![diopside stx](/~hamilton/Images/diopside.gif)
This is a polyhedral representation of the crystal structure of the
pyroxene diopside (CaMgSi2O6), which is part of a
solid solution series of
pyroxene compositions. The blue polyhedra represent the Si-O tetrahedra
that are linked to create the basic chain structure of all pyroxenes.
The magenta polyhedra represent the M1 cation site (filled by Mg), and
the green atoms (shown with their bonds) are Ca atoms filling the M2
cation site. All of the molecules in the structure vibrate (stretching and
bending along bonds), resulting in absorption features in the middle
infrared portion of the EM spectrum. This view is looking along the
C-axis of the mineral, parallel to the chains of Si-O tetrahedra.
![diopside spectrum](/~hamilton/Images/diopside_spec.gif)
This is the thermal infrared emission spectrum of diopside.
If you are more familiar with reflectivity spectra, simply invert the
spectrum. If you are
familiar with transmission (absorption) spectra, this is not a
comparable spectrum, although it may look similar.
Due to the nature of emission (or reflection) spectroscopy, and the
complex nature of vibrational interactions in crystalline solids, the
absorption features in this type of spectrum do not represent the
actual vibrational frequencies of the mineral. Regardless, they are
still diagnostic of mineral type, even in the minerals of a solid
solution series. This characteristic allows us to distinguish all
minerals from each other, including silicates, oxides,
carbonates, etc. The same is true for rocks - every rock contains a
different percentage of constituent minerals. Mineral spectra add
linearly (in proportion to their abundance) in the spectra of rocks. The
variable percentages of minerals in rocks allow us to discriminate not
only between basalt, sandstone, schist, and granite, for example, but
also between different compositions within each rock type. We can use
linear
deconvolution to determine the composition of Martian meteorites
from their spectra.
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