An example of heavy minerals study to support chemical analysis of soil, arsenic in soil (Pirkkala, Southern Finland)

Geochemical survey was conducted in the area to assess the risk of arsenic present in the soil. The samples were systematically analyzed by partial and total dissolution to evaluate the potential for bio intake of arsenic. Sampling site for the heavy mineral work was selected on the basis of chemical results that showed typically low arsenic contents in the upper part of till bed and high contents up to 80ppm at the base of the till bed.

Sampling was conducted as a three sample profile representing the entire till bed. Each samples was weighing 45kg (-2mm) and heavy mineral concentrates were recovered through successive preconcentration and heavy media separation systems. Each concentrate was examined by scanning electron microscope supported by mineral mapping program. 

The main result was that arsenic bearing minerals were absent at the top of the sample horizon and their content start increasing towards the bottom. At the bottom, the arsenic content of the concentrate was several thousands of ppms. Primary arsenic mineral grains consists of arsenopyrite and lollingite that commonly showed increasing alteration into secondary phases i.e., arsenogoethite and scorodite. However, alteration rims around primary arsenic mineral grains were seldom complete indicating a high risk of arsenic dissolution in the event of disturbing the till bed by human activity like construction.

Sample position

surface
middle
base
Mineral
Mineral weight (%)
Mineral weight (%)
Mineral weight (%)
Garnets
22,96
4,55
5,04
Zirkon
4,32
4,18
3,98
Ilmenite
57,34
80,37
72.85
Pyrrhotite
0,04
0,18
5,64
Scorodite
0,00
0,05
0,01
As_goethite
0,01
0,03
0,16
Arsenopyrite
0,03
0,58
0,57
Lollingite
0,00
0,05
0,03
Other heavies
15,30
10,01
11,72
Total
100,00
100,00
100,00

 Modal mineral composition of heavy mineral concentrate
 

Data acquired by MLA (Mineral Liberation Analyzer) techniques  (Red – arsenopyrite, lilac – lollingite, orange – Scorodite, light blue – As-goethite).

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