Skeletal carbonates

Representation of Chemical Reactions 1185 Table 3-2. The 16 possible unit exchanges at any skeletal carbon atom. 

PHOSPHAZENES CONTAINING SKELETAL CARBON, SULFUR, AND METAL ATOMS... 

Table 2 illustrates this point where, by using mercury porosimetry, carbon densities at 0.1 MPa and 404 MPa have been used to calculate the Ifactional volumes of macro/meso, micropore and skeletal carbon for some carbons based on the following ... 

Why does C4H8 adopt puckered D2d geometry As shown in Fig. 3.82, the skeletal carbon atoms twist out of planarity (with dihedral cccc = 17.9°), allowing each methylenic hydrogen to be distinguished as axial (with cccH(a) = 94.9°) or equatorial (with cccH(e) = 138.7°). The puckered equilibrium structure lies only 0.8 kcal mol-1 below the transition-state D4h structure. 

Walter, L. M., and J. W. Morse (1984b), "Reactive Surface Area of Skeletal Carbonates During Dissolution Effect of Grain Size", J. Sediment. Petrol 54,1081-1090. 

Williams [1] has given an excellent review on Early Carboranes and Their Structural Legacy and he defines carboranes as follows Carboranes are mixed hydrides of carbon and boron in which atoms of both elements feature in the electron-deficient polyhedral molecular skeleton . According to the electron counting rules [2] for closo- (2n + 2 SE), nido- (2n + 4 SE) and arachno-clusters (2n + 6 SE SE = skeletal electrons, n = number of framework atoms) and the An + 2 n electron Hiickel rule, small compounds with skeletal carbon and boron atoms may have an electron count for carboranes and for aromatics (see Chapters 1.1.2 and 1.1.3). 

Shell-secreting organisms to be used for paleotemperature studies must have been precipitated in isotope equilibrium with ocean water. As was shown by studies of Weber and Raup (1966a, b), some organisms precipitate their skeletal carbonate in equilibrium with the water in which they live, but others do not. Wefer and... 

Farkas J, Buhl D, Blenkinsop J, Veizer J (2007) Evolution of the oceanic calcium cycle during the late Mesozoic evidence from 5 / Ca of marine skeletal carbonates. Earth Planet Sci Lett 253 96-111... 

This reaction was used to introduce the final two skeletal carbons in a total synthesis of maytansine (4).2 The reaction of the 2,/ -unsaturated aldehyde (2) with I (R = C6H5) gives the desired 4,5-unsaturated 3-hydroxy ester 3 in 80% yield. The ratio of the desired (S)-alcohol to the epimer is 93 7. The resulting amino acid was cyclized to the lactam in 80% yield with mesitylenesulfonyl chloride (8, 318-319). Epoxidation by the Sharpless procedure (9, 78 79) was also highly stereoselective, giving the desired epoxide and the undesired epimer in the ratio > 200 1. 

Phosphazene Containing Skeletal Carbon, SulfUr. and Metal Atoms... 

The triterpenoid skeletons may be subjected to a variety of structural modifications as already illustrated. However, the particular modifications considered in this section are those that lead to loss of several skeletal carbon atoms. Preeminent amongst such degraded triterpenoids are... 

Slaframine.—Slaframine (37) is produced by the phytopathogen Rhizoctonia leguminicola. It has been known for some time that (37) derives in part from lysine via pipecolic acid (33), which is incorporated intact the earliest bicyclic intermediate identified is (38) (c/. Vol. 5, p. 9 and ref. 2). New results have shown that the two skeletal carbons in (37), and also in the metabolite (36), not accounted for by pipecolic acid, derive from malonate (and acetate).13 The labelling of (37) by, in particular, [2-2H2]acetate was deduced to be of C-2 on the basis of mass spectral evidence (which is not entirely convincing). The acyl-CoA derivative (34) has been suggested as an intermediate in the biosynthesis of (37) and also of (36). It is to be noted that condensation between malonyl-CoA and pipecolic acid (33) to give (34) must be simultaneous with decarboxylation of malonyl-CoA, since two deuterium atoms of acetate are retained at C-2 in (37) (later intermediates with a double-bond to C-2 are also excluded by these results). 

Solubilities of annealed skeletal carbonates were estimated by Land (1967) by allowing the system to reach a steady state pH and by determining dissolved Ca2+ and Mg2+ concentrations. Dissolution of the magnesian calcites in Land s experiments appeared to be congruent in the sense that the steady state solution had a molar Ca2+/Mg2+ ratio similar to that of the solid. 

In a further section, we emphasize the mineralogy and chemistry of carbonate skeletal grains, because of their abundance in Phanerozoic sediments, and their potential usefulness in paleoenvironmental interpretation. To provide the reader at this point with some idea of the complexity of skeletal carbonate grains, Tables 5.2 and 5.3 summarize the chemistry, mineralogy and structure of major... 

Because of the general complexity of factors controlling mineralogy, stable isotope composition and the chemistry of skeletal carbonates, it has been found that the best approach is to use these parameters in combination, rather than relying on any one of them (e.g., Lowenstam, 1961 Schifano and Censi, 1986). This approach is especially necessary in cases where early diagenesis may have taken place. Because, as will be discussed later in this chapter, this process can take place nearly concurrently with skeletal deposition, considerable care must be taken in... 

A few experiments have been successfully performed at low temperatures to simulate carbonate diagenetic processes for example, cements have been precipitated on skeletal carbonate sands in experimental reaction chambers designed to mimic vadose and phreatic meteoric cementation (Thorstenson et al., 1972 Badiozamani et al., 1977). These cements are remarkably similar in composition and morphology to those found in rocks cemented in the meteoric... 

Gaffey Susan J. (1988) Water in skeletal carbonates. J. Sediment. Petrol. 58, 397-414. 

Land L.S. (1966) Diagenesis of metastable skeletal carbonates. Ph.D. Dissertation, Lehigh Univ., Bethlehem, PA. 

Sandberg P.A. (1975) New interpretation of great salt lake ooids and of ancient non-skeletal carbonate mineralogy. Sedimentology 22, 497-538. 

Sandberg P.A. (1983) An oscillating trend in Phanerozoic non-skeletal carbonate mineralogy. Nature 305, 19-22. 

Thorstenson D.C., Mackenzie F.T. and Ristvet B.L. (1972) Experimental vadose and phreatic cementation of skeletal carbonate sand. J. Sediment. Petrol. 42, 162-167. 

Walter L.M. (1985) Relative reactivity of skeletal carbonates during dissolution implications for diagenesis. In Carbonate Cements (eds. N. Schneidermann and P.M. Harris), pp. 3-16. Soc. Econ. Paleontologist and Mineralogists, Spec. Pub. 2. Tulsa, OK. 

Walter L.M. and Morse J.W. (1984b) Reactive surface area of skeletal carbonates during dissolution Effect of grain size. J. Sediment. Petrol. 54, 1081-1090. 

Weber J.N. (1973) Incorporation of strontium into reef coral skeletal carbonates. Geochim. Cosmochim. Acta 37, 2173-2190. 

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