Chapter 5 Skeletal V e r s u s Nonbiogenic C a r b o n a t e s UV-Visible-Near IR (0.3-2.7-µm) Reflectance Properties
Spectroscopic Characterization of Minerals and Their Surfaces Downloaded from pubs.acs.org by EAST CAROLINA UNIV on 03/13/19. For personal use only.
Susan J. Gafffey Department of Geology, Rensselaer Polytechnic Institute, Troy, NY 12181 Skeletal carbonates, unlike non-biogenic carbonates, contain organics and comparatively large amounts (up to 3 weight %) of water. Water and organic compounds, which play central roles in carbonate diagenesis, are an intrinsic part of carbonate skeletons incorporated during growth. Organics and water are intimately associated and are disseminated throughout skeletons in minute inter- and intracrystalline voids. VNIR spectroscopy can be used to monitor changes in forms and abundances of water and organics during diagenetic alteration. Skeletal samples prepared for mineralogical and chemical analyses by routine procedures are not representative of these materials as they occur in nature, nor are they 100% aragonite and/or calcite. The chemical and mineralogical composition of carbonate skeletons is more varied and complex than the usual terms "aragonite" and "high" or "low Mg calcite" imply. Water and organics are an intrinsic part of all carbonate skeletons and can play a major role in the diagenesis of carbonate sediments. Liquid water provides the medium for all diagenetic reactions Presence of HoO- and 0H~containing mineral phases can affect the stability and solubility of carbonate skeletons (e.g. 2± 3)· Organic compounds can facilitate the precipitation of carbonates, and can affect the mineralogy, chemical composition, and crystal form of the precipitate (e.g. 4-6). Organic compounds form complexes with metal cations, affecting their mobility (5^ 7). Breakdown of organic compounds can alter pore water chemistry, affecting Eh and pH, as well as concentrations of dissolved species such as NH^ , SO/ , P O ^ , and C0 (7J_ 8) which can, in turn, affect the solubility and stability of carbonate phases (9)· Both water and organics may remain in reduced amounts in ancient carbonate sediments, even after considerable diagenetic alteration (e.g. 10-15)» Despite the importance of water and organic compounds in diagenetic reactions, and although organic compounds and water in various forms (liquid and bound H 0 and 0H~) are known to be +
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0097-6156/90/0415-0094$06.75/0 ο 1990 American Chemical Society
5. GAFFEY
Skeletal Versus Nonbiogenic Carbonates
95
important constituents not only of skeletal but also of non-skeletal grains such as ooids and peloids (e.g. 2± 5± 10-13t 16-24) there are at present no analytical techniques routinely in use by carbonate petrologists which enable identification and determination of abundances of these components. Visible and near infrared (VNIR) (0.3 - 2.7 μπι) spectroscopy provides a rapid, inexpensive, non-destructive technique for characterizing significant aspects of the mineralogical and chemical composition of geologic samples. Like X-ray diffraction i t is non selective, giving information on a l l optically active phases present in a sample. Unlike X-ray diffraction (as routinely employed by geologists for mineral identification) i t can be used to study many liquid and amorphous phases and to identify many of the atoms or molecular ions which occur in solid solution rather than as discrete mineral phases, providing a useful compliment to X-ray diffraction analyses. VNIR spectroscopy can also be used to study the lighter elements such as C, 0, H, and N, which are not amenable to study by techniques more commonly employed by geologists such as microprobe or XRF. This paper deals only with skeletal carbonates, those which are clearly produced by carbonate-secreting organisms such as corals, mollusks, or calcareous algae (biologically-controlled mineralization, as defined by Mann, [25]), and with non-biogenic carbonates. It does not deal with the products of biologicallyinduced precipitation, as defined by Mann (25) and Lowenstam (26), although the limited data available suggests that the spectral properties of such biologically-induced products as stromatolites are very similar to those of skeletal carbonates. Materials and Methods Most modern and fossil material used in this study was collected on Oahu, Hawaii, between 1980 and 1984, and from San