Agriculture Reference
In-Depth Information
Box 2.4
Model Layer-lattice Structures
Phyllosilicates are composed of two-dimensional silica and alumina sheets
stacked in regular arrays in the
c
direction. Figure B2.4.1 shows the generalized
structure of a phyllosilicate. The following terminology is used:
•A
plane
comprises covalently bonded atoms (such as O and OH) one atom
thick.
•A
sheet
is a combination of planes of atoms (such as a silica sheet).
•A
layer
is a combination of sheets (such as two silica sheets combined with
one alumina sheet, as in mica).
•A
crystal
is made up of one or more layers.
• Planes of atoms are repeated at regular distances in multilayer crystals, which
gives rise to a
d
spacing, or
basal spacing
, characteristic of a particular mineral.
• Between the layers is the
interlayer
space, which may be occupied by water
and solute molecules.
• Phyllosilicates generally have a large planar surface and small edge surfaces.
Figure B2.4.1
A model layer-lattice crystal structure (White 1997). Reproduced with permission of
Blackwell Science Ltd.
c
axis
a
b
Planar (cleavage) face
Lamella or layer
Crystal
Interlayer space
secondary mineral formed by weathering. Secondary silica initially exists as amor-
phous opal that dehydrates over time to form microcrystalline quartz, such as
flint
or
chert
. Black flints are found in Cretaceous Chalk beds underlying vineyard soils
of the middle Loire Valley in France. As the CaCO
3
dissolves during weathering,
the very durable flints remain embedded in the clay impurities of the Chalk to
form a shallow clay-with-flints soil. The insolubility of quartz and flint means that
these minerals are abundant in the sand and silt fractions of many soils.
