Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Calcium sodium magnesium iron aluminium silicate hydroxide
Chemical Formula: (Ca,Na)0.3(Mg,Fe)3(Si,Al)4O10(OH)2•4H20
Method(s) of Verification: Llŷn - XRD (Roberts, 1981); Llanelwedd Quarry - EMPA (Garvie & Metcalfe, 1997; Metcalfe, 1990).
- Metamorphic: low-grade
Introduction: saponite belongs to the smectite group of clay minerals. It has a greasy lustre and is found in a variety of colours from white or cream through to blue, green or red-tinged. Saponite is found as a low-temeprature alteration mineral in hydrothermal mineralized veins; infilling vesicules in basalt; in fissures cutting calc-silicates, iron-rich skarns, amphibolites, and in serpentinite (Anthony et al., 1995). The presence of saponite is generally only confirmed by X-ray diffraction or microprobe analysis.
Occurrence in Wales: as with many other low-grade metamorphic and hydrothermally produced secondary silicate minerals there are just a few reported occurrences in the literature. It is however likely that the distribution of such minerals, including saponite, is far wider than is currently documented.
- Llanelwedd Quarry, Builth Wells, Powys: saponite has been described from quartz-calcite veins cutting altered basic lavas at Llanelwedd Quarry, near Builth Wells as part of the hydrothermal assemblage (Garvie & Metcalfe, 1997, Metcalfe, 1990).
- Llŷn, Gwynedd: Roberts (1981) reports saponite-bearing metadolerite sills from near Pwllheli and Dinas, on Llŷn, North Wales, forming intergranular patches or pseudomorphing olivine. It occurs with an assemblage containing actinolite ± albite ± ankerite ± calcite ± chlorite ± prehnite ± quartz ± titanite. A specimen of saponite from Gimlet Quarry is in the collections of the Natural History Museum ( BM 1909,462).
- Anthony J., Bideaux R., Bladh K. & Nichols M., 1995. Handbook of Mineralogy, vol. 2 (parts 2/1 and 2/2), Silica, Silicates
- Garvie, L.A.J. & Metcalfe, R., 1997. A vein occurrence of co-existing talc, saponite, and corrensite, Builth Wells, Wales. Clay Minerals, 32, 223-240.
- Metcalfe, R., 1990. Fluid-rock interaction and metadomain formation during low-grade metamorphism in the Welsh marginal basin. Unpublished Ph.D. thesis, University of Bristol.
- Roberts, B., 1981. Low grade and very low grade regional metabasic Ordovician rocks of Llyn and Snowdonia, Gwynedd, North Wales. Geological Magazine, 118, 189-200.