Salvador Island, Bahamas, in March, 1986, and January and June, 1988. Mercenaria shells are from the Pleistocene of Virginia. After collection, skeletal samples are usually either dried, frozen, or cleaned in 5 % sodium hypochlorite solution so they can be shipped and/or stored. For this study calcareous green algae such as Halimeda and Penicillus, and gorgonians were dried in an oven at temperatures of about 40°C. Whole skeletons of coralline red algae, scleractinian corals, hydrozoans, octacorals, and echinoids which were collected live were cleaned in 5 % sodium hypochlorite solution before returning them to the laboratory for analysis. Sands were collected both from beaches and the marine environment so that tests of foraminifera, plates of Halimeda, and segments of coralline red algae could be separated from them. Tests of Homotrema rubrum were removed from coral heads or rock slabs they encrusted. Tests of some foraminifera were also removed from Penicillus and Halimeda. Skeletons of dead echinoids, algae, and corals were collected from bottom sediments and from beaches. Mineralogy of samples was determined using X-ray diffraction. Modern and fossil samples were examined in thin section and/or with SEM. Treatment of samples from which VNIR (0.3 - 2.7 μπι) reflectance spectra were obtained varied. Spectra of skeletons of a l l groups of organisms studied were obtained from bleached and/or unbleached
96
SPECTROSCOPIC CHARACTERIZATION OF MINERALS AND THEIR SURFACES
powders. Spectra were also obtained from bleached and unbleached coral slabs, bleached slices of mollusc shells, whole mollusc shells, bleached whole echinoid plates, foraminifera, serpulids, bryozoans, and ostracods, and unbleached fossils. Either H 0 buffered to a neutral pH with NaOH or 5% sodium hypochlorite solution was used to bleach samples. Spectra were obtained using the spectrophotometers at RELAB at Brown university, described by Pieters (27) and at the U.S.G.S i n Denver, described by Clark, R. N.; King, T. W.; Klewja, M.; Swayze, G. Α., ( J . Geophys. Res., i n press). 2
2
Background When light interacts with a compound certain wavelengths are preferentially absorbed. The number, positions, widths, and intensities of these absorptions are diagnostic of the mineralogical and chemical composition of the sample. Several different processes in compounds and crystals can cause absorption of light: (1) electronic transitions in molecular orbitals; (2) electronic transitions between unfilled d-shells in transition metal ions such as F e and Fe^ ; (3) internal vibrational transitions in molecules and molecular ions such as H 0 or NH^ in which bond lengths and bond angles change; and (4) external vibrational transitions in which the molecule or molecular ion moves as a unit. 2+
+
+
2
Advantages of the VNIR Region. The VNIR region of the spectrum employed in the present study has a number of advantages over the more commonly used mid-infrared (MIR) (3 to 20 μτα) and far infrared (FIR) (>20 um) regions. (1) Because the types of particle size effects which hamper work in the MIR and FIR (28-31 ) are virtually absent in the VNIR, spectra can be obtained from samples in any form including powders, sands, and broken, sawed, or polished surfaces (32). Unstable or soluble phases such as organics and hydrated mineral phases can be studied in situ, without subjecting materials to grinding, heating, or extraction techniques which may alter, remove, or destroy the phases of interest. (2) The VNIR region contains absorptions due to a l l four types of transitions listed above, unlike the MIR and FIR which only contain features due to vibrational transitions. (3) A great deal of overlap between absorption bands occurs in the 3 Jim region which contains absorptions due to the fundamental 0-H, C-H, and N-H stretching modes and the f i r s t overtones of the H-O-H and H-N-H bending modes. However, in the VNIR energy differences between different vibrational modes are amplified, and a useful separation exists between absorptions due to overtones and combination tones of these modes (33t 34)» Spectral Properties of Anhydrous Carbonate Minerals. Spectra of a l l anhydrous carbonate minerals show a series of sharp, narrow absorptions at wavelengths >1.6 }im (Figure 1), whose precise positions and widths vary with mineralogy and chemical composition. These absorption bands are due to combinations of translational lattice modes with internal modes, and in general, bands shift to longer wavelengths with increasing mass of the major cation ( 3 5 ) · Solid substitution for the major cation, common in calcite-group minerals, can also produce shifts in band position. For example, in spectra of ferroan dolomites, carbonate bands shift to longer
