Continuous reaction series

The plagioclase branch of Bowen's reaction series, in which crystals progressively react with the melt to become sodium-rich as temperature falls.

Plagioclase feldspar solid solution series: A continuous mineral series from anorthite (CaAl2Si2O8) to albite (NaAlSi3O8), composition varying with temperature. The series is the reaction track: each crystallized composition is a point along this solid solution.
Norman L. Bowen: Petrologist who experimentally determined crystallization sequences of silicate melts in the early twentieth century. His 1913–1928 experiments at the Geophysical Laboratories defined the continuous plagioclase branch.
Discontinuous reaction series: The mafic branch in which one mineral is wholly replaced by another, from olivine through pyroxene to biotite. The paired branch: discrete mineral replacements versus the plagioclase branch's gradual compositional shift.
Normal zoning: A plagioclase crystal with a calcic core grading outward to a sodic rim, matching the reaction series trend. The textbook outcome of the continuous series when cooling is faster than diffusion.
Zone zoning in plagioclase crystals: Concentric compositional bands within a single plagioclase crystal recording changing melt conditions during growth. Incomplete reactions leave a compositional record, making zoned crystals direct evidence of the series.
Geophysical Laboratory: Carnegie Institution lab in Washington, D.C. founded in 1905 for experimental study of rocks at high temperature and pressure. Where Bowen's quenching experiments on melts produced the reaction series concept.
Fractional crystallization: Separation of crystals from melt so early-formed minerals cannot react, skewing remaining liquid compositions. Its failure to complete reactions explains why natural plagioclase is commonly zoned rather than uniform.
Sieve texture: Plagioclase cores riddled with melt inclusions, formed when crystals dissolve back into a changed melt. Evidence of reversal reactions when the melt composition changed faster than crystals could follow.
Calcium-sodium ion exchange: Substitution of Na+ plus Si for Ca2+ plus Al in the feldspar lattice, changing composition without changing structure. The crystal-chemical substitution that lets plagioclase react continuously instead of being replaced.
Phenocryst studies of Mount Shasta: Bowen's 1928 monograph 'The Evolution of the Igneous Rocks' synthesizing his experimental crystallization results. The publication that set out the continuous branch as a general law of igneous differentiation.

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Bowen's reaction seriesBroader topic: The continuous branch: each plagioclase composition reacts with the melt to become more sodium-rich.