Orthoclase

Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Widespread
Chemical Composition: Potassium aluminium silicate
Chemical Formula: KAlSi206
Method(s) of Verification: Station Quarry - XRD (Natural History Museum, x10843); St. David’s - polarizing microscope.

Chemical Group:

  • Silicates

Geological Context:

  • Igneous
  • Metamorphic
Introduction: orthoclase belongs to the alkali feldspar family and may contain up to 15% albite (NaAlSi206) component (expressed as Or85). Although alkali feldspars form a complete solid solution at magmatic crystallization temperatures, on cooling the sodic and potassic components in potassium-rich feldspar ‘unmix’ to form orthoclase microperthite. At even lower temperatures orthoclase can convert to microcline, which is triclinic. Orthoclase is most commonly found as a rock-forming mineral in igneous rocks, particularly in granitoids and pegmatite and is also characteristic of high grade metamorphism of pelite.
Occurrence in Wales: orthoclase is probably widely developed in Wales, although specific references to it in the literature are relatively few; most commonly reference is made just to the presence of ‘alkali feldspar’ . In many instances this may be because there is uncertainty as to the structural state of the feldspar (microcline or orthoclase). Documented occurrences are listed below.

Key Localities:

  • St. David’s, Pembrokeshire: Bloxham & Dirk (1988) refer to the presence of rare anhedral orthoclase, associated with microcline and microcline-perthite as an interstitial phase to quartz and plagioclase, in the Precambrian, St David’s granophyre.
  • Station Quarry, Arenig, Llanycil, Gwynedd: orthoclase has been identified from a Natural History Museum specimen (B.M. 1946, 38) from Station Quarry, and verified by X-ray diffraction anaysis.

References:

  1. Bloxham, T.W. & Dirk, M.H.J., 1988. The petrology and geochemistry of the St. David’s granophyre and the Cwm Bach rhyolite, Pembrokeshire, Dyfed. Mineralogical Magazine, 52, 563-576