Work

Heimaeyite, Na 3 Al(SO 4 ) 3 , a new mineral from the fumaroles on Eldfell volcano, Iceland

Year: 2025

Type: article

Abstract: The new mineral, heimaeyite, was found among the fumarolic encrustations collected on Eldfell volcano, on the island of Heimaey, Iceland. The mineral formed as microscopic rounded aggregates (<50 µm) and rounded grains (<10 µm) mixed with tiny needles of koryakite and a mineral phase with an NaMgAl(SO 4 ) 3 composition. It was formed in scoria consisting of anhydrite, hematite, and cristobalite. Other associated minerals are tamarugite, hexahydrite, löweite, and langbeinite. Heimaeyite is orange with a yellowish streak. It is translucent with a vitreous lustre. The calculated density is 2.783 g cm −3 . The chemical analysis gives a composition of Na 2.93 Al 0.82 Fe 0.25 S 2.99 O 12.05 . The ideal formula is Na 3 Al(SO 4 ) 3 with 10 % to 25 % replacement of Al with Fe. Single-crystal X-ray diffraction analysis shows that it is trigonal and has a space group of R3‾ with unit cell parameters of a =13.4326(9) Å, c =8.9818(7) Å, V =1403.5(2) Å 3 , and Z =6. The X-ray powder diffraction resembles that of Na 3 V(SO 4 ) 3 (PDF 39-0243), with which it is isostructural. They both belong to the K 7 Nb(SO 4 ) 6 structure type. In the crystal structure, the octahedral coordinations of the cation (in heimaeyite this is Al) are connected by sulfate tetrahedra in a “pin-wheel” arrangement into chains extending along [001]. The chains are interconnected by [NaO 6 ] distorted trigonal prism coordinations. Fe substitutes for Al in the larger of the two symmetry-independent octahedral sites. Synthetic pure Na 3 Al(SO 4 ) 3 was prepared through a solid-state reaction at 550 °C, and its structure was refined by the Rietveld method. It shows a smaller difference in size between the two octahedral sites than the natural sample.

Source: European Journal of Mineralogy

DOI: 10.5194/ejm-37-79-2025