Microcline

Crystal System: Triclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Widespread
Chemical Composition: Potassium aluminium silicate
Chemical Formula: KAlSi3O8
Method(s) of Verification: Llŷn (Sarn Granite) - polarizing microscopy & XRD (Horák, 1993).

Chemical Group:

  • Silicates

Geological Context:

  • Igneous
  • Metamorphic
Introduction: microcline belongs to the alkali feldspar family and, like orthoclase, may contain a minimum of 85% orthoclase component (expressed as Or85) and a maximum of 15% albite (NaAlSi206) component. Microcline, except in extremely potassium-rich compositions (more than Or92), occurs as perthite (or microperthite), an intergrowth of microcline and albite. Microcline is a common constituent of acid plutonic igneous rocks such as granites, granodiorites, pegmatites and aplites or their high-grade metamorphic equivalents.
Occurrence in Wales: microcline is probably present in many of the acidic intrusive rocks in Wales, although specific records are few; the more general term ‘alkali feldspar’ being commonly used.

Key Localities:

  • Central Anglesey: microcline forms large crystals, up to 40 mm in length, within the porphyritic facies of the Precambrian Coedana Granite. In thin section the microcline shows characteristic ‘cross-hatch’, in addition to both Carlsbad and Baveno twinning. The history of potassium feldspar growth is recorded by the incorporation of small (around 300 μm) plagioclase cystals towards the core of the microcline crystals and larger (up to 600 μm) crystals towards the rim (Horák, 1993).
  • Llŷn, Gwynedd: microcline has been noted from the acidic rocks of the Sarn Complex on Llŷn (e.g. Sarn Granite), where both perthitic and non-perthitic forms have been described (e.g. Horák, In: Gibbons & McCarroll, 1993). Microcline in the granite varies from anhedral crystals up to 7 mm across, with larger grains containing plagioclase inclusions. Whereas more potassium rich crystals lack perthitic textures (e.g. Or94) others display a range of textures, all of which can be classified as microperthites using the scheme of Laves & Soldatos (1963), as they contain bands of albite no wider than 50 μm.
  • North East Anglesey: in the area around Bryn Fuches farm metamorphic microcline is found within the amphibolite grade calc-silicate rocks of north-east Anglesey. It is particularly abundant in the clinopyroxene-feldspar gneiss, where it occurs with clinopyroxene, biotite, and titanite, but is also found in the calc-silicate gneiss (Horák, 1993).
  • St. David’s, Pembrokeshire: Bloxham & Dirk (1988) specifically record the presence of rare microcline in the St. David’s granophyre.

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
  2. Gibbons, W. & McCarroll, D., 1993. Geology of the country around Aberdaron, including Bardsey Island. Memoir of the British Geological Survey, sheet 133 (England and Wales).
  3. Horák, J.M., 1993. The Late Precambrian Coedana and Sarn Complexes, Northwest Wales - a Geochemical and Petrological study. Unpublished Ph.D. thesis, University of Wales, 415pp.