BOWEN'S REACTION SERIES

The explanation for why specific minerals tend to occur with each other and not with other minerals in igneous rocks was worked out and summarized by N.L. Bowen in 1928.  Mineral occurrence in igneous rocks is related to the actual temperature at which they crystallize (not the rate).

Among the minerals discussed above, olivine has the highest melting/freezing point at nearly 1600C.  Pyroxene and Ca plagioclase form at somewhat lower temperatures, amphibole and mixed plagioclase somewhat lower still, biotite and Na plagioclase even lower, and K feldspar, muscovite and quartz at the lowest temperatures of all the common igneous minerals.

Bowen's illustration points out the sequential nature of crystallization of these minerals with declining temperature as well as the two separate processes that accomplish the sequence for the feldspars and ferromagnesian minerals.  Each of these pathways is called a "reaction series".  The diagram below summarizes the two series.

According to the diagram the ferromagnesian minerals form in a "discontinuous series" as the temperature of the magma drops.  Olivine crystallizes at the highest temperatures, pyroxene at somewhat lower temperatures, and so on.  The "discontinuous" nature of this series lies in the way the transitions from one mineral to the next one down the scale are accomplished.  At some specific temperature the olivine atomic arrangement becomes unstable and that of pyroxene stable.  At this point the olivine re-dissolves in the melt and, simultaneously, pyroxene begins to form from the ions thus liberated.  With further temperature decrease the same thing happens to cause amphibole to replace pyroxene, and finally happens again to cause biotite to replace amphibole.  At this point the discontinuous series stops because biotite is the stable ferromagnesian mineral at this and all lower temperatures.

The "continuous" series of plagioclase feldspars behaves differently.  At the highest temperature that any feldspar is stable (approximately the same as for pyroxene) only Ca ions are taken into the crystals.  With continued temperature decrease Na ions replace Ca ions in the structure in an essentially one-by-one fashion.  Thus, the transition from Ca plagioclase at high temperatures to Na plagioclase at low ones is gradual and can produce a feldspar with any proportion of the two ions.  There is no rapid and instantaneous change from one mineral to another as in the discontinuous series.  Pure Na plagioclase is the stable composition at about the same temperature that biotite forms and all lower temperatures.

The last minerals on the diagram, K feldspar, muscovite, and quartz all form at slightly lower temperatures from the remaining liquid not yet used up in the minerals of the two reaction series.

CONTINUE

RETURN TO IGNEOUS ROCK PAGE