Axinite

Crystal System: Triclinic
Status of Occurrence: Confirmed Occurrence
Distribution: Locally Abundant
Chemical Composition: Calcium manganese magnesium iron aluminium borosilicate hydroxide.
Chemical Formula: (Ca,Mn)4(Mn,Fe,Mg)2(Al,Fe)4B2Si8O30(OH)2
Method(s) of Verification: electron microprobe analysis.

Chemical Group:

  • Silicates

Geological Context:

  • Metamorphic: low-grade
Introduction: axinite is a mineral group, containing four members, ferroaxinite, magnesioaxinite, manganaxinite and tinzenite. Axinite typically occurs in low and medium-grade metamorphic rocks and may be the only boron-bearing mineral where the rocks are aluminium poor and calcium rich. Whereas ferroaxinite and manganaxinite form a continuous series, there does not seem to be a similar relationship between magnesioaxinite and manganaxinite and only limited substitution with ferroaxinite. Tinzenite is formed where manganese substitutes for calcium (Andreozzi et al., 2000).
Occurrence in Wales: of the four occurrence of axinite in Wales, three have been identified as ferroaxinite (see ferroaxinite for further details). An occurrence of axinite from Garn Turne Rocks, Fishguard, Pembrokeshire has been identified optically as axinite but not further characterized by microprobe analysis. It is associated with prehnite-quartz-white mica in very thin veins cutting altered dacite at Garn Turne Rocks (Bevins, 1994).

Key Localities:

There are no key localities for this specimen.

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. Senero, E. & Gottardi, G., 1968. Classification nomenclature and crystal chemistry of axinites. American Mineralogist, 53, 1407-1411.