As shown in spectra of the mineral standards and natural samples
analyzed in this study, the dominant Raman band of both common
crystal structures of calcium carbonate is at ∼1085 Δcm−1 due to the
symmetric stretching (ν1 vibration) of carbonate (CO3). A minor,
lattice mode band is also present at ∼155 Δcm−1. Because calcite and
aragonite have different lattice structures, the positions of some of
their minor Raman bands are different. Calcite has minor bands at
∼282 and 713 Δcm−1; aragonite has minor bands at ∼207 and
704 Δcm−1 (Fig. 3). The bands below 300 Δcm−1 are lattice modes,
while the bands near 700 Δcm−1 are due to the in-plane bending (ν4
vibration) of CO3 (Bischoff et al., 1985; Urmos et al., 1991; Stopar et al.,
2005). A veryweak band at ∼1435 Δcm−1 for calcite and ∼1462 Δcm−1
for aragonite was sometimes observed in spectra of sufficient signal
strength; this is due to the anti-symmetric stretch (ν3 vibration). The
Raman band positions of both calcite and aragonite were very
consistent regardless of the sampling optic or excitation wavelength.
However, the normalized intensities (band height divided by the
height of the ∼1085 Δcm−1 band) showed some variation for calcite.
This variation appeared to be somewhat related to the area being
analyzed (i.e., whether the area was transparent or more opaque).
Large, single, translucent crystals are more susceptible to optical
scattering effects caused by anisotropy within the crystal. In opaque
aggregates of fine grained crystals, the laser spot illuminates a large
number of crystals in various orientations thus averaging out
orientation effects. Spectra of aragonite were collected from both
the inside and outside of the shell sample. Raman band positions and
normalized intensities were very consistent for all of the aragonite
spectra. Table 3 shows the band positions and the variations in
normalized intensity for six spectra of each sample collected with the
following optics: calcite — 10× (1), NCO (3), IO (1), InPhotote (1);
aragonite — 10× (1), NCO (3), InPhotote (2).
The spectrum of geological calcite collected with green excitation
shows an inclined baseline due to some fluorescence of the sample
(Fig. 3). However, the intensity of the fluorescence is not sufficient to
obscure the Raman bands. The broad fluorescence observed from
calcite in this study peaks at ∼2900 Δcm−1 (∼629 nm) (not shown).