Barrerite |(Na,K,Ca0.5)8(H2O)26| [Al8Si28O72]

BarreriteBarrerite, Rocky Pass, Kuiu Island, Sitka Borough, laska, USA. Image width 3 cm. © Volker Betz.

  Orthorhombic, mmm, platy crystals with prominent {010} face, modified by {100}, {001}, {111}
Physical properties:

Cleavage: {010} perfect
Hardness: 4 - 4.5
D = 2.13 gm/cm3
Luster: vitreous, pearly on {010}
Streak: white

Optical properties:                                         Bikitaite
  Color: Colorless to white. Colorless in thin section.
Biaxial (-).  α = 1.479, β = 1.485, γ = 1.489, δ = 0.010, 2Vx  = 78°. a = X, b = Y, c = Z.  O.A.P. || (010)
  Unit cell:
a  13.643,  b  18.220,  c  17.842 Å
Z = 2, Space group Amma or Ammm                       
  Passaglia and Pongiluppi (1974, 1975) described barrerite as a new mineral from Capo Pula, Sardinia, Italy. Name honors Professor Richard M. Barrer (1910-1996) of Imperial College, London, recognizing his contributions to the chemistry of molecular sieves. Barrerite is a very rare zeolite, presently known from two localities.
Crystal structure:  
  BarreriteThe framework topology of barrerite is the same as stilbite (STI). However barrerite is orthorhombic, space group Amma, and has (Si,Al) disordered over five distinct tetrahedral sites Galli and Alberti 1975, Sacerdoti et al. 1999). Five partially occupied channel cations can be resolved. Two of these are similar to the Ca site in stilbite and stellerite (C1, C1p) and have the highest Na occupancies,0.72 and 0.61 (Galli and Alberti 1975). The other two sites are somewhat similar to the Na sites in stilbite (C2, C2p) with lower occupancies, 0.14 and 0.25. The fifth site (C3) is specific to barrerite, with an occupancy of 0.25 (Galli and Alberti 1975). The figure shows the channel site distribution. Galli and Alberti (1975) also found 14 channel H2O sites ranging in occupancy from 0.2 to 0.91. The cation and H2O distribution in barrerite is thus quite complex and may vary from sample to sample (Galli and Alberti 1975, Sacerdoti et al. 1999).  Upon dehydration barrerite transforms to heat-collapsed phases characterized by major changes in the framework (Alberti and Vezzalini 1978, Sani et al. 1998).
    Channel occupancy of barrerite from Capo Pula, Sardinia, Italy (Galli & Alberti 1975b), Na and minor other cations are distributed in five different sites, C1, C1p, C2, C2p, and C3, accommodating a maximum of 16 cations.
Chemical composition:
Barrerite shows little variation around the average 28 Si per cell. Na is the most common non-framework cation and K occurs rarely, while other elements like Mg, Sr, and Ba are absent or very minor in amount.
  Because the morphology of barrerite is indistinguishable from stilbite, a chemical analysis is necessary to prove the high Na content, and X-ray diffraction is necessary to determine the orthorhombic symmetry.
  Barrerite has been reported from only a few localities. The two well described localities are in altered volcanic rock, very near sea level. At the type locality from Capo Pula, Sardinia, Italy, the host rock is deeply weathered lava, either andesite or rhyolite that is exposed nearby in a less altered state (Passaglia and Pongiluppi 1974). The other occurrence is in the basalt tidal zone of Kuiu Island in southeastern Alaska (Di Renzo and Gabelica 1997). Experimental evidence suggests that barrerite cannot be derived from stilbite by Na-ion exchange, and must therefore, have crystallized from Na-rich fluids. Barrerite has been reported from others localities in Nagasaki Pref., Japan; Bay of Fundy, Nova Scotia, Canada; Cedar Mountain Stone Corp. quarry, Virginia, U.S.A.; but mineral data or analyses have not been reported.
  Alberti, A. and Vezzalini, G. 1978. Crystal structures of heat-collapsed phases of barrerite. In Natural Zeolites, Occurrence, Properties Use. Sand, L.B. and Mumpton, F.A. (eds) Pergamon Press, Oxford, 85-98.

Di Renzo, F. and Gabelica, Z. 1997. Barrerite and other zeolites from Kuiu and Kupreanof Islands, Alaska. Can. Mineral., 35, 691-698.

Galli, E. and Alberti, A. 1975. The crystal structure of barrerite. Bull. Soc. fr. Minéral. Cristallogr. 98, 331-340.

Passaglia, E. and Pongiluppi, D. 1974. Sodian stellerite from Capo Pula, Sardegna. Lithos, 7, 69-73.

Passaglia, E. and Pongiluppi, D. 1975. Barrerite, a new natural zeolite. Min. Mag. 40, 208.

Sacerdoti, M., Sani, A., and Vezzalini, G. 1999. Structural refinements of two barrerites from Alaska. Microporous Mesoporous Mater. 30, 103-109.