GAFFEY
Skeletal Versus Nonbiogenic Carbonates
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SPECTROSCOPIC CHARACTERIZATION OF MINERALS AND THEIR SURFACES +
wavelengths at high Fe^ concentrations, again reflecting the influence of cation mass on band position (11 ). Spectral Properties of H^O and OH". h^O has three internal fundamental vibrational modes: a symmetric stretch (^), a bending mode (^2)» * antisymmetric stretch (Vj), as well as an intermolecular mode, V*^ (36). Approximate wavelengths of these fundamental modes are given in Table I, and wavelengths and band assignments for absorptions produced in the NIR are given in Table II. A spectrum of liquid h^O, obtained from the RELAB general collection, is shown in Figure 1. When h^O is incorporated in a crystal, hydrogen bonding causes the absorption bands to broaden, intensify, and shift to longer wavelengths (37-39) OH" produces a single stretching fundamental near 2.78 jim (39)· In addition to the absorptions listed in Table II, spectra of 0H~containing minerals display a number of sharp, narrow absorptions around the 0-H stretching fundamental due to combinations of the 0-H stretching vibration with lattice modes (39t 4-0)· Spectra of h^Ocontaining phases show absorption features near 1.2 and 1.9 Jim which, because they involve the H-O-H bending mode, do not appear in spectra of 0H"-containing phases. Spectra of minerals containing OH", however, w i l l show an absorption feature between 2.0 and 2.3 Jim due to a combination of the 0-H stretch and the M-O-H bending mode, M being the atom to which the OH group is bound (34i 4-1-4-4.)· The spectra of some h^O- and 0H"-containing minerals are shown in Figure 2. The spectrum of hydrocerussite (PbjtCO^tOH^) (4-5) actually a reagent grade "PbCOo" sample, as well as those of brucite (MgtOHlp) and portlandite (CafOHU) illustrate absorption features due to OH". The hydromagnesite (kg^tCO^COHU^h^O) (£5) spectrum provides an example of a mineral containing both HgO and OH". The spectra in Figure 2 illustrate the usefulness of reflectance spectroscopy in studies of water in minerals. While the composition, crystal structure, and spectral properties of CaCOH)? and MgiOH)? are well known, those of many hydrated carbonate minerals are not. The hydrozincite spectrum in Figure 2 is that of a reagent grade "ZnCOy, shown by X-ray diffraction to be the mineral hydrozincite, composition Zn^CO^WOH)^ ( 4 5 ) · However, the spectrum displays a strong 1.9-jim band, indicating the presence of hydrogen-bonded HoO. The shapes and positions of the water bands in the magnesite spectrum in Figure 2 show that bound HpO is present in addition to liquid h^O in fluid inclusions, despite the fact that only magnesite is detected by X-ray diffraction analysis of this sample. As these spectra show, and as other workers have stated (e.g. 4-6, 4.7), the chemical composition of many hydrated carbonate mineral phases are incompletely known, making i t difficult to relate hydrated phases encountered in skeletons to specific mineral phases. a n c
a
n
Spectral Properties of Organics - General. The following discussion is not intended to give a complete description of the spectral properties of a l l organics, but to briefly outline the spectral characteristics of broad classes of organic compounds found in carbonate skeletons as well as in the soft, uncalcified tissues of carbonate-secreting organisms. Large organic molecules can be thought of as consisting of rings and/or long chains of atoms joined by covalent bonds to form the backbone of the molecule, and of side
5. GAFFEY
99
Skeletal Versus Nonbiogenic Carbonates
