Dear Daniel,

I am working on several medieval sites in Zurich (Switzerland). At some places, organic reach archaeological layers with embedded bones and coprolites cover a calcareous, water logged loam, which was deposited by Lake Zurich. In this homogenous loam, several well developed vivianite microcrystals can be observed – mainly on pores and on surfaces of calcareous gravels. Those microcrystals are built due to the reducing environment (water logged sediment) on the one hand and because of the overlying archaeological sediments, which act as phosphate source.

I attached some microphotographs of vivianite microcrystals in PPL and XPL. The sediments I am working with are quite different than yours, but I hope this is useful for you anyway.

Best,

David

Dear Daniel,

I have no pictures of vivianite microcrystals, but you can find some at pages 535 and 536 in: G. Stoops, V. Marcelino & F. Mees (eds.) 2010, Interpretation of Micromorphological Features of Soil and Regoliths, Elsevier, Amsterdam (chapter 23, Phosphatic Features, by P. Karkanas & P. Goldberg).

Best,

Diego

Hello Diego , thank you for the answer. I had read the chapter before . My sample is a "Terra Preta de Indio" from the Amazon, a sandy soil, which own a mineral like the book, picture but in a different matrix... assocition frequantly close to char, iron nodules and in voids, I have doubts on which mineral is , although they look a bit like the book picture. The mineral is blue with polarized light shows green , yellow , red colors. I have not done mineral analysis with SEM- EDS, but i will; i did analyzed by micro-Raman, and i  found a  PO4 peak and Iron peak, but the spectrum is different from the standard geological sample, i believe due to decomposition. 

I can send you some pictures so you can give me your opinion ? if not too much to ask

Dear Mr. Sousa,

you will be faced with big problems to find well crystallized vivianite XX in soil, excluding paleosols which underwent some kind of "soil diagenesis" . In this profile you may find three types of Fe concentration, from bottom to top: "Blue Earth" (enriched in vivianite or different types of phosphate bearing bivalent Fe), "White Earth" (enriched in sideritic minerals) and "Brown Earth" (enriched in Fe oxide hydrate, mainly goethite) I could find these accumulations and identify them by eye based on their color, you may get XRD graphs of poorly-crystallized minerals but you may hardly find in young soil types well-shaped crystals. I studied vivianite from well-crystallized mineral aggregated which have been derived from hydrothermal deposits to those from supergene processes in soils affected by oscillating ground water levels.

Harald G. Dill

Dear Daniel,

I am working on several medieval sites in Zurich (Switzerland). At some places, organic reach archaeological layers with embedded bones and coprolites cover a calcareous, water logged loam, which was deposited by Lake Zurich. In this homogenous loam, several well developed vivianite microcrystals can be observed – mainly on pores and on surfaces of calcareous gravels. Those microcrystals are built due to the reducing environment (water logged sediment) on the one hand and because of the overlying archaeological sediments, which act as phosphate source.

I attached some microphotographs of vivianite microcrystals in PPL and XPL. The sediments I am working with are quite different than yours, but I hope this is useful for you anyway.

Best,

David

Dear colleagues,

below attached to this answer you will find a set of vivianite X aggregates and its crystallographic labeling under the petrographic microscope (ppl+crossed polars), SEM-EDX and as micromount. What does it mean to be well-crystallized or well-shaped ?

Best regards

H.G.Dill

Dear Daniel,

yes, the best you can do is uploading some pictures of yours in RG, to make them available to other 'micromorphology people' (see, for instance, the nice examples David has uploaded). Good luck with the "Terras Pretas de Indio", they are amazing anthrosols!

Diego

Dear colleagues,

Thank you for your help... the pictures are really amazing... I have some mineral look like the pictures.. I have attached some pictures of the features that I think is vivianita . I would ask the opnion of you who are on the "Geoarcheology road" longer than i. I 'm just ... PhD student.

The Pedoclimate regime that the soil is subjected is a Udic regime (in the soil taxonomy) , and the climate is tropical monsoon ... the soil is rich in P , guano , and this layer is antrhopic, and is located in the plains of the Amazon... I always have these microcrystals inside pores , and associated with iron nodules , micro coals and ashes near...

With the experience of you , these microcrystals have the optical characteristics of Vivianites ? thank you for your attention