Crystal System: Tetragonal
Status of Occurrence: Confirmed Occurrence
Chemical Composition: Titanium dioxide
Chemical Formula: TiO2
Method(s) of Verification: all occurrences cited are based on confident visual identifications
- Hydrothermal: alpine type veins
Prismatic rutile crystal in quartz from Clogau Mine. Field of view 15 mm across. J.S. Mason Collection (JM 2540). Photo D.I. Green, © National Museum of Wales.
Introduction: rutile is a frequently-observed accessory mineral in igneous (especially intrusive) rocks and as a detrital mineral in sedimentary rocks: it is often present in the metamorphosed equivalents of both. In mineral veins it is less frequent, except in those of the alpine fissure-type, where it is sometimes common, and is invariably associated with quartz, albite, chlorite, apatite, the other TiO2 polymorphs (anatase and brookite) and rare-earth minerals like synchysite. Rutile is readily identified when well-crystallized owing to its tendency to form dense aggregates of golden hairlike crystals, which when embedded in clear quartz, constitutes the semi-precious gemstone known as sagenite or rutilated quartz. Coarser crystals tend to be darker in colour but again the prismatic to acicular habit and the association is diagnostic.
Occurrence in Wales: early reports of rutile in Wales were limited to descriptions of its occurrence as a rock-forming mineral, as in its presence in the basaltic tuffs of the Bedded Pyroclastic Formation on Lliwedd in the Snowdon range (Williams, 1927). More recently, the attention given to alpine fissure-type veins by mineral collectors has resulted in many more discoveries, which are listed below with references. In igneous rocks, rutile often develops by the breakdown of primary iron-bearing minerals such as ilmenite, as described in dolerites from the Preseli hills of SW Wales by Ixer (in Thorpe et al., 1991) and at Parys Mountain on Anglesey (Pointon & Ixer, 1980). A common product of such a reaction is a mixture of quartz and rutile which is generally referred to as leucoxene.
- Central Wales Orefield: the early (A1 of Mason, 1997) polymetallic vein breccia mineralization occasionally reveals open microfractures within clasts of brecciated sedimentary rock locked in a quartz cement. These fractures carry an alpine fissure-type mineral assemblage comprising microcrystalline quartz, albite, apatite and rutile (Rust, 1993), the latter in 1-2 mm hair-like crystals. In some cases the fractures run along clast-quartz interfaces so that they clearly post-date the cementing of the breccia by quartz: a mechanism to explain how they have formed has yet to be derived. The microfractures have been recorded at a number of mines (Bevins & Mason, 1997) but appear to be most abundant at Brynyrafr, Darren and Castell mines.
- Clogau Mine, Bontddu, Gwynedd: locally common in alpine fissure-type veins which crosscut the gold-lodes (the so-called White Elephant quartz veins). Rutile occurs in two habits: firstly it is widespread as hairlike crystals in vugs lined with chlorite and quartz and secondly, an earlier generation occurs locally as metallic black prisms to 30 mm in length embedded in milky quartz and chlorite. Some specimens of the latter association constitute the richest known from Wales (J.S. Mason/M.J. Liezers, unpublished data). Rutile (or a TiO2 polymorph) was also recorded by Naden (1988) as individual needles to 10 µm and aggregates of needles to 100 µm associated with arsenopyrite, pyrite/pyrrhotite and cobaltite - suggesting it is also a component, albeit minor, of the main gold-lode paragenesis.
- Cwmorthin Quarry, Blaenau Ffestiniog, Gwynedd: black to brown groups of parallel acicular rutile crystals occur rarely with quartz, anatase, xenotime and synchysite (National Museum of Wales/Manchester Museum unpublished data).
- Hendre Quarry, Glyn Ceiriog, Clwyd: silvery, metallic acicular microcrystals associated with quartz, albite, anatase and brookite (Starkey et al., 1991).
- Manod Quarry, Blaenau Ffestiniog, Gwynedd: as sagenitic replacements of small brookite crystals (J.S. Mason, unpublished data).
- Prenteg, Tremadog, Gwynedd: much less common than anatase or brookite, rutile has been reported as silvery-yellow acicular crystals associated with quartz and chamosite and as inclusions in quartz, albite, brookite and fluorapatite. This indicates that it is early in the paragenesis: however a second, secondary generation also occurs forming sagenitic replacements of brookite (Starkey & Robinson, 1992).
- Tanygrisiau Station, Blaenau Ffestiniog, Gwynedd: common as hair-like crystals to 20 mm forming inclusions in quartz, albite and anatase (Green & Middleton, 1995).
- Bevins, R.E. & Mason, J.S., 1997. Welsh metallophyte and metallogenic evaluation project: Results of a minesite survey of Dyfed and Powys. CCW Contract Science Report No. 156. National Museums & Galleries of Wales.
- Green, D.I. & Middleton, D., 1996. Alpine-type vein minerals from Tanygrisiau, Gwynedd. U.K. U.K. Journal of Mines and Minerals, 16, 30-33.
- Mason, J.S., 1997. Regional polyphase and polymetallic vein mineralisation in the Caledonides of the Central Wales Orefield. Transactions of the Institution of Mining and Metallurgy (Section B: Applied Earth Science), 106, B135-B144.
- Naden, J., 1988. Gold mineralisation in the Caledonides of the British Isles with reference to the Dolgellau Gold Belt and the Southern Uplands of Scotland. Unpublished Ph.D thesis, University of Aston, UK.
- Pointon, C.R. & Ixer, R.A., 1980. Parys Mountain mineral deposit, Anglesey, Wales: geology and ore mineralogy. Transactions of the Institution of Mining and Metallurgy (Section B: Applied earth science), 89, B143-B155.
- Rust, S., 1993. Alpine minerals from the Bryn-yr-Afr Mine, Mid Wales. British Micromount Society Newsletter No. 35, 7
- Starkey, R.E. & Robinson, G.W., 1992. Famous mineral localities, Prenteg, Tremadog, Gwynedd, Wales. Mineralogical Record, 23, 391-399.
- Starkey, R.E., Hubbard, N. & Bayley, M.P., 1991. Mineralization at Hendre Quarry, Glyn Ceiriog, Clwyd, Wales. U.K. Journal of Mines and Minerals, No. 10, 48-51.
- Thorpe, R.S., Williams-Thorpe, O., Jenkins, D.G. & Watson, J.S., 1991. The geological sources and transport of the bluestones of Stonehenge, Wiltshire, U.K. Proceedings of the Prehistoric Society, 57, 103-157.
- Williams, H., 1927. The geology of Snowdon (North Wales). Quarterly Journal of the Geological Society of London, 83, 346-431.