branches or chains. Functional groups such as amino (NH ), hydroxyl (OH), and methyl (CH^) groups are the reactive portions of the molecule and may occur either within the main chain or as side branches. Despite the large size and structural complexity of organic molecules, small groups of atoms such as NH^ , OH, or CH within those molecules w i l l behave semi-autonomously, always producing absorptions in the same general region of the spectrum with similar intensities. These are called the characteristic group frequencies. Precise positions of these absorption features w i l l vary depending on the chemical environment of the atomic group (4-2, 4.8-50). In many cases these oscillator groups which give the molecule its characteristic spectral properties are the same as the functional groups which determine many of its chemical properties. Vibrations of these large molecules can be grouped into three basic types: (1) vibrations of portions of the main chain; (2) vibrations of side branches or groups; and (3) wagging, twisting, or rocking vibrations in which the side branches move as a unit relative to the main chain. Wavelengths of absorptions due to fundamental vibrational modes of important oscillator groups are given Table I. Only fundamentals whose overtones and combination tones produce absorptions in the NIR region are included. The reader is referred to (^2) for excellent illustrations of these vibrational motions. Following the conventions of Herzberg (51 ) indicates a symmetric stretch, V a symmetric bending mode, an antisymmetric stretch, and Vi an antisymmetric bend. The antisymmetric stretching and 2
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Table I. Fundamental Modes of Oscillator Groups (33j. 42j> 52-54·) Assignment
Wavelength (μπι)
H0
ν
Λ
V I>3 1>L 2
NH
Assignment CH
2
3.05 6.08 2.94 20.00
Wavelength (μπι)
2
ΙΛ, V V wag (w)
3.57 6.90 3.45 7.49
twist (t)
7.82
2
3
2
ν v i>3
2.99 6.25 2.90
*Ί V V 1/4
3.23 6.67 3.03 6.25
Λ
2
NHo
CH ν 3
2
3
Amide I (C=0 stretch) Amide II (C-N-H bend) Amide III (C-N-H bend)
5.92 6.45 8.00
vi
3.45 7.20 3.39 6.84
rock
8.70
Λ
l/ I/o
2
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SPECTROSCOPIC CHARACTERIZATION OF MINERALS AND THEIR SURFACES
bending modes are doubly degenerate in molecular groups of four atoms such as NH^ and CEy The near infrared portion of spectra of organic compounds is dominated by overtones and combination tones of the N-H, C-H, and 0-H stretching fundamentals (4.2). VNIR absorptions of oscillator groups in organic compounds are listed in Table II. Absorption bands due to NH, NH , and NHo tend to be broad and diffuse with multiple peaks, and the symmetric stretching mode is stronger than the asymmetric stretch (4-2, 55). Absorption bands produced by CH, CH , and CH groups tend to be narrow and sharp (4-2, 54.)* In addition to the strong absorptions listed for CH and CH in the 2.25 - 2.4.5 Mm region, CH, CHo and CHo groups may produce a series of weaker absorptions at slightly longer wavelengths due to combinations of the C-H stretching modes with wagging and twisting vibrations (33)· OH groups in organic compounds generally produce intense, broad absorptions. However, where hydrogen bonding is weak, these absorptions are narrow, sharp, and occur at shorter wavelengths than those produced by more strongly bound OH groups (56). As with OH-groups, hydrogen bonding causes absorptions due to a l l these oscillator groups to broaden and shift to longer wavelengths. C-H and N-H absorptions are several orders of magnitude weaker than those produced by an equal concentration of H 0 or 0H~ in the NIR spectral region (36, 57, 58). +
+
2
2
3
2
3
2
Table II. Band assignments and approximate positions of VNIR absorption features produced by water and organics (33. 42, 44, 49. 51. 53. 54. 59) Assignment H0 3V 3Ρ ν +p +p 2l/ 2V^ P + p^ + p P ^3 3 L
Wavelength (μιη)
2
V
Λ
Λ
2
3
1f
2
L
+
1
2
+
0.9 1.2 1.4 1.8 ·9 '5
2
+
NHo 3 ^ , 3i>3 2Ρ
Λ$
2Po
2
1.1
CH3 3v
1.5
ν
NH 3ν 2l/ 2ν P +P^ P +Pj 2
3
Λ
9
2
Peptide linkage "NH + Amide I + Amide II NH + Amide III 2
N H
N H
+ νΛ
Λ
1.2
3
ν
Λ
+ΙΛ
2.3
-2.4
^ CH 3 ^ t 3^Q 2ν 2P P +P 2
1.02 1.45 1.49 1.95 - 2.05
Λ
2
3i>
19
2.0 - 2.2
+ iy
V
Assignment Wavelength (μιη) OH 31Λ| 0.9 ZV^ 1.4 IA| + M-O-H bend 1.9 - 2.3
1
·45 2.01 2.06 2.17
Λ%
3
3
2
1.2 1.7 2.3 - 2.4
5. GAFFEY
Skeletal Versus Nonbiogemc Carbonates
0.5
1.0 1.5 2.0 2.5 W A V E L E N G T H (pm)
Figure 2. Spectra of non-biogenic mineral samples containing OH* (brucite, portlandite, hydrocerussite), liquid H 0 (aragonite speleothem) and bound H 0 (magnesite, hydrozincite). Hydromagnesite contains both bound H 0 and OH". 2
2
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SPECTROSCOPIC CHARACTERIZATION OF MINERALS AND THEIR SURFACES
