by Patrick Gaffikin
Go, my sons, buy stout shoes, climb the mountains, search the valleys, the deserts, the sea shores, and the deep recesses of the earth. Mark well the various kind of minerals, note their properties and their mode of origin....
Petrus Severinus, a Danish physician. (1542-1602)
What are minerals?
A mineral is a naturally occurring inorganic solid with a definite chemical composition.
from rock n gem magazine issue 70
If a mineral can be mined profitably it is called an ore. An example of a mineral found in Ireland is sphalerite, also known as zinc blende, which has the chemical name zinc sulphide (ZnS). That is, each molecule of the mineral is composed of one ion of zinc combined with one ion of sulphur. (An atom is electrically neutral but when an atom loses or gains one or more electrons it becomes an ion. For example, in the case of ZnS, when a Zn atom loses its two outer electrons it becomes a zinc ion, Zn++ , and when a S atom gains two outer electrons it becomes a a sulphur ion, S--.) Sphalerite is also an ore – the largest working zinc mine in Europe is at Navan in Co. Meath where millions of tons of sphalerite-bearing rock form an ore deposit.
The atoms of minerals are arranged in a regular pattern and this determines many of each mineral’s characteristic properties like crystal shape, cleavage (the way it breaks) and density. For instance the mineral halite (sodium chloride), also known as common salt, has a cubic crystal shape and cubiodal cleavage due to the arrangement of the sodium and chlorine ions.
Note that ‘minerals’ in food supplements are not minerals in the strict sense; they are actually trace elements like zinc, potassium, calcium and so on.
The difference between rocks and minerals
Most rocks are composed of a mixture of minerals that have crystallised together or lots of particles cemented or compacted together. For instance, Mourne Granite is a rock mainly composed of the crystallised minerals quartz, feldspars and mica whereas Chalk is composed of tiny grains of calcite (calcium carbonate) that have been compacted together. Some rocks consist of one mineral, for example pure sandstone may be comprised of over 99% quartz. However, this is still classed as a rock since the sandstone is made of lots of individual particles of quartz rather than a single crystal. Note that some rocks are not made of minerals as they are organic in origin – coal being an example.
The earliest mining for metals in Ireland
The Bronze Age succeeded the Stone Age. It is usually accepted that the Bronze Age in Ireland commenced around 4,500 years ago and ended circa 2,300 years ago (300 B.C.) to be followed by the Iron Age, which lasted until around 400 A.D. As the name implies, the Bronze Age was the time when prehistoric people were able to produce bronze, which is an alloy of copper and tin.
During this period our ancient ancestors were firstly able to produce copper by heating a copper mineral, such as malachite, and cooling the molten residue.
Furthermore, probably by experimentation, they found that they could manufacture a harder and more durable metallic substance, bronze, by mixing molten copper with a small amount of tin.
The Bronze Age people in Ireland may have obtained the tin from indigenous sources such as the Mournes and other granite regions. Currently the geologists are investigating this.
They are also researching that gold may have been recovered from river sediment in the Mournes, as a by-product of tin streaming, during the Bronze Age.
‘Streaming’ involved channelling water into streams where tin-bearing sediments could be collected.
(However, the best example of ancient workings in the Mournes is actually the Ballincurry River, near Rostrevor, which is outside the granite outcrop and has no tin enrichment.
It is thought that this area was mined for gold, not as a by-product of tin streaming.)
Bronze was used for making tools, weapons and ornaments and evidence of bronze working in Ireland exists in places like Co. Antrim and Co. Cork.
For example a casting workshop was discovered at White Park Bay, Co. Antrim. At Mount Gabriel, near Schull, Co. Cork, over 30 small Bronze Age copper mines have been found in sandstone.
Minerals found in Ireland
Ireland has a wide range of rock-types and ore minerals. Some occur in substantial quantities like gypsum at Kingscourt, Co. Cavan; zinc and lead ores at Navan, Co. Meath and halite forms rock salt deposits at Kilroot, Co. Antrim. But small amounts of other minerals for instance beryl, calcite, quartz, dolomite and zeolites also are present. Zeolite minerals, for example, highly valued by collectors, have been found in cavities in the Co. Antrim Basalt.
How do minerals form?
In recent times, the Irish economy has greatly benefited from mining deposits of zinc and lead ores.
These were concealed in the Carboniferous Limestones of Ireland’s central plain – for example at Navan, Co. Meath and Tynagh, Co. Galway.
There is now good consensus among exploration geologists as to how mineral deposits of various types have formed. For instance, it is now known that these zinc and lead deposits formed when the Earth’s crust, below central Ireland, was being stretched and faulted during the Carboniferous Period (around 340 million years ago). At that time, Ireland was covered by a shallow sea. Hot fluids circulated deep within the crust and, when conditions were right, minerals like sphalerite and galena precipitated out of the fluids to be deposited on the sea floor or within the limestones below the sea bed.
Conservation of minerals
Since the Bronze Age, and arguably before, minerals have been extracted from the Earth and these have enhanced the lives of people. The Earth’s resources, generally, have contributed to making our species the dominant one on the planet. However, as with other resources from the Earth, mineral resources are finite. It is for our individual and collective benefit that profligate consumption should be avoided and re-cycling becomes commonplace. Removing minerals from the Earth in vast quantities inevitably damages the environment – and this, ultimately, affects us all detrimentally.
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