Retrieved January 2, 2011. "Calcite"Granite Gap "Several variety names exist for calcite. Iceland Spar is an ice-clear variety that demonstrates the effect of double refraction or birefringence ... Young mountain ranges in Mexico and South America also host fine localities for calcite. They include Chihuahua, Chihuahua; the Santa Eulalia Dist., Chihuahua; Mapimí, Durango; Guanajuato, Guanajuato; and Charcas, San Luis Potosí; all Mexico"
This article incorporates text from this source, which is in the public domain. Webster, Noah (1828). "Birefringence". Webster's Dictionary. Springfield, Massachusetts: C. & G. Merriam Co.
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"Helgustaðanáma". Umhverfisstofnun (in Icelandic). Retrieved 2020-08-20.