Does lava have a chemical formula?

Anonymous

Dear reader,

Minerals have chemical formula. For example, the formula for olivine, a common mineral in basalt, is (Fe,Mg)2 SiO4. That means for each little unit of olivine there is 2 atoms of iron (Fe), or 2 atoms of magnesium (Mg), or one of each plus one atom of silicon (Si), plus 4 atom of oxygen (O).

Most rocks are made of the following elements:

Element (Symbol)	Weight percent
Oxygen (O)	46.6
Silicon (Si)	27.7
Aluminum (Al)	8.1
Iron (Fe)	5.0
Calcium (Ca)	3.6
Sodium (Na)	2.8
Potassium (K)	2.6
Magnesium (Mg)	2.1
	98.5

The amount of these different elements in a rock can be determined. Geologists report the values as oxides (the element combined with oxygen). For example, a rock might have 6 weight percent FeO (iron oxide).

Different types of lava have different chemical compositions. Here are a few examples:

		Basalt	Andesite	Dacite	Rhyolite	Trachyte
SiO2		49.20	57.94		65.01	72.82		61.21
TiO2		1.84	0.87		0.58	0.28		0.70
Al2O3		15.74	17.02		15.91	13.27		16.96
Fe2O3		3.79	3.27		2.43	1.48		2.99
FeO		7.13	4.04		2.30	1.11		2.29
MnO		0.20	0.14		0.09	0.06		0.15
MgO		6.73	3.33		1.78	0.39		0.93
CaO		9.47	6.79		4.32	1.14		2.34
Na2O		2.91	3.48		3.79	3.55		5.47
K2O		1.10	1.62		2.17	4.30		4.98
P2O5		0.35	0.21		0.15	0.07		0.21

Sometimes if the lava cooled too quickly to form crystals (minerals) it is classified by its chemistry.

To learn more about minerals and volcanic rocks see Minerals, Magmas, and Volcanic Rocks.

Steve Mattox, University of North Dakota

Source of Information:
LeMaitre, R.W., 1976, The chemical variability of some common igneous rocks: Journal of Petrology, v. 17.

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