Irish Zeolite Localities
The Antrim Basalt Formation in County Antrim, Northern Ireland has long been a source of varied and interesting zeolite minerals. A combination of rugged coastline cliffs and multiple ‘road stone’ quarries has meant that good quality specimens can be obtained from multiple locations, serving to show the diversity of zeolite mineral occurrence across the formation.
In the following articles it is my intention to introduce those interested in Irish minerals in general, as well as zeolite minerals in particular, to some of the more spectacular and interesting localities.
Beginning with one of the most dramatic localities!
1. Fallowvee, Waterfoot, County Antrim
The Fallowvee locality is situated on the dramatic north coast of County Antrim approximately 2 miles from the village of Waterfoot. At the locality the Antrim Basalt Plateau forms cliffs and scarps, which fall away to the waters of the Irish Sea. Beautiful views along the coastline as well across the Irish Sea to Scotland can be obtained from the locality (depending on the very unpredictable Irish weather!!).
The cliffs are topped by the Upper and Lower Basalt Formations, which together form the Tertiary Antrim Basalt Formation. Below this are the contrasting, bright white beds of the Cretaceous Ulster White Limestone (chalk) Formation (UWLF).
Although not visible at Fallowvee, the UWLF is itself underlain by very soft Jurassic age sediments. The instability of these sediments have contributed to the dramatic rotational landslips which have affected overlying basalt and limestone strata at different localities, most noticeably at the nearby Garron Head.
The name Fallowvee or ‘Falavee’ itself may well have originated from the dramatic cleft in the cliffs at the locality. It is possible to use this as a route to the Garron Plateau at the top of the cliffs, but this should only be attempted by the very fittest and adventurous of visitors!! This geographical feature represents a post Tertiary age fault which has down-thrown the Upper Basalt Formation against the Lower Basalt Formation on the left of the cleft when viewed from the road.
In addition, approximately 10 years ago, a substantial cliff fall occurred at the locality and it is this material which provides a rich source of nice zeolite specimens.
**** Please note if visiting the locality, that the cliffs remain unstable and care and common sense should always be exercised ****
As previously stated the rock fall has dislodged a large amount of basaltic material which shows a nice suite of zeolite minerals including thomsonite, mesolite, chabazite, garronite, cowlesite, levyne and analcime. Calcite crystals are also found at the locality in a number of different habits and I have also found one example of small white apophyllite crystals on a bed of grey bladed thomsonite crystals.
Chabazite is perhaps the most abundant mineral present, forming colourless crystals lining amygdales. It is frequently associated with white clusters of thomsonite crystals with an intergrowth of colourless mesolite needles. These amygdales can be up to 10cm across and where the thomsonite/ mesolite crystals are undamaged, can be very attractive.
Thomsonite also occurs as amygdale linings of light grey, tightly packed clusters of bladed crystals. These amygdales are usually spanned by delicate hair-like crystals of mesolite and often also contain straw brown coloured crystals of calcite.
Levyne occurs as white platy and colourless tabular crystals. Occasionally the white platy crystals also show the classic epitaxial overgrowth of white fibrous erionite/ offretite micro crystals.
Cowlesite is one of the rare zeolite minerals found at the locality. It typically forms light grey spherules which line amygdales and often displays a partial overgrowth of colourless calcite crystals. It generally occurs as the only zeolite mineral within the amygdale apart from the occasional colourless rhombdohedral chabazite crystal.
Phillipsite is another uncommon zeolite mineral at the locality. It tends to form as aggregates of white crystals forming an amygdale lining.
Perhaps the rarest zeolite mineral found at Fallowvee is garronite, which takes its name from the Garron Plateau is directly above the Fallowvee cliff line. The garronite completely fills the amygdales and is white with the characteristic radial and concentric fracture lines.
Although widespread at other zeolite localities in the Antrim Basalt Formation, amygdales containing analcime crystals are generally hard to find at Fallowvee. The crystals tend to be of the normal trapezohedral habit and colourless, occasionally taking on an almost black appearance due to dark underlying saponite material.
Fallowvee is a super zeolite locality with copious amounts of material to work with. Its ease of access and dramatic coastal location help to make it a great location to both hunt for minerals and to simply soak in the scenery and enjoy a pleasant picnic…once again, weather permitting!!
More ‘Collecting Ireland’ articles coming soon.