Zeolites in basalts of Iceland and elsewhere

Zeolites in basalts from Iceland (and beyond)

Zeolites infilling voids (amygdales) in basalt lavas

Fig. 1: Sample I-12, collected southeast of Egilsstadir in eastern Iceland on 02 August 2022. The rock is a fine-grained basalt with roughly 5 volume percent amygdales. These mineral-filled voids are commonly spherical, 2-5 mm in diameter, though some appear to be flattened, the cavities deformed during the solidification of the cooling host rock. Aqueous fluids, bearing silica and other major elements (Al, Ca, Na, etc), pass through the volcanic pile when it has cooled far below the temperature range of the freshly-erupted magma. The void filling is white, and does not react with dilute acid. The mineral is not calcite, and does not appear to be quartz, but rather another white silicate, such as a zeolite or (less likely) feldspar. The rock is appreciably magnetic (magnetic susceptibility circa 12x10^-3 SI units). A quick look at a thin section reveals a Ti-rich basalt with plagioclase and clinopyroxene (titanaugite) with Fe-Ti oxides (ilmenite and lesser magnetite). Opaque, black iron-rich glass occurs in association with the amygdale fillings.

The locality is a cliff in the vicinity of the Holmanes nature reserve, which occupies a peninsula on the north coast of Reydarfjordur, accessed along highway 92, just south of the small town of Eskifjordur. The cliff, some 20 metres high, displays reddened paleosols (oxidized, weathered soil layers between lava flows) and one or more pahoehoe basalt lava flows, successive layers being massive or distinguished by variable abundance of the prominent white amygdales. The area is part of the Tertiary Basalt Formation, dating back to the period 15-3 Ma. The immediate area is part of the extinct Reydarfjordur volcanic system, dated at 11.5 Ma (Thordarson and Hoskuldsson, 2014, pp.153-156).

"Rock of the Month # 274, posted for April 2024" ---

Lavas, including basalts, often trap a portion of the volatiles that exsolve from the magma as it nears the surface of the Earth. Much of the gas escapes into the atmosphere, but some is trapped, and when the lava solidifies the gas pressure is rendered visible in the form of bubbles in the crystallizing melt. These bubbles are termed vesicles. Often, percolation of fluids in the cooling lava flows causes these voids to be infilled with minerals, common examples being carbonates (calcite) or silicates (such as zeolites). Such crystal-filled vesicles are termed amygdales (amygdules), the resultant "spotty" texture is "amygdaloidal", and in some cases, the rock gets an informal name "amygdaloid", as in the Keweenaw peninsula of Michigan.

Figs. 2-3:
Two more examples from the same Holmanes locality as sample I-12, found / collected by Tom Hart on 02 August 2022. Note the acicular (elongate, prismatic) crystal habit, typical of certain species of zeolite. These examples resemble the species mesolite, based on crystal habit and occurrence alone.

Figs. 4-5:
Left: amygdaloidal, zeolitic basalt I-11, collected in southeast Iceland en route from Hofn north to Egilsstadir, to the north of the inland pass at Oxi, on 01 August 2022. The rock is appreciably magnetic (magnetic susceptibility circa 25x10^-3 SI units).

Right: zeolitic basalt I-03, from the north coast of the Snaefellsnes peninsula in western Iceland, collected 27 July 2022, between Kirkjufell and Olafsvik. The pieces are variably, appreciably magnetic (magnetic susceptibility circa 8-30x10^-3 SI units). A few open voids are present, up to at least 10x5x12 mm. The white void fillings may be just a thin coating on the walls of such voids, and may in turn be coated in a dark thin film of Fe hydroxyoxides. Three other factors are worth noting: a) small (1 mm) vesicles remain as open bubbles - only the larger voids are filled; b) some of the larger voids show evidence of two-stage filling, with a thin white rim and a white to greyish interior filling the bulk of the cavity; and c) a flattened tabular habit can be discerned in the coarsest white material, in contrast to the acicular forms that may grow into open spaces, where the finest collectible specimens occur.

Zeolites are a large family of several dozen hydrated silicate minerals, which can be hard to distinguish in hand specimen. They are covered in every field guide to minerals, including a unique Icelandic one by Saemundsson and Gunnlaugsson (2014, pp.88-133). Zeolites are widespread, with many of the most famous localities and spectacular samples coming from the end-Cretaceous Deccan Traps of west India. The word "zeolite" means "boiling stone" in ancient Greek, and these hydrous minerals give off water at about 200°C, readily readsorbing H₂O at room temperature (Gottardi, 1978). Coombs et al. (1997) tackled the classification and nomenclature, and detailed descriptions can be found in Deer et al. (2004).

