Laboratory Manual for Introductory Geology 4e

FIGURE 6.1 Physical weathering of coarse-grained granite.
(a) A rock breaks along the dashed lines.
(b) The rock fragment preserves
grain sizes and relationships.
(c) The fragment breaks into
smaller rock fragments and,
eventually, mineral grains.
sediment contains. For example, sediment derived from physical weathering of
granite would contain clasts of quartz, potassium feldspar, plagioclase, and biotite,
whereas sediment derived from physical weathering of basalt would contain clasts
of plagioclase, pyroxene, and olivine.
Chemical weathering is the process by which rock chemically reacts with
air, water, and acidic solutions. In other words, chemical weathering involves
chemical reactions, the breaking of chemical bonds in the rock’s minerals and
the formation of new bonds in the weathering products. These reactions destroy
some of the original minerals and can produce new ones. Minerals that are
easily weathered are called unstable or nonresistant, whereas those that can survive
weathering or are produced by weathering are said to be stable or resistant.
Chemical weathering has hardly any effect on quartz, for example, because
quartz is stable, but it transforms feldspar, which is an unstable mineral, into clay
and ions (K 1 , Ca 21 , Na 1 ). Similarly, chemical weathering converts ferromagnesian
minerals such as olivine, pyroxene, amphibole, and biotite into hematite
and limonite (iron oxide minerals) along with ions of silicon and oxygen that
are removed in solution.
Relatively few minerals are stable at the Earth’s surface, so chemical weathering
generally reduces the number of minerals in sediment over time. As a result, weathering
of very different source rocks can yield surprisingly similar sedimentary
mineral assemblages (FIG. 6.2). Weathering of granite, for example, produces
quartz, clay minerals, and iron oxides. Gabbro also weathers into clay minerals
and iron oxides. Note that only the proportions of minerals (more iron oxides in
the weathered gabbro) and the occurrence of quartz (none in weathered gabbro)
help us distinguish sediments derived by chemical weathering of granite from
those derived from gabbro.
6.2.3 Mineralogical Maturity
Not all sediments have undergone the same amount of weathering. Sediments that
contain nonresistant minerals that are readily weathered chemically are said to be
mineralogically immature. Sediments that lack these minerals and instead consist
only of those resistant to chemical weathering are said to be mineralogically mature.
The longer and more extensive the chemical weathering, the more mineralogically
mature the resulting sediments will be. The effects of weathering are examined
further in Exercise 6.1.
6.2 SEDIMENT FORMATION AND EVOLUTION
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