Spectra of some organic materials are shown in Figure 3· The Halimeda spectrum was obtained from young, lightly calcified plates. Although the plates were bleached in sodium hypochlorite solution, large amounts of organics remain in the sample. The sapropel sample is from a saline lake in the Bahamas. The coral polyp spectrum was obtained from coral tissue exposed by slabbing a Diploria strigosa head. The coral organics, also from Diploria strigosa, were removed from the skeleton by dissolving i t in 1 M HC1. The dead leaf spectrum comes from the RELAB general collection at Brown university. The gorgonian spectrum i s that of a lavender sea pen. Proteins. Spectra of proteins are dominated by absorptions due to vibrations of the peptide or protein chain, although absorptions due to side groups may also occur in spectra (Λ9. 52). Amino acids generally exist in the zwitterion form in which an acidic carboxyl group (-C00H) donates a proton to the amino group (NH ) forming an ammonium ion (NHQ ) (Λ2, 60). Thus protein spectra can contain 2
+
features due to "NH^"" as well as to NH , NH, and CH (Λ2, 49. 59. 61). 1
2
2
Absorption features due to proteins occur in the coral polyp, coral organics, and gorgonian spectra in Figure 3· Polysaccharides. NIR spectra of polysaccharides contain features due to OH, CH^, and CH^. OH groups produce broad bands near 1.4 and 1.0 um, the f i r s t and second overtones of the 0-H stretching fundamental. Sharp narrow 0-H features may occur i f OH groups are weakly hydrogen bonded, as, for example, i n the sapropel spectrum i n Figure 3* In addition, two or more broad bands occur between 1.9 and 2.3 Jim due to combinations of 0-H stretch and C-O-H bending modes (42, 44. 56, 62). The Halimeda, sapropel, and leaf spectra i n Figure 3 contain features due to polysaccharides. Lipids. NIR spectra of lipids are dominated by CH, CH , and CH^ features (33. 53). However, as these features also occur in spectra of proteins and polysaccharides, no features in the spectra in Figure 3 can be unequivocally assigned to lipids at this time. 2
Organic Pigments. Organic pigments such as chlorophyll and carotene produce very intense absorption features in the visible region due to electronic transitions in molecular orbitals (e.g. 63). These pigments and/or the products resulting from their breakdown can persist i n sediments for geologically long periods of time. They serve as biological markers or chemical fossils in organic geochemical studies, and absorptions due to these pigments are found in oils and kerogen extracted from ancient sediments (64). These biomarkers may be studied i n situ using VNIR reflectance spectroscopy (65). Spectra containing absorptions due to organic pigments are shown in Figure 3· Organics in Carbonate Skeletons Organic compounds occur not only in the soft tissues of carbonatesecreting organisms, but also within their skeletons. A l l organic compounds can potentially play important roles in carbonate diagenesis. The large number of overlapping features due to carbonate, water, and organics in the VNIR makes identification of
5. GAFFEY
Skeletal Versus Nonbiogenic Carbonates
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W A V E L E N G T H (pm)
Figure 3· Spectra of organic samples illustrating absorption features due to C-H (1), 0-H (2), N-H (3), and organic pigments (4)· (Halimeda spectrum offset vertically by 0.3, sapropel spectrum by 0.04, coral polyp by 0.04, gorgonian by -0.04, dead leaf by 0.04, and coral organics by - 0 . 0 6 5 · )
104
SPECTROSCOPIC CHARACTERIZATION OF MINERALS AND THEIR SURFACES
specific phases difficult. However, each compound or functional group produces a distinct set of absorption features. Although a single band cannot be assigned to a given compound with certainty, suites of features considered together reduce ambiguities in identification of compounds (66, 67). H 0, OH", and CO^ " produce the most intense absorptions %n the VNIR, H 0 and OH" because they are strong absorbers (36), CO^ " because i t is the most abundant phase in these samples. Because C-H and N-H absorptions are comparatively weak in this spectral region, features due to NH- and CH-containing functional groups are most evident in the more transparent regions of the spectrum where features due to OH", H 0 and COo are weak or absent. There are four principle sources for the organic compounds detected in VNIR spectra of carbonate skeletons: (1) organic matrices; (2) organic pigments produced by the carbonate-secreting organism; (3) endoliths; and (4) soft tissues and body fluids of the organism (68, 69)» Many organisms