Zeolites may display vertical zonation within a volcanic pile, as in New Jersey, Northern Ireland, and eastern Iceland (Deer et al., 1966, pp.393-402). The zeolitization of the primary mineral assemblages may be discordant to the igneous stratigraphy, and be more reflective of the temperature distribution in the cooling pile. Much of the early work on zeolite zonation in volcanic fields was undertaken by volcanologist George P.L. Walker, whose career involved work in countries such as Iceland, India (the Deccan), Scotland, Italy and Mexico. In Iceland, the zonation of zeolites is well understood, with individual species forming from 250 down to as little as 50°C (see the excellent, compact summary by Saemundsson and Gunnlaugsson, 2014, pp.15-23). Individual zeolite species form at different temperatures, e.g., heulandite (70-160°C) and mesolite (80-120°C).

So, what mineral species comprise the Icelandic void fillings? One set (Figs. 2-3) have a suggestive crystal habit (mesolite?). The more massive, fine-grained white infills are harder to identify, or at least to guess at. Not calcite (no acid reaction) and not quartz (somewhat softer than a knife blade). The mineral is quite resistant, so that amygdales often stand proud on the exposed surfaces of the host lava. In the case of I-12 (Fig. 1) a polished thin section was prepared. The basic physical and optical properties of the void mineral include: a blocky, tabular habit, with no obvious tapering; maximum grain size of at least 1.4x1.1 mm; low birefringence (similar or a bit lower than quartz); and a lack of twinning. A first guess, based on available information, is heulandite, as opposed to stilbite or another zeolite species. Amygdales may be zoned, with a turbid zeolite layer on the cavity wall, then a clear layer within, and in some cases an opaque glass in the centre (all three zones have very low reflectivity, so no coarse oxide inclusions are involved).

REFERENCES

Coombs,DS plus 18 (1997) Recommended nomenclature for zeolite minerals: report of the subcommittee on zeolites of the International Mineralogical Association, Commission on New Minerals and Mineral Names. Can.Mineral. 35, 1571-1606.

Deer,WA, Howie,RA and Zussman,J (1966) An Introduction to the Rock-Forming Minerals. Longmans, 528pp. (N.B.: 3rd edition published in 2013).

Deer,WA, Howie,RA and Zussman,J (2004) Rock-Forming Minerals. Volume 4B. Framework Silicates, Silica Minerals, Feldspathoids and Zeolites. Geological Society, London, 2nd edition, 982pp.

Gottardi,G (1978) Mineralogy and crystal chemistry of zeolites. In "Natural Zeolites: Occurrence, Properties, Use" (Sand,LB and Mumpton,FA editors), Pergamon Press, Oxford, 546pp., 31-44.

Saemundsson,K and Gunnlaugsson,E (2014) Icelandic Rocks & Minerals. Mal og Menning, Reykjavik, 2nd edition, 232pp., translated from the Icelandic by Anna Yates, with photography by G. Eiriksson.

Thordarson,T and Hoskuldsson,A (2014) Classsic Geology in Europe, 3. Iceland. Dunedin, Edinburgh, 2nd edition, xv+256pp.

Fig. 6: A zeolitic basalt (sample SCGO) sampled from aggregate used as a railway bed, on the GO line at Danforth station of the GO suburban commuter network, Scarborough, eastern side of the Greater Toronto metropolitan area. A good example of the vital role of industrial minerals (like basalt, used as aggregate), less glamourous than gold and other exotic metals, but arguably more vital for the infrastructure of modern life. This vesicular basalt is probably from the North Mountain basalt in Nova Scotia, presumably brought by train to form a safe, firm bed for more trains! The vesicle fillings effervesce intensely in 10% HCl, indicating calcite. In contrast, none of the amygdales in the Icelandic samples shown here shown here display any reaction to dilute acid. In thin section, twinned tabular crystals of plagioclase feldspar stand out against black glass. Olivine and alteration products are visible, plus oxides (ilmenite, magnetite). The amygdale fillings are all granular calcite (which is soft, readily cut with a knife). Not appreciably magnetic (magnetic susceptibility circa 0.2x10^-3 SI units), in contrast to all the Icelandic samples.

Graham Wilson, 16-18,20 February 2024

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