* [https://www.usgs.gov/centers/national-minerals-information-center/strontium-statistics-and-information Ober, Joyce A.; Polyak, Désirée E. «Mineral Yearbook 2007: Strontium» (PDF). United States Geological Survey] | * [https://www.usgs.gov/centers/national-minerals-information-center/strontium-statistics-and-information Ober, Joyce A.; Polyak, Désirée E. «Mineral Yearbook 2007: Strontium» (PDF). United States Geological Survey] |