

K₂O ∙ Al₂O₃ ∙ 2 SiO₂

Mineral name

Symbol

Molecules

Diopside

di

CaO(Mg,Fe)O ∙ 2 SiO₂

Wollastonite

wo

CaO ∙ SiO₂

Hypersthene

hy

(Mg,Fe)O ∙ SiO₂

Olivine

ol

2(Mg,Fe)O ∙ SiO₂

Acmite

ac

Na₂O ∙ Fe₂O₃ ∙ 4 SiO₂

Magnetite

mt

FeO ∙ Fe₂O₃

Hematite

hm

Fe₂O₃

Ilmenite

il

FeO ∙ TiO₂

Apatite

ap

3 (3 CaO ∙ P₂O₅)

Pyrite

pr

FeS₂

Calcite

cc

CaO ∙ CO₂

Tab. 1) The CIPW normative minerals (after Matthes, 1987)

Important plutonic rocks, their average chemical composition (oxides, wt%) and their normative mineral content (wt%)

Oxides

Gabbro

Granite

SiO₂

48.36

72.08

TiO₂

1.32

0.37

Al₂O₃

16.84

13.86

Fe₂O₃

2.55

0.86

FeO

7.92

1.67

MnO

0.18

0.06

MgO

8.06

0.52

CaO

11.07

1.33

Na₂O

2.26

3.08

K₂O

0.56

5.46

P₂O₅

0.24

0.18

H₂O⁺

0.64

0.53

Total

100

100

Normative minerals

Gabbro

Granite

Quartz

29.31

Corundum

0.90

Orthoclase

3.31

32.27

Albite

19.12

26.1

Anorthite

34.15

5.42

Leucite

Nepheline

Kaliophilite

Diopside

15.57

Wollastonite

Hypersthene

13.41

3.15

Olivine

7.04

Acmite

Magnetite

3.70

1.25

Hematite

Ilmenite

2.51

0.70

Apatite

0.56

0.42

Pyrite

Calcite

Total

99.37

99.52

Tab. 2) The average chemical composition of gabbro and granite (Matthes, 1987) and the resulting normative mineral content in wt%; * Sum without water.

Important volcanic rocks, their average chemical composition (oxides, wt%) and their normative mineral content (wt%)

Oxides

Basalt

Rhyolite

SiO₂

50.83

73.66

TiO₂

2.03

0.22

Al₂O₃

14.07

13.45

Fe₂O₃

2.88

1.25

FeO

9.05

0.75

MnO

0.18

0.03

MgO

6.34

0.32

CaO

10.42

1.13

Na₂O

2.23

2.99

K₂O

0.82

5.35

P₂O₅

0.23

0.07

H₂O⁺

0.91

0.78

Total*

100

100

Normative minerals

Basalt

Rhyolite

Quartz

3.71

33.08

Corundum

0.85

Orthoclase

4.85

31.62

Albite

18.87

25.3

Anorthite

25.96

5.15

Leucite

Nepheline

Kaliophilite

Diopside

19.83

Wollastonite

Hypersthene

17.31

0.83

Olivine

Acmite

Magnetite

4.18

1.81

Hematite

Ilmenite

3.86

0.42

Apatite

0.53

0.16

Pyrite

Calcite

Total*

99.1

99.22

Tab. 3) The average chemical composition of basalt and rhyolite (Matthes, 1987) and the resulting normative mineral content in wt%; * Sum without water


The Streckeisen diagram

The Streckeisen diagram, also known as a QAPF-diagram, is a schematic diagram for the classification of igneous rocks. It is named after the Bernese petrographer Albert Streckeisen (1901-1998).

Streckeisen diagram

Schematic Streckeisen-Diagram

Presentation

The representative point in the diagram is usually determined by the modal (microscopic) population of the felsic (light) minerals and their relative occurrence in the rock (wt-%). Consequently, only the mineral phases quartz (Q), alkali feldspars (A), plagioclase (P) and foides (Feldspathoid, F) are taken into account for this classification. The diagram is made up of two concentration triangles. The two triangles touch each other on the feldspar baseline, because free quartz and foides in a magma can not crystallize at the same time, but would react to feldspar. The vertices of the Streckeisen diagram are formed by rocks in which the respective minerals form the only felsic part, but this does not exclude other mafic minerals. A separate classification diagram is used for both plutonic and volcanic rocks.

Rock types

The rocks of the Streckeisen diagram for plutonic rocks are granite, quartzite, diorite, gabbro, tonalite, monzonite, syenite, anorthosite, foidolite and their mixed forms. The volcanic rocks are rhyolite, dacite, trachyte, latite, basalt, andesite, phonolite, tephrite, foidite and transitional forms. As a rough guide, the rule is that the proportion of mafic (dark) minerals usually increases from the vertices A (alkali feldspar) to P (plagioclase) and from Q (quartz) to F (foides, Feldspathoids).

Applicability and limitations

The Streckeisen diagram may only be used for the classification of igneous rocks if:

Since volcanic rocks are often very finely crystallized and contain rock glass, the proportion of individual felsic mineral phases can not usually be determined by microscopic methods (modal inventory). Therefore, other classification systems based on the chemical analysis of rock samples, such as the TAS diagram, are often used for volcanics. Alternatively, a normative mineral population can be calculated from a chemical rock analysis, for example using the CIPW standard, which is then used for classification in the Streckeisen diagram.

Source:

The TAS diagram

TAS-diagram

TAS-Diagram (Legend)

For the designation of the individual fields (rock names) see the explanations on the page \"Calculation & Classification\". Furthermore, mean:

UB: Ultrabasic; B: Basic; IM: Intermediate; A: Acid; HAlc: Hyper-Alkaline Series; Alc: Alkaline Series; SAlc: Sub-Alkaline Series;

Application

The Total Alkaline vs. Silica (TAS) diagram is used to classify volcanic rocks based on the chemical composition of the total rock. The contents in percent by weight of Na₂O + K₂O in the Y direction are plotted against SiO₂ in the X direction. This diagram is used when a determination of the modal mineral population of the rock, as required for the classification in the Streckeisen diagram, is not possible. This is often the case with fine-grained or glassy volcanics.

The above-mentioned chemical parameters are useful because the relative proportions of alkalis to SiO₂ play an important role in determining the actual and normative mineralogy. This classification can be a simple method for rocks that have been chemically analyzed.

Limitations

However, this classification can not be applied to all volcanic rocks (Le Maitre et al., 2002). Certain rocks can not be named with this diagram. For other rocks further chemical, mineralogical or textural criteria must be applied, e.g. for lamprophyre.
The TAS classification should only be applied to rocks whose mineral composition can not be determined (otherwise classifications based on mineralogy are used (for example, the Streckeisen- (QAPF-) or other diagrams for igneous rocks).

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For more information and explanations concerning geology go the help page, please.

Help and infos



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