Laboratory Manual for Introductory Geology 4e

points melt first, whereas others remain solid. The entire rock melts only when the
temperature rises enough to melt all the minerals—over a range of several hundred
degrees in some cases.
■ Some minerals, such as quartz, melt simply, like ice. When heated to their
melting temperatures, these minerals melt completely to form a liquid with the
same composition as the mineral. This is not surprising, but the next two findings
are.
■ Plagioclase feldspars melt continuously over a range of temperatures. When
plagioclase containing equal amounts of calcium and sodium begins to melt, more
sodium atoms than calcium atoms are freed from the crystalline structure. The first
liquid to form is thus richer in sodium than the original plagioclase. More and more
calcium is freed from the residual mineral as temperature rises, however, so the
compositions of both the liquid and the residual mineral change continuously. Only
when the entire mineral has melted does the liquid have the same composition as
the original mineral.
■ Ferromagnesian minerals melt discontinuously, and new minerals form in the
process. Melt ice and you get water; but melt pyroxene, for example, and when the
first liquid forms, the remaining solid mineral changes from pyroxene to olivine.
Keep heating it, and the olivine gradually melts. Similar things happen to the
other common ferromagnesian minerals: biotite, amphibole, and olivine. As with
plagioclase, the melt has the same composition as the starting material only when
the process is complete.
Bowen found patterns of cooling and crystallization to be the exact opposite
of these patterns of melting. He summarized these findings graphically in what
is now named Bowen’s reaction series in his honor (FIG. 5.8). Note the relationship
between the melting temperatures of the common rock-forming minerals and
the minerals found in the four major igneous rock groups: igneous rocks in the same
compositional group generally contain minerals that crystallize at similar temperatures. In
Exercise 5.8, you will use the insights learned from these and other melting and
crystallization experiments.
FIGURE 5.8 Bowen’s reaction series, showing the sequence of crystallization of
minerals from magma.
High
Temperature
1400°
Olivine
First minerals to crystallize
(Ca-rich)
Ultramafic
1050°
Discontinuous series
Pyroxene
Amphibole
Biotite
(Na-rich)
Plagioclase
Continuous series
Mafic
Intermediate
K-Feldspar
800°
Muscovite
Felsic
Low
Temperature
Quartz
Last minerals to crystallize
5.4 ORIGIN AND EVOLUTION OF MAGMAS
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