Ferroaxinite

Crystal System: Triclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Calcium iron aluminium borosilicate hydroxide
Chemical Formula: Ca2FeAl2(BO3OH)(SiO3)4
Method(s) of Verification: Graiglwyd Quarry, Penmaenmawr Quarry, Llanddwyn Island & Carreg Ddu - electron microprobe (National Museum of Wales/A. Tindle, unpublished data).

Chemical Group:

  • Silicates

Geological Context:

  • Hydrothermal
  • Metamorphic: low-grade
Mauve-tinted bladed crystals of ferroaxinite associated with quartz from Graiglwyd Quarry, Penmaenmawr. Specimen National Museum of Wales (NMW 27.111.GR.248), ex G.J. Williams Collection. Photo D.I. Green, © National Museum of Wales.
Platy brown ferroaxinite crystals associated with quartz and epidote. Carreg Ddu. Specimen National Museum of Wales (NMW 78.33G.M.100). Photo D.I. Green, © National Museum of Wales.
Introduction: ferroaxinite is the iron-rich member of the axinite group and typically occurs in low and medium-grade metamorphic rocks and it may be the only boron-bearing mineral where the rocks are aluminium poor and calcium rich. It forms a continuous series with manganaxinite but there is only limited substitution by magnesium (Andreozzi et al., 2000).
Occurrence in Wales: Sargent (1916) provided the first description of axinite in Wales from the Penmaenmawr Intrusion in North Wales and there have been three subsequent recordings of its occurrence. Three of these occurrences have been identified as ferroaxinite, and all occur in a similar context of thin veins, bearing a low-grade metamorphic assemblage, which cut basic or intermediate composition igneous rocks.

Key Localities:

  • Carreg Ddu, Porth Dinllaen, Llŷn, Gwynedd: a similar occurrence to that at Llanddwyn Island, of axinite cutting basic lavas, is known from the Monian Supergroup of Llŷn. Platy brown ferroaxinite crystals up to 14 mm across occur in veins (up to 140 mm wide) in association with quartz and epidote. Platy mauve-brown ferroaxinite is also present as specimens in the National Museum of Wales Collection (e.g. NMW 87.43G.M.54), although these specimens do not show matrix or associations with other minerals.
  • Graiglwyd Quarry, Penmaenmawr, Conwy: Sargent (1916) described axinite in thin veins from loose blocks of Penmaenmawr Intrusion at Graiglwyd Quarry. In a more detailed work (Sargent, 1925) he described three mineral assemblages: axinite+prehnite+quartz; axinite+pectolite; axinite+zoisite+quartz. These were contained in veins, up to 5 cm wide, bearing massive mauve axinite, and fissures and cracks with pale brown axinite forming thin radiating blades up to 2.5 cm long. In both modes axinite occurs associated with prehnite. National Museum of Wales specimens show mauve brown ferroaxinite associated with prehnite (e.g. NMW 27.111.GR.249 & NMW 28.473.GR.221) and brown ferroaxinite associated with quartz and epidote (NMW 27.457.GR.1). Well formed crystals of brown ferroaxinite are up to 10mm long and 5 mm wide.
  • Llanddwyn Island, Anglesey: Greenly (1919) reported the presence of axinite in a 15 cm wide vein cutting altered basic lavas of the Monian Supergroup at the northern end of the island, mistakenly attributing this as the first occurrence in Wales.
  • Penmaenmawr, Conwy: in Penmaenmawr Quarry the host lithologies and vein assemblages are essentially the same as those described from Graiglwyd Quarry. National Museum of Wales specimens, NMW 30.620.GR.1 & NMW 97.47G.M.1 show a fine-grained pale green altered microdiorite matrix with a thin vein of prehnite coated by massive mauve-brown ferroaxinite.

References:

  1. Andreozzi, G.B., Ottolino, L., Lucchesti, S, Graziano, G & Russo, U., 2000. Crystal chemistry of the axinite group minerals: a multi-analytical approach. American Mineralogist, 85, 698-706.
  2. Bevins, R.E., 1994. A Mineralogy of Wales National Museum of Wales, Geological Series No. 16, Cardiff, 146pp.
  3. Greenly, E., 1919. The Geology of Anglesey. Memoirs of the Geological Survey of Great Britain, 980pp (2 volumes).
  4. Sargent, H.C., 1924. Notes on the petrology of Penmaenmawr Mountain (Part 1). Proceedings of the Liverpool Geological Society, 14, 82-89.
  5. Sargent, H.C., 1916. Axinite veins in the Penmaenmawr Porphyrite. Geological Magazine, New Series, Decade 6, Vol. 3, 5-7.
  6. Sargent, H.C., 1925. Notes on the petrology of the Penmaenmawr Intrusion (Part II). Proceedings of the Liverpool Geological Society, 14, 123-142.