Introduction: baryte is a common mineral that occurs in a range of geological settings. It occurs as a primary mineral in a variety of hydrothermal mineral deposits but particularly Mississippi Valley Type vein/cavity-fill deposits and sedimentary-exhalative deposits, both of which can carry sufficient baryte to be mined economically for it. Common associates in such deposits include calcite, sphalerite and galena. Baryte also occurs with cavity-fill hematite deposits, where it is regarded as a gangue mineral. In the supergene environment, baryte may be present both in natural, in situ weathering zones and in post-mining assemblages where it tends to be microcrystalline. Baryte is also present in sedimentary rocks, where it tends to occur in cavities within septarian nodules in mudstone sequences. Detrital baryte may occur in placer deposits in areas where rocks carrying primary Ba mineralization are being eroded. Baryte has a high density, hence the old miners' name for it - 'heavy spar'. This assists in identification and it may be distinguished from the similarly heavy barium carbonate, witherite, by its resistance to dilute hydrochloric acid (witherite will effervesce). It may contain significant strontium and there is a mineral series between baryte and the strontium sulphate, celestine.

Occurrence in Wales: baryte occurs abundantly in many of Wales' hydrothermal ore deposits, particularly those of Mississippi Valley Type (MVT), where it is associated variably with calcite, fluorite, witherite and base-metal sulphides. At some localities it occurs in small quantities as a gangue mineral, but it was formerly sufficiently abundant at others to be mined in its own right. Baryte was mined in the Llanengan district of North Wales, at the Pennant Mine near St Asaph, at mines in the Middletown area on the Powys-Shropshire border, at three mines on the Van Lode in Central Wales and at the Vale of Towy and Cystanog Mines near Llandeilo (Carruthers et al., 1915). In comparison to other important baryte-producing areas of the UK, the Welsh occurrences are generally massive, with few good crystals being produced. The chief exceptions to this are the cavity-fill iron-ore deposits of South Wales, which have intermittently produced excellent display specimens over the years, and the septarian nodules of the South Wales Coalfield, from which fine single crystals are occasionally recorded. The abundance of baryte dictates that the specific localities listed here are only those notable either for fine specimens or for the paragenetic interest of the occurrence

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