Sodium (symbol Na; atomic number 11; relative atomic mass 22.98976928) [Link to webelements.com]
Na has one oxidation state, +1, and one natural isotope (23Na).
Sodium is the most abundant of the alkali metals and is the sixth-most abundant element in the Earth’s crust.
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All Na-compounds in nature include oxygen, chlorine, or fluorine.
Common element:mineral associations of Na
Most common mineral associations: Na-plagioclase (albite, oligoclase), clay-minerals of the smectite group (montmorillonite, beidellite), and mica in siliciclastic sediments. Halite is the most common evaporite mineral.
Although Na is present as a major and/or minor component of many phosphates, halide, carbonates, nitrates, borates, and sulfates, it commonly has only significance for chemostratigraphy when present as silicate minerals or halite.
Na in siliciclastic rocks
The most important mineral association of Na is with plagioclase feldspars, clay minerals, and mica.
Albite (NaAlSi3O8) is the Na-rich endmember of the plagioclase feldspars, but a solid solution series exists between Na-rich (albite) and Ca-rich (anorthite, CaAl2Si2O8) plagioclase.
For the alkali feldspar series, a high-temperature solid solution exists between NaAlSi3O8 and KAlSi3O8, where sanidine is the high-temperature K-feldspar that can include up to 63 mole % albite, and anorthoclase is the high-temperature Na-feldspar with up to 37 mole % sanidine.
Montmorillonite ((Na,Ca)0,3(Al,Mg)2Si4O10(OH)2•n(H2O)), beidellite (Na0.5Al2(Si3.5Al0.5)O10(OH)2•n(H2O)), and nontronite (Na0.3Fe2(Si,Al)4O10(OH)2•n(H2O)) are the most common Na-bearing clay minerals. They occur as alteration products of volcanic tuffs and ashes (bentonites), mafic and ultramafic rocks, hydrothermal alteration products, and authigenic minerals in marine sediments.
Paragonite (NaAl2(Si3Al)O10(OH)2) is an example of a Na-rich mica, but its occurrence is rather uncommon and associated with low- to medium-grade metamorphic schists and phyllites.
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Na in carbonates and evaporites
In carbonate sediments, Na concentrations vary and are attributed to the influx of seawater.
In evaporites, mainly halite (NaCl) is of importance for chemostratigraphy. Although Na is present as a major and/or minor component of many halide, carbonate, sulfate, phosphate, nitrate, borate minerals, it commonly has only significance.
Na in igneous rocks
Sodium is a major component in all igneous rock types except the ultramafic rocks. In contrast to K, Na is more abundant in intermediate/basic igneous rocks (diorite and andesite) in the form of Na-plagioclase than in acidic igneous rocks.
A major component in igneous rocks is plagioclase, where a a solid solution series between NaAlSi3O8 (albite) and CaAl2Si2O8 (anorthite) exist. Like outlined above, for the alkali feldspar series, a high-temperature solid solution exists between NaAlSi3O8 and KAlSi3O8, exists. Anorthoclase is the high-temperature Na-feldspar with up to 37 mole % sanidine, the latter is the high-temperature K-feldspar (with up to 63 mole % albite).
Other Na-bearing minerals in igneous rocks are amphiboles, i.e. the hornblende group (K,Na)0-1 (Ca,Na,Fe,Mg)2 (Mg,Fe,Al)5 (Al,Si)8O22 (OH)2, and pyroxene / clinopyroxenes, e.g. of the aegirine-augite series (aegirine, NaFeSi2O6; augite, (Ca,Na)(Mg,Fe,Al,Ti)(Si,Al)2O6)).