Skip to main content
U.S. flag

An official website of the United States government

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

January 1, 1997

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.

Citation Information

Publication Year 1997
Title Fe-Ca-phosphate, Fe-silicate, and Mn-oxide minerals in concretions from the Monterey Formation
DOI 10.1016/S0009-2541(96)00170-2
Authors M.D. Medrano, D. Z. Piper
Publication Type Article
Publication Subtype Journal Article
Series Title Chemical Geology
Series Number
Index ID 70019860
Record Source USGS Publications Warehouse
USGS Organization