The only calc-alkaline volcano on Iceland.
Ref.: Palache, Charles, Harry Berman & Clifford Frondel (1951), The System of Mineralogy of James Dwight Dana and Edward Salisbury Dana Yale University 1837-1892, Volume II, John Wiley and Sons, Inc., New York, 7th edition, revised and enlarged, 1124 pp.: 17.
http://www.tmm.utexas.edu/npl/mineralogy/Mineral_Genesis/Hekla.htm