Fe-Ca-phosphate, Fe-silicate, and Mn-oxide minerals in concretions from the Monterey Formation

Chemical Geology
By:  and 



Concentrically zoned phosphatic-enriched concretions were collected at three sites from the Monterey Formation. The following minerals were identified: vivianite, lipscombite, rockbridgeite, leucophosphite, mitridatite, carbonate fluorapatite, nontronite, todorokite, and barite. The mineralogy of the concretions was slightly different at each of the three collection sites. None of the concretions contains all of the minerals, but the spatial distribution of minerals in individual concretions, overlapping mineralogies between different concretions, and the geochemical properties of the separate minerals suggest a paragenesis represented by the above order. Eh increased from the precipitation of vivianite to that of rockbridgeite/lipscombite. The precipitation of leucophosphite, then mitridatite, carbonate fluorapatite and todorokite/Fe-oxide indicates increasing pH. Concretion growth culminated with the precipitation of todorokite, a Mn oxide, and minor amounts of barite along microfractures. Conspicuously absent are Fe-sulfide and Mn-phosphate minerals. The concretions are hosted by finely laminated diatomite. The laminations exhibit little to no deformation around the concretions, requiring that the concretions formed after compaction. We interpret this sediment feature and the paragenesis as recording the evolving pore-water chemistry as the formation was uplifted into the fresh-ground-water zone.
Publication type Article
Publication Subtype Journal Article
Title Fe-Ca-phosphate, Fe-silicate, and Mn-oxide minerals in concretions from the Monterey Formation
Series title Chemical Geology
DOI 10.1016/S0009-2541(96)00170-2
Volume 138
Issue 1-2
Year Published 1997
Language English
Publisher Elsevier
Larger Work Type Article
Larger Work Subtype Journal Article
Larger Work Title Chemical Geology
First page 9
Last page 23
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