Crystal System: Monoclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Barium potassium aluminium silicate hydrate
Chemical Formula: (Ba,K)1-2(Si,Al)8O16.6H2O
Method(s) of Verification: most Welsh harmotome occurrences appear to be based purely on visual identification, based on the characteristic cruciform-twinned habit of the prismatic crystals. Minor harmotome was identified by XRD (NMW X-1611) at Benallt Mine.
Prismatic cruciform-twinned harmotome crystals from Dolyhir Quarry. N. Hubbard Collection. Photo D.I. Green, © D.I. Green.
Scanning electron micrograph of a spray of cruciform-twinned prismatic harmotome crystals from Dolyhir Quarry. National Museum of Wales Collection (NMW 2002.51G.M.9). Image T.F. Cotterell, © National Museum of Wales.
Introduction: harmotome is a zeolite group mineral that occurs in cavities in basalts, phonolites and trachytes, in gneisses, and in some hydrothermal ore veins.
Occurrence in Wales: the first published account of harmotome in Wales was made by Campbell Smith et al. in 1944, briefly describing harmotome encrusting the new mineral banalsite from the Benallt Manganese Mine near Rhiw on the Llŷn. Harmotome had, in fact, been recorded much earlier than this by A. Russell (in unpublished manuscripts) from Cwm Orog Mine near Llangynog in the northern extremes of the Central Wales Orefield (Morgan & Starkey, 1991; Bevins, 1994). An original Russell specimen dated as October 1912 is in the mineral collection of the National Museum of Wales (NMW 78.85G.M.19), while G.J. Williams, the former H.M. Mine Inspector for North Wales had in his collection (Specimen no. Mo.26) a specimen collected in 1913 (now National Museum of Wales specimen no. NMW 27.111.GR.442). Harmotome has since been discovered at a number of other mines in NW Powys and in recent years frequent rich examples of prismatic cruciform-twinned harmotome crystals have been collected coating fractures in Precambrian sediments exposed at Dolyhir Quarry. This latest locality has surpassed all earlier localities in terms of abundance and quality.
- Benallt Mine, Llŷn, Gwynedd: occurs as minute (up to 0.03 mm across) crystals, with barite encrusting cream-coloured coatings associated with banalsite from the no. 2 ore body (Campbell Smith et al., 1944) and as minute, brilliant crystals overgrowing a pale-brown mica in a thin platy band of banalsite from the no. 5 ore body (Campbell Smith, 1945).
- Craig Rhiwarth Mine, Llangynog, Powys: Bevins (1994) reported that harmotome had been collected here.
- Cwm Orog Mine, Llangynog, Powys: recorded in unpublished Russell manuscripts as occurring in a barium vein dominated by witherite (Morgan & Starkey, 1991). The earliest known specimens were collected by A. Russell and G.J. Williams in 1912/1913. At Cwm Orog harmotome typically forms small (< 4 mm long) prismatic cruciform twinned crystals coating fractures in gritty sediments. Secondary barite in the form of minute tabular crystals frequently encrusts the harmotome.
- Dolyhir Quarry, Old Radnor, Powys: relatively common at this locality, particularly in fractures cutting Precambrian sediments. The only habit observed, is cruciform twinned crystals, typically 1-2 mm in length, but exceptionally 5 mm. Crystals are often colourless and completely transparent with a glassy lustre, although a number of white, somewhat corroded crystals have been collected from veins cutting micaceous sandstone near the entrance to the quarry. Occasional specimens have been collected that show harmotome completely encrusted by aggregates of minute tabular barite crystals.
- Gorn Mine, Llanidloes, Powys: rare at this locality. Morgan & Starkey (1991) noted a single crystal on witherite-bearing matrix (found by N. Hubbard). National Museum of Wales specimen NMW 98.35G.M.472 exhibits cruciform twinned harmotome crystals up to 3.5 mm in length on a bladed barite matrix.
- Llanymynech, Welshpool, Powys: a specimen in the National Museum of Wales Mineral Collection (NMW 75.38G.M.15) labelled simply as ‘near Llanymynech’ is likely to be from the Llangynog area, as visually it looks similar to material from Cwm Orog.
- Penyclun Mine, Llanidloes, Powys: occurs as colourless, transparent euhedral crystals, almost invariably exhibiting the characteristic cruciform twin habit (Morgan & Starkey, 1991). Crystals are typically less than 2 mm long by about 0.5 mm across.
- Bevins, R.E., 1994. A Mineralogy of Wales National Museum of Wales, Geological Series No. 16, Cardiff, 146pp.
- Campbell Smith, W., 1945. Banalsite crystals from Wales. Mineralogical Magazine, 27, 63-64.
- Campbell Smith, W., Bannister, F.A., & Hey, M.H., 1944b. Banalsite, a new barium-felspar from Wales. Mineralogical Magazine, 27, 33-46.
- Morgan, D. & Starkey, R., 1991. Harmotome from Pen-y-Clun Mine, Llanidloes, Dyfed, Wales, U.K. Journal of Mines and Minerals, No. 10, 4-6.