1. 3http://en.wikipedia.org/wiki/Abundance_of_the_chemical_elements elements: relative abundance of elements in Earth's crust graphic from public domain source http://geopubs.wr.usgs.gov/factsheet/fs087-02/)
2. 2 Godart C. APNFM, Dresden,Germany, 23–25.01.2008
3. 1 Vining C. , European conference on thermoelectrics, Odessa, Ukraine, 10.-12. Sept. 2007, priv. communication