Denudation, weathering and mass wasting - Fundamentals of the Physical Environment - page 294

OLIVINE

ANORTHITE
Ca—feldspar

PYROXENE
Augite

Ca—Na feldspar

AMPHIBOLE
Horrnblende

BIOTITE ALBITE
Na—feldspar

ORTHOCLASE
K—feldspar

MUSCOVITE

QUARTZ

Plate 13.7 Tafoni, or cavernous weathering pits, on marine
sediments in the San Gabriel mountains, California. They are
thought to form through mechanical action associated with
de/rehydration cycles, salt weathering and wind in the semi-
arid and coastal environment.
Photo: Ken Addison

Figure 13.8 Mineral susceptibility to chemical weathering,
closely linked with magma geochemistry and melt
temperatures.

Most minerals dissolve slowly in water, but those
dominated by ionic bonds, e.g. mafic minerals Mg ++ ,Fe ++ ,
CaAl 2 Si 2 O 8 (calc-plagioclase), K + (potassium), Na +
(sodium) and Ca + (calcium), are more susceptible to
solution than felsic minerals dominated by covalent
bonds, e.g. KAlSiO 3 O 8 (orthoclase), KAl 2 (OH) 2 Si 3 AlO 10
(muscovite) and (SiO 2 ) quartz. Al 2 SO 3 (alumina) is

solvent . Hydrogen ion concentration is expressed as the
pH of water, which is neutral when pH = 7·0, acid at less
than 7·0 and alkaline at over 7·0. Chemical weathering
potential increases inversely with pH and proportionally
with increasing equilibrium solubility of minerals and
temperature, until saturation is reached.

Ti
Fe
Al
Si

Anatase
Goethite
Haematite
Gibbsite
Boehmite

Amorphous

OLIVINE
AUGITE
HORNBLENDE

^

Hydrous
Oxides

^

VOLCANIC
GLASS (basic)
ZEOLITE

Amorphous Fe
Hydrous Al
Oxides Si

Gibbsite
Allophane

H + ,Ca 2+
K +
Ca 2+

Ca 2+

Illite (3-octohedral)

BIOTITE -SiO 2

Montmorill-
onite

Vermiculite Clay

+SiO 2

MUSCOVITE

H + ,Ca 2+

Illite (2-octohedral)

K +

VOLCANIC
GLASS (acid)
FELDSPARS

Amorphous Al
Hydrous
Oxides Si

Gibbsite

Figure 13.9
Weathering sequence and
secondary products of primary
rock-forming minerals.
Source: After Selby (1993)

QUARTZ Silicic acid Chalcedonite Silicic acid

Secondary
Quartz