Student Guest Post: Haotian Cao on Chemical Analysis of the Soil with Vivianite-Positive Bones
Over the summer, the Arch Street Project team was busy cleaning human remains. The work was spread over three sites: The College of New Jersey, Rutgers-Camden, and the basement of the College of Physicians of Philadelphia (home of the Mütter Museum and Mütter Research Institute). Quite a few students and volunteers worked with us on this monumental task. The following post is from one such volunteer - Haotian Cao, a student from the Springside Chestnut Hill Academy, Philadelphia, PA.
Chemical Analysis of the Soil with Vivianite-Positive Bones
by Haotian Cao, Springside Chestnut Hill Academy
Seven bodies from the Arch Street project that were cleaned during the summer contained blue stains, which we assumed to be the crystal vivianite (ref.1) (Figure 1). Previous studies in the literature have shown that vivianite forms on human bone in environments that contain soil that is rich in iron, phosphorus, and water (ref 2). Examination of vivianite-positive bones from other historical grave sites has shown vivianite appearing on bones from 20+ to 500 years old (ref 2 and 3). In order to determine the identity of the blue stains, we tested soil collected from the vivianite-positive graves to see if the samples contained elevated levels of iron and phosphorus. I (Haotian Cao) conducted chemical tests for levels of iron and phosphorous (using LaMotte Field Study kits) on ten vivianite-negative samples (called "control samples") from the Arch Street site, and on seven vivianite-positive soil samples in a classroom laboratory at Springside Chestnut Hill Academy (Figure 2, and Figure 3). We found that there was a high phosphorus concentration in the soil from both from the control group and the experimental group, and a very scattered pattern of iron concentration which did not differ between the two groups (Figure 4). These results surprised us initially because one might assume that these levels of iron and phosphorous would be elevated in the vivianite-positive soil, perhaps leading to the formation of this crystal. Imaging studies done by the Mutter Research Institute revealed that there were elevated levels of iron within the blue crystals on the bone, supporting the notion that they are in fact vivianite. There are a few possible explanations for our results: 1) the iron that was previously in the soil was absorbed by the bone and used to form the vivianite; therefore, the iron level in the vivianite-positive samples should be lower than the vivianite-negative samples; 2) The soil was disrupted by the repeated burials that happened over the past 300+ years as well as the recent excavation at the Arch Street construction site; therefore, the soil closest to the the bone has changed over time so that it is not possible to locate the soil which originally produced the vivianite. Finally, it is important to remember that one of the conditions related to vivianite formation is soil hydration. It is not possible to measure the amount of water in these soil samples since they were collected over a year ago. Although we can say that most of the vivianite sample came from area 3 and area 4, there is no way to know the depth of each grave and how close to the water table each might have been. According to Dr. George Leader “The soil is heavily disturbed and certainly not in a context that one would be able to easily understand without intensive taphonomic study at a high resolution. (And that can't be done now.) The only "known" stratigraphic layer that was present was a top fill that was laid down very, very late in the history of the cemetery and the vast majority of the remains would have remained under this layer.” As such, direct analytical analysis of the possible vivianite needs to be done to directly confirm its identity.
Vivianite-positive samples -
- Area 3:G-157, 126, 132, 141,
- Area 4:G-102,
- Area 6:G-288, 262,
- Area 8:G-260
Vivianite-negative samples (controls) -
- Area 2:G-91, 138, 316,
- Area 4:G-102, 350,
- Area 5: G-111, 201, 310, 367,
- Area 6: G-279, 187, 232, 172,
- Area 7:G-322,
- Area 8:G-237
1. Springside Chestnut Hill Academy students help identify bones in a major archeological project - Chestnut Hill Local Philadelphia PA. (2018). Chestnut Hill Local Philadelphia PA. Retrieved 17 September 2018, from https://www.chestnuthilllocal.com/2018/07/27/springside-chestnut-hill-academy-students-help-identify-bones-major-archeological-project/
2. McGowan, G., & Prangnell, J. (2005). The significance of vivianite in archaeological settings. Geoarchaeology, 21(1), 93-111. doi:10.1002/gea.20090.
3. Hrala, J. (2018). This Strange Mineral Grows on Dead Bodies And Turns Them Blue. ScienceAlert. Retrieved 10 October 2018, from https://www.sciencealert.com/vivianite-the-blue-mineral-that-eerily-turns-buried-bodies-blue