We present new information on the chemical structure of some natural and synthetic Mo compounds. Molybdenum K-edge EXAFS spectra of 12 high-Mo black shales and euxinic sediments yield two basic spectral types. One, associated with Mo ores from China, is that of an MoS2 phase, possibly x-ray amorphous jordisite. This structure is identical to molybdenite in its first and second shells around molybdenum. The other structure, associated with Cretaceous deep sea sediments and with black shales ranging in age from Cambrian to Jurassic, is characterized by short Mo-O distances (1.66-1.71 Å), by Mo-S distances of 2.14-2.35 Å and in some cases by second shell Mo and Fe interactions which suggest the presence of transition metal-rich phases. Based on comparisons to EXAFS spectra of synthetic amorphous materials prepared by scavenging Mo from HS- solutions with Fe(II), FeOOH (a-goethite), and humic acid, we suggest that the second type of spectrum arises from Mo chiefly in two forms. One is a compact Mo-Fe-S "cubane"-type cluster with Mo-Sm3 distance of ~2.36 Å, and the other is an organic form characterized by Mo-Oterm double bonds (~1.68 Å) and Mo-Sbridg bonds which tie Mo to the organic structures. Humic acid
+ HS- scavenging products yield spectra quite similar to the second spectral type observed in the natural samples, including indications of Mo-Fe interactions. Mo L-edge spectra indicate that the mean oxidation state in the sediments and shales is between IV and VI. One of the implications of this work is that although molybdenite is the thermodynamically stable mineral phase for Mo-S in many sedimentary and geologic settings, it does not form at low temperature even under geologic times that exceed 108 y. The form of Mo in black shales is more similar to that found in modern euxinic sediments and is likely a mixture of the two forms that we have reproduced synthetically: an organically bound MoO(±S) form and a cubane-type cluster form of Mo-S-Fe. This work demonstrates the usefulness of EXAFS for obtaining structural information, where components are present in x-ray amorphous forms which defeat conventional mineralogical methods.