Silicon saturation

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In addition to the SiO 2 content, silicon saturation is the most important parameter for characterizing igneous rock .

introduction

Since igneous rocks, with the exception of relatively rare rocks such as carbonatites, are mainly composed of silicates , the SiO 2 (silica) content in the respective melts is of decisive importance for the crystallizing mineral stock. It enables a primary classification of igneous rocks into four categories:

The concept of silicon saturation ( English silica saturation ) in igneous rock goes beyond the purely quantitative SiO 2 content. Rather, it is a measure of the ability of a magma to form SiO 2 -saturated silicates. For example, feldspars (silica / oxide ratio = 6: 1) are far more saturated in SiO 2 than foids (2: 1), pyroxenes (1: 1) are more SiO 2 -rich than olivine (1: 2).

definition

"The silicon saturation in igneous rock is the concentration quotient , which is determined from the ratio of SiO 2 content to the content of all other silicate-forming components."

Classification

Three cases can be distinguished for silicon saturation:

  • Silicon supersaturated
  • Silicon-saturated
  • Silicon-undersaturated

In the case of silicon-oversaturated magmatites, more silica is present than is required for all other main silicate minerals, the excess SiO 2 can therefore crystallize out as quartz or its polymorphic representative cristobalite or tridymite . Significantly, the component quartz (q) appears in the CIPW standard . Granitoids usually meet this criterion.

In the case of igneous magmatites saturated with silicon, the existing silica exactly balances the requirements of all other silicates (there is neither excess nor deficiency). There is therefore no crystallization of quartz, but on the other hand also the crystallization of feldspar representatives or olivine. These magmas are hypersthene normative (hy). Rock examples are diorite and andesite .

In the case of silicon-undersaturated magmatites, there is a lack of silica in order to be able to meet the requirements of all main silicates, which is why feldspar representatives, such as nepheline , analcime or leucite , which require less SiO 2 in their structure, are now formed instead of feldspars . Furthermore, magnesium-rich olivine, perovskite , melanite and melilite crystallize . Characteristic standard minerals are olivine (ol) and possibly nepheline (ne). Rock examples are nepheline syenites .

CIPW standard

When calculating the CIPW standard, the principle of silicon saturation is used, ie the standardization process tries to follow the natural magmatic processes as much as possible. After the formation of the feldspars (components ab, an, or) and pyroxenes (components di and hy), it is checked whether the amount of SiO 2 present is actually sufficient for this. If so, then we have a silicon-saturated igneous rock before us. If more SiO 2 is present than necessary, normative quartz can be formed with the excess and the rock is supersaturated with silicon. If, on the other hand, the amount is insufficient, more economical silicates must be formed from the saturated. The following conversions come into consideration, among others:

  • Orthopyroxene (hy) ==> olivine (ol)
  • Albite (ab) ==> nepheline (ne)
  • Orthoclase (or) ==> leucite (lc)

The amount of newly formed components is an indicator of the silicon undersaturation.

Peccerillo (2002) made use of this principle in his parameter ΔQ, which he defines as follows:

ΔQ = q - (lc + ne + kal + ol)

On silicon-oversaturated rocks have ΔQ> 0, saturated ΔQ = 0 and undersaturated ΔQ <0. Plotted against K 2 O / Na 2 O, this new parameter enables a well-resolved differentiation of magma provinces .

Silicon saturation in basaltic rocks

The basalt tetrahedron with the three areas of silicon saturation

In basaltic rocks , by far the most common igneous rocks on earth, the individual areas of silicon saturation can be found very nicely in the basalt tetrahedron clinopyroxene- nepheline- plagioclase- quartz- orthopyroxene- olivine or Di-Ne-Ab, which goes back to Yoder and Tilley (1962) -Q-Hy-Ol (see adjacent figure). The green area clinopyroxene-plagioclase-orthopyroxene (Di-Ab-Hy) represents the compositions of silicon-saturated basalt magmas. In the adjoining room to the right of this interface (room Di-Ab-Hy-Q) there are quartz and hypersthene-normative basalts that are supersaturated with silicon - quartz tholeiites . Olivine- and nepheline-normative basalt magmas that are undersaturated with silicon occupy the space Di-Ab-Ol-Ne located to the left of the violet Di-Ab-Ol interface - alkali olivine basalts and basanites . The interstitial space Di-Ab-Hy-Ol occupies a special position; these are critically undersaturated basaltic magmas - olivine tholeiites with normative oil and hy. Ultramafic mantle rocks also fall into this range, so they are saturated to critically undersaturated in silicon.

Among the basalt relatives , nephelinites , olivine nephelinites and related foidites , as well as basanites and tephrites , are undersaturated in silicon.

Individual evidence

  1. ↑ Peccerillo, A .: Quaternary magmatism in Central-Southern Italy: a new classification scheme for volcanic provinces and its geodynamic implications . In: Bolletina della Società Geologica Italiana . Volume Speciale n.1, 2002, p. 113-127 .
  2. ↑ Yoder, HS and Tilley, CE: Origin of basalt magmas: An experimental study of natural and synthetic rock systems . In: Journal of Petrology . tape 3 , 1962, pp. 342-532 .