use an organic matrix to facilitate precipitation of their skeletons (e.g. 24t 26, 70). In others, such as the mollusks, organic compounds also play a structural role by adding strength and toughness to the skeleton (17, 24)» Features attributable to organic matrices are most evident in spectra of whole, unpowdered skeletal samples. For example, the Homotrema rubrum spectrum (Figure 4a) contains features near 1.2 and 1.7 μιη, and a weak inflection on the 2.3-μπι CO^ band, due to C-H. While not, strictly speaking, a part of the organic matrix, portions of the organism's soft tissues and/or body fluids may be trapped during growth within and/or between carbonate crystals making up the skeleton (68j, 71-73)» Some organic compounds in skeletons are introduced by endoliths. Endolithic algae, bacteria, and/or fungi can infest skeletons during the l i f e or following the death of an organism. The spectrum of the coral Dichocoenia shows very strong absorptions near 0.4 and 0.67 μιη due to chlorophyll in the endolithic alga Ostreobium (74t 75)» Endoliths can add, alter, or remove organics in carbonate skeletons (68). However, the exact nature of the changes in organic content caused by endoliths isn't well documented. Although this study is concerned primarily with carbonate skeletons, rapid breakdown of soft tissues after death of the organism can have a major effect on porewater chemistry and thus, on diagenetic processes (7^ 8,_ 76). The spectrum of the slab of the coral Dichocoenia stokesii shows features near 1.2 and 1.7 μπι due to C-H, and weak N-H features near 1.25, 1»55, 2.1 and 2.2 jim, due i n large part to incomplete breakdown of coral tissues in the unoccupied portions of the skeleton (Zabielski and Gaffey, Proc 4th Symp. Geol. Bahamas, in press). Even after grinding and bleaching, samples s t i l l contain organic compounds. Organic pigments provide the most striking examples. The spectrum of the powdered, bleached Homotrema rubrum sample (Figure 4b) s t i l l shows an intense absorption feature near 0.5 μια due to the pigments which give this foraminifera its distinctive red color, although the C-H features seen in the spectrum of the whole tests (Figure 4a) are less evident. Portions of organic matrices also remain in treated samples, and some absorption features originally attributed to inorganic hydrated 2
2
2
2
5. GAFFEY
105
Skeletal Versus Nonbiogenic Carbonates
a
b 0.5
1.0 1.5 WAVELENGTH
2.0 (pm)
2 5
Figure 4. Spectra of bleached whole (a) and powdered (b) samples of skeletal carbonates.
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SPECTROSCOPIC CHARACTERIZATION OF MINERALS AND THEIR SURFACES
mineral phases (13), appear to be better explained as absorptions due to organic compounds. The Nautilus spectrum provides an example. Nautilus shells contain >5 % organic matrix by weight, composed of proteins rich in aspartic acid, and polysaccharides (24. 77). The strong absorption feature near 2.05 um can be attributed to 0-H absorptions of polysaccharides, and the sharp narrow features between 2.1 and 2.3 μιη to C-H absorptions of polysaccharides and/or proteins. The C-H absorptions near 1.7 and 1.2 μιη and the broadening of the 1.4-um 0-H band support this interpretation. Additional work is needed to establish detection limits for various organic compounds in carbonates. Because H 0 is such a strong absorber in this spectral region, organic compounds present in low concentrations may not be detected in reflectance spectra. Lytechinus, like other echinoids, contains very l i t t l e organic material, the organic matrix comprising < 0.1 weight % of the test (78), and its spectrum (Figure 4a) lacks any absorption features attributable to organics. Although small amounts of organic material remain in coral skeletons after grinding and bleaching (as low as 0.1 % of the skeleton by weight) (68, 71, 72, 79)> absorptions due to organics are masked, and a l l the absorption features in the Montipora spectrum (Figure 4b) can be attributed to liquid H 0 and CaCO^. 2
2
Water in Skeletal Carbonates Liquid H 0 in Fluid Inclusions. Spectral data show that liquid water in fluid inclusions is nearly ubiquitous in carbonate mineral samples and is particularly abundant in carbonate skeletons (10-13t 18). Electron microscopy studies of a variety of skeletal materials reveal minute inter- and intracrystalline voids, apparently containing water and ranging in size from 0.5 μπι to
