Introduction:

perhaps the most familiar mineral to many (because it is so frequently met with), quartz brings to mind images of rock crystal, amethyst, smoky and rose-coloured varieties and the cryptocrystalline varieties typified by jasper, agate and chalcedony. Yet much quartz goes unseen, occurring in countless millions of small grains, cemented together in sedimentary rocks. Quartz is hard and highly resistant to both physical and chemical weathering and thus it survives in abundance in sedimentary environments: in some sedimentary rocks such as sandstones the grains may be large enough to see while in others, such as the Triassic red marls of South Wales (composed primarily of quartz and iron oxides), they are microscopic in size. Quartz is also an important rock-forming mineral in igneous and metamorphic rocks. It is additionally an extremely frequent component of mineral veins of all types, with or without carbonates and sulphides. The hardness and crystal morphology of quartz make it an easy mineral to identify in hand specimens: its resistance to acids is also notable (with the exception of hydrofluoric acid, which dissolves it). Quartz is particularly useful when investigating mineral deposits as the textures in thin section yield information about subsequent deformation (see images below) and it is also particularly adept at trapping and preserving inclusions of the original mineralizing fluids (so-called fluid inclusions) which, when analysed, can tell us a great deal about the temperature and composition of the fluid and the pressure at which the mineral crystallized. This in turn helps us to work out how a particular mineral deposit was formed.

Occurrence in Wales:

quartz is present in one form or another everywhere in Wales: it is true to say that virtually every square metre of the surface of the Welsh landscape has some quartz present, and in some places (e.g. parts of Snowdonia) it is so abundant in veins as to give rock outcrops a white-streaked appearance. This abundance contrasts strongly with the distribution of some of the minerals categorized as 'rare' in this database, some of which are only known from one or maybe two specimens! Outside of the myriad rockforming occurrences, some fine specimens of quartz are also known from Wales. Rock crystal from North Wales used to be sold, as long ago as the early 19th Century, as 'Snowdon Diamonds', which were bought as curios by tourists. Since then, many fine specimens have surfaced, with many of the best, in terms of form and clarity, coming from the alpine fissure-type veins of Snowdonia. The metalliferous hydrothermal lodes of the various Welsh orefields have likewise produced large numbers of quartz specimens which make up in size what they lack in clarity. In Central Wales, the later (A2) stage of vein mineralization includes an assemblage in which coarsely-crystalline quartz is a major feature (Mason, 1997). Coloured varieties of crystalline quartz are less of a feature but hydrothermal lodes in South, Central and North Wales have all locally produced smoky or amethystine crystal groups. The cryptocrystalline varieties are less of a feature, with the exception of jasper which is particularly abundant on the Llŷn Peninsula (Matley, 1928); agate only occurs sporadically as in the 'Potato Stones' occurring in Triassic rocks in South Wales and as rare, small vesicle fillings in Lower Palaeozoic lavas in some areas. Because it is so widespread, only sites that have produced fine specimens of quartz are listed.

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