EXTENSION MODULE 4.1

EM4.1-1

A. The Reaction PrincipleDuring crystallization of silicate magmas, reactions takeplace between crystallized minerals and the remainingmolten liquid.These reactions change the compositionsof the minerals or consume one mineral while produc-ing a new mineral.As a result of these reactions, miner-als appear and disappear in a specific order as crystal-lization and reactions progress. This reaction principle,which explains the minerals found in igneous rocks, isbased on both laboratory experiments and evidence ofmineral reactions preserved in real rocks. Norman L.Bowen presented the principle in 1928, and his researchwas supported by nearly two decades of research at theCarnegie Institution of Washington.

Figure EM4.1-1 diagrammatically illustrates themineral reactions. In continuous reactions, the miner-al composition changes gradually as crystallizationoccurs. In discontinuous reactions, one mineral reactswith the liquid to produce a new mineral at theexpense of the first one.

B. Why Reactions HappenWhy should a mineral, once it crystallizes from magma,then react with the magma? The key thing to remem-ber is that the crystallizing minerals do not have theexact same composition as the magma. For example,quartz consists entirely of silica, but no magma is 100percent silica. Another example is olivine, which com-monly crystallizes first in cooling mafic magma. Olivinecontains less silica and more magnesium than maficmagma. As more and more olivine crystals form, theremaining liquid contains progressively more silica andless magnesium than before the olivine crystallized.Eventually, the magma is sufficiently silica rich andmagnesium poor that the olivine crystals are no longerstable and these crystals react with the magma to formpyroxene, which contains less magnesium and more sil-ica than the original olivine.

C. Understanding Each SeriesThe two sides of the reaction series diagram (EM4.1-1) depict two different types of reactions betweenminerals and melt. Bowen named these reactions thecontinuous and discontinuous series.

The continuous series involves only the plagioclase

feldspars. Relatively calcium-rich (Ca) plagioclasefeldspar crystallizes first in a cooling magma. As tem-perature decreases during crystallization, plagioclasefeldspar reacts with the remaining liquid to becomeprogressively more sodium rich (Na), as illustratedschematically in Figure EM4.1-2. The sodium-rich

Bowen’s Reaction SeriesThis module illustrates the reactions observed to occur between minerals and magma, and the order of these reactions

during magma crystallization that was described by Norman Bowen in the 1920s.

Continuous series Discontinuous seriesCa-rich

plagioclasefeldspar

Na-richplagioclase

feldspar

Potassium feldsparMuscovite

Quartz

Early Olivine

Pyroxene

Amphibole

Biotite

Order of crystallization

Late

▲ Figure EM4.1-1 A diagram of the reaction series.Bowen used this diagram in 1928 to illustrate two different mineral series that result from reactionsbetween early crystallized minerals and magma. Arrows show the chemical reactions that produce newminerals (discontinuous series) and gradual changing composition within the plagioclase feldspars (con-tinuous series) during crystallization of a silicate magma.

Dec

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Com

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of p

lagi

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ldsp

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Calcium-rich

Plagioclasecrystal

Magma

Sodium-rich

Initial crystal is calcium-rich

Crystal becomes more sodium-rich as it grows

Figure EM4.1-2 Visualizingthe continuous series.The first plagioclase feldspar crys-tals to form in a cooling magmaare relatively rich in calcium. Asthe magma cools, the feldsparcrystals react with the melt andbecome more sodium rich as theycontinue to grow. If crystallizationtakes a sufficiently long time,then the reactions will homoge-nize the crystal into a single com-position. Otherwise, the center ofeach crystal remains more calci-um rich while the edges are moresodium rich.

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Smith/Pun How Does Earth Work? Second Edition, Pearson Prentice Hall © 2010

EM4.1-2

feldspar also contains more silica and less aluminumthan the calcium-rich feldspar. The descriptive term“continuous” emphasizes that the reactions are ongo-ing and modify the composition within single crystalsof plagioclase feldspar.

The discontinuous series describes reactionsbetween the common Fe-Mg silicate minerals—olivine, pyroxene, amphibole, and biotite. After crys-tallization of each mineral, reaction with the remain-ing liquid causes that mineral to dissolve and bereplaced by the next mineral shown beneath it on thediagram (EM4.1-1). Figure EM4.1-3 schematicallyillustrates how olivine and magma react to formpyroxene. The term “discontinuous” applies to reac-tions in which one mineral is consumed and a newmineral formed, unlike the continuous series wherecompositional change happens continuously within asingle mineral group.

