Chemistry Reference
In-Depth Information
trioctahedral silicates. The spectra of biotites show consistent peak positions for
Fe
2+
in M2 sites and for Fe
2+
in M1 sites with broader variation in hyperfine
parameters for the latter [
247
,
249
]. Additional Fe
3+
components that appear in the
spectra are resolved as two distinct doublets with hyperfine parameters repre-
senting the M2 and M1 sites [
249
,
250
].
Chlorites exhibit the main features of biotites. Well-known representatives of
the chlorite group are clinochlore, (Mg,Fe
2+
)
5
Al(Si
3
Al)O
10
(OH)
8
and its iron-rich
variant chamosite. The major Fe
2+
doublet in the spectra of chlinichlore and
chamosite has the typical values d
Fe
= 1.13 mm/s and D = 2.70 mm/s whereas a
minor Fe
3+
doublet gives d
Fe
= 0.39 mm/s and D = 0.67 mm/s [
243
]. A third
doublet is observed at lower temperatures, but has nearly the same hyperfine
parameters as the M2 ferrous iron at RT. This doublet might be attributed to
substitutions in the brucite layer [
251
]. The hyperfine parameters of trioctahedral
micas obtained in about fifty studies of these silicates are summarized in a review
by Dyar [
250
].
Finally, Table
3.18
presents a survey of d
Fe
and D values that have been
reported so far in the literature for a number of selected phyllosilicate species. It is
obvious from these data that the interpretation of the Mössbauer spectra of micas is
not straightforward, the more so nature has provided us with such silicates with an
enormous variety in chemical composition.
3.6 Phosphates
3.6.1 Introduction
A large variety of phosphate minerals are found on Earth. Among these, apatite,
Ca
5
(PO
4
)
3
(F,OH,Cl), is the most abundant one and consequently serves as the
major source from which other naturally occurring phosphates originate by diverse
transformational processes. The structure of apatite can accommodate numerous
metal cations that substitute for Ca. By far most of the phosphates exhibit a
structure that is based on a polymerization of (PO
4
) tetrahedra and (M/
6
) octa-
hedra, with / = O
2-
or OH
-
. As such one distinguishes structures with finite
clusters of (PO
4
) tetrahedra and (M/
6
) octahedra (e.g. anapaite), structures with
infinite chains of tetrahedra and octahedra (eosphorite), structures with infinite
sheets of tetrahedra and octahedra (strunzite, vivianite), and finally structures with
infinite frameworks of tetrahedra and octahedra (triphylite, heterosite, leu-
cophosphite). Iron, both as a divalent or trivalent cation, is a common substituent
and even complete solid solutions may be encountered. As a particular iron
phosphate generally occurs in low concentrations and as fine-grained material in
association with other mineral species, including other phosphates that may or may
