Crystal System: Trigonal
Status of Occurrence: Confirmed Occurrence
Chemical Composition: Lead sulphate carbonate hydroxide
Chemical Formula: Pb4(SO4)(CO3)2(OH)2
Method(s) of Verification: Bwlch-glas Mine - XRD (Rust, 1996); Esgair Hir Mine - XRD (Rust & Mason, 1988); Frongoch Mine - XRD(NMW, X-1155); Llechweddhelyg Mine - XRD (NHM); Nantmelyn Mine - visual; Nant-y-creiau Mine - visual
- Supergene: post-mining oxidation & weathering deposits
- Supergene: in situ natural oxidation & weathering deposits
Pale yellow pseudohexagonal susannite crystal 0.5 mm across from Frongoch Mine, in the Central Wales Orefield. National Museum of Wales Collection (NMW 2003.1G.M.248), ex J.S. Mason Collection. © National Museum of Wales.
Introduction: susannite is a rare secondary mineral polymorphous with leadhillite and macphersonite. It is found typically as a late stage post-mining phase in lead-bearing mine dumps and in the oxidized portions of lead-bearing orebodies. Susannite is frequently found in association with caledonite.
Occurrence in Wales: generally regarded as rarer than its polymorph leadhillite, susannite is actually the more common species in Wales. Confusion exists over some occurrences as many identifications are referred to as leadhillite or leadhillite-group before further analysis has been carried out. Therefore it is recommended that both the leadhillite and susannite records are consulted. All confirmed records of susannite in Wales are from the Central Wales Orefield, where the vast majority of occurrences are formed by post-mining supergene processes within lead-bearing mine spoil. Typical of post-mining minerals, crystals are small - 2 mm being considered large - but frequently well formed. Rust and Mason (1988) produced the first account of susannite from Wales, at Esgair Hir Mine where, susannite forms groups of rod-like to prismatic crystals within supergene dump-formed mineralization. Subsequent occurrences are noted by Bevins (1994) at other Central Wales minesites including, Llechweddhelyg, Hendrefelen and Frongoch. The discovery at Frongoch soon led to a reputation as the source of the finest Welsh susannite specimens (Green et al., 1996) with crystals reaching 2 mm across. The crystals are not just the largest recorded from Wales, but also the most attractive, frequently displaying stepped growth patterns. Rust (1996) provided an interesting account of the first in-situ susannite, from underground workings at Bwlch-glas mine. Further occurrences of both susannite and leadhillite are listed by Bevins & Mason (1997). However, as no confirmation has been made, most should be regarded as leadhillite-group minerals until further analysis has been made.
- Bodcoll Mine, Devil’s Bridge, Ceredigion: leadhillite associated with caledonite is listed by Bevins & Mason (1997). Further investigations are required to verify this identification, which, is likely to have been made based on the visual appearance of pseudohexagonal leadhillite-like microcrystals. The presence of susannite cannot be ruled out.
- Brynyrarian Mine, Tre-Taliesin, Ceredigion: leadhillite is listed by Bevins & Mason (1997), but as with Bodcoll further analysis is required to determine the true identity of this leadhillite-group mineral.
- Bwlch-glas Mine, Tal-y-bont, Ceredigion: Rust (1996) produced the first account of in-situ susannite from Wales at Bwlch-glas mine. Susannite was discovered in brecciated galena-sphalerite-quartz veinstone with minor chalcopyrite, exposed near to the famous pyromorphite-zone in the shallow adit level. Crystals are tabular to blocky, colourless to white and reach 0.8 mm, occasionally associated with blue-green prismatic caledonite crystals to 0.4 mm, anglesite, cerussite and malachite.
- Darren Mine, Pen-bont Rhydybeddau, Ceredigion: leadhillite is listed by Bevins & Mason (1997). No description is given, but due to visual similarities between leadhillite and susannite the possibility remains that this could represent susannite.
- Esgairhir Mine, Tal-y-bont, Ceredigion: groups of rod-like susannite crystals (up to 0.3 mm) and a slightly concave prismatic form are described by Rust & Mason (1988) within highly oxidized veinstone on the dumps of the mine. Susannite forms a rare part of the supergene assemblage in association with caledonite, leadhillite and anglesite.
- Frongoch Mine, Devil's Bridge, Ceredigion: susannite is relatively common at Frongoch, forming microcrystals representing a late-stage post-mining phase. Crystals reach 2 mm and occur in a variety of habits. Green et al. (1996) describe typical tabular to blocky pseudohexagonal forms along with a more unusual elongated prismatic habit. One particular feature of Frongoch susannite is that, some crystals display step-like growth formations, interrupted growth, and overgrowths of several different crystal forms producing some wonderful forms. Colour varies from colourless to white and cream with green, blue or yellow tints or occasionally toffee brown. As with many localities caledonite is a common associate on a characteristic matrix of corroded galena.
- Gorlan Mine, Trefriw, Gwynedd: leadhillite is listed by Bevins & Mason (1998), but no further details are given. Until this record is verified as leadhillite, there remains the possibility that it could be susannite. This would represent the first occurrence outside of the Central Wales Orefield.
- Hendrefelin Mine, Ysbyty Ystwyth, Ceredigion: confirmed by Clark & Rust (1993) from the same dump as bottinoite.
- Llechweddhelyg Mine, Penrhyncoch, Ceredigion: susannite has several modes of occurrence: as yellowish transparent pseudohexagonal platy crystals (up to 1 mm); colourless pseudohexagonal crystals (up to 0.6 mm) covering areas of 1 cm|2; and as minute prismatic crystals (Rust, 1990) associated with caledonite.
- Llettyhen Mine, Bontgoch, Ceredigion: susannite is listed without description by Bevins & Mason (1997).
- Nant-y-creiau Mine, Devil’s Bridge, Ceredigion: small blocky pseudohexagonal leadhillite-group crystals occur within heavily weathered pyritic lead-zinc-copper bearing veinstone dumped within the bed of a dried up stream.
- Nantmelin Mine, Central Wales: a leadhillite-group mineral richly encrusting highly altered galena-bearing veinstone from the shaft dumps shows visual similarities to susannite from Frongoch Mine. Pale cream, platy pseudohexagonal microcrystals with a pronounced stepped growth pattern, form layers associated with linarite (T.F. Cotterell, unpublished data).
- Bevins, R.E., 1994. A Mineralogy of Wales National Museum of Wales, Geological Series No. 16, Cardiff, 146pp.
- Bevins, R.E. & Mason, J.S., 1998. Welsh Metallophyte and metallogenic evaluation project: Results of a Minesite Survey of Gwynedd. National Museums of Wales, Cardiff.
- 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.
- Clark, A.M. & Rust, S.A., 1993. Bottinoite, a mineral new to Britain. Mineralogical Magazine. 57, 543-544.
- Green, D.I., Rust, S.A. & Mason, J.S., 1996. Classic British mineral localities: Frongoch Mine, Dyfed. UK Journal of Mines & Minerals, 17, 29-38.
- Rust, S., 1996. Susannite and caledonite in-situ in Central Wales. U.K. Journal of Mines and Minerals, 17, 18.
- Rust, S.A., 1990b. Susannite and sulphatian schmeiderite from Llechwedd Helyg Mine, Tir-y-Mynach, Dyfed, Wales. U.K. Journal of Mines and Minerals, 8, 48
- Rust, S.A. & Mason, J.S., 1988. The minerals of Esgair-Hir mine, Dyfed, Wales. UK Journal of Mines & Minerals, 5, 35-43.