D. Illustrations in Real RocksBowen realized that reactions like those produced inlaboratory experiments were observed in many vol-canic rocks. Rapid cooling of lava flows and near-surface magma intrusions can prevent the reactionsfrom being completed. Therefore, Ca-rich plagio-clase can be surrounded by more Na-rich plagioclase(continuous series) and Fe-Mg silicates that formedearly can be surrounded by those that formed bylater reaction (discontinuous series). Figure EM4.1-4 illustrates these reactions in real rocks.

E. Relationship of the ReactionSeries to Rock CompositionWhere crystallization begins and ends on Bowen’sdiagram depends on the initial composition of themagma and whether or not fractional crystallizationoccurs (see Section 4.7 of text). A mafic magma maybegin with crystallization of olivine and Ca-plagio-clase and be completely crystallized by the time pyrox-ene and slightly more sodium-rich plagioclase form.

Quartz, potassium feldspar, and muscovite typical-ly crystallize if the magma has not completely solidi-fied after formation of sodium-rich feldspar andbiotite (see Figure EM4.1-1). These minerals do notform as a result of chemical reactions but simply crys-tallize from the remaining magma, which typically isvery rich in silica, potassium, and aluminum. Thesethree minerals are not, therefore, part of either thecontinuous or discontinuous series.

Plagioclase crystal has a calcium-rich center surrounded by more sodium-rich zones.

This plagioclase feldspar in thisandesite has distinct zones.Analyses show that the interior of the crystal is calcium rich and the outer zones are sodium rich. these zones reflect the reactions of the continuous series.

The olivine crystals in this gabbro have rims of pyroxene,documenting the reaction of the olivine with the magma to producepyroxene at the time crystallization of the rock ended.

Olivine reacted withmagma to form a rim of pyroxene.

2 mm

Figure EM4.1-4 Reactions observed in rocks.These microscope photographs show evidence for reactions inigneous rocks. (Source: Aurora Pun)

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Olivine crystallizesfrom cooling magma

Olivine reacts with magma to form pyroxene

Olivine crystal

Magma

Olivine Pyroxene

Pyroxene crystal

Figure EM4.1-3 Visualizingthe discontinuous series.Olivine crystallizes early in maficmagma. As crystallization proceedswith decreasing temperature, theolivine reacts with the magma toform calcium-poor pyroxene. Thetemperature does not change untilthe reaction is completed. After thereaction consumes all of theolivine, the temperature continuesto decrease and pyroxene contin-ues to crystallize.

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Smith/Pun How Does Earth Work? Second Edition, Pearson Prentice Hall © 2010

Figure EM4.1-5 illustrates how the reaction seriesexplains why mineral associations are different foreach compositional group. Minerals widely separatedon the diagram (e.g., Mg-rich olivine and quartz) arenot expected to be found together because they do notform at the same stage of magma crystallization or inmagmas of comparable composition. Typically, onlyadjacent minerals on the diagram are found together.

Bowen used an analogy to a childhood game that waspopular in his time to explain the mineral associations:“Suffice it to say here that those minerals that belong tothe same general period of crystallization tend to beassociated and those belonging to remote periods ordi-narily fail to associate. The controlling factors are thusanalogous to those which determine that little girls ordi-narily play ‘London Bridge’ with other little girls, occa-sionally with their mothers, seldom with their grand-mothers and never with their great grandmothers.”

Putting It Together

■ Bowen’s reaction series diagrammatically illus-trates the minerals that commonly occur togeth-er in rocks in accordance with rock and labora-tory studies that show early minerals to reactwith magmas during crystallization.

■ In the continuous series, reactions producegradual changes in the composition of plagio-clase feldspar during magma crystallization.

■ In the discontinuous series, an earlier formedmagnesium-rich (and calcium-poor) silicatemineral is consumed by a reaction that producesa different magnesium-rich silicate.

EM4.1-3

Continuous series Discontinuous series Rock compositionCa-rich

plagioclasefeldspar

plagioclasefeldspar

Potassium feldsparMuscovite

Quartz

Early OlivineMafic

Intermediate

FelsicBiotite

Late

Figure EM4.1-5 Relating Bowen’s reaction series to rockcomposition.The broad horizontal bands on the diagram show the ranges ofapproximate boundaries between the mineral compositions ofmafic, intermediate, and felsic rocks. The types of minerals found ineach rock type, also illustrated in text Figure 4.3, result from mineralreactions. Olivine, typical of mafic rocks, does not occur with quartz,typical of felsic rocks, because these two minerals form at differentstages in magma crystallization.

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Smith/Pun How Does Earth Work? Second Edition, Pearson Prentice Hall © 2010