1. Padilla AJ (2019) Tantalum. U.S. Geological survey mineral commodity summaries
2. Reichl C, Schatz M (2019) World mining data minerals production
3. CREDENDEO (2019) Länderrisiken.
https://www.credendo.com/de/country-risk
. Accessed 31 July 2019
4. MARSH (2019) Political Risk Map.
https://www.marsh.com/at/de/campaigns/political-risk-map-2019.html
. Accessed 31 July 2019
5. World Bank Group (2019) Worldwide Governance Indicators (WGI).
https://info.worldbank.org/governance/wgi/#doc
. Accessed 31 July 2019