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Celadonite-family mica minerals occurring in the Triassic Gavenwood Tuffs. Murihiku Supergroup. Hokonui Hills. Southland. New Zealand, have been analyzed by XRD. TEM. AEM. and EMPA. Packets a few unit cells to several hundred nanometers thick are inti­mately intergrown with chlorite, berthierine. and corrensite. Analyses of homogeneous packets, combined with analyses from the literature, imply complete or nearly complete solid solution between end-members of the celadonite family defined by octahedral ex­change involving MgFe 3+ , Fe 2+ Al, Fe 2+ Fe 3+,  and probably MgAl, and show that EMPA analyses are commonly contaminated by mixtures. Two new minerals of the celadonite family are defined: ferroceladonite. K 2 Fe 2 ) Fe 3  2  Si 8 O 20 (OH) 4,  and ferroaluminoceladonite. K 2 Fe 2+  2 Al 2 Si 8 O 20 (OH) 4 . The former occurs largely as submicrometer (≤ 200-300 nm thick) grains in vesicle rims, and the latter admixed with chlorite and mixed-layered minerals in vesicle interiors and replacing glass shards. Heulandite is intimately associated with both ferroceladonite and ferroaluminoceladonite. Both new minerals are blue-green in thin sec­tion and occur as IM polytypes. Powder X-ray diffraction patterns of mixtures show only one set of mica peaks, with only a few peaks exhibiting slight broadening. The strongest lines in the X-ray powder diffraction patterns are [d (I, hkl)]: 3.65 (52. 112̅); 3.358 (86. 022); 3.321 (100. 003); 3.090 (60. 112); 2.584 (50. 131). A composite sample composed of ferroceladonite and ferroaluminoceladonite gives the following unit-cell data: space group C2/m, Z = 2, with refined averase lattice parameters a = 5.270(5), b = 9.106(7), c = 10.125(8) β = 100.27 (14)°, V= 478.1(4) Å 3 . The calculated densities are 3.045 (3) and 2.928 (2) g/cm 3  for ferroceladonite and ferroaluminoceladonite, respectively. Cel­adonite mineral-aluminous clay mineral and celadonite mineral-Ca-rich zeolite assem­blages of the zeolite facies are related to illite-chlorite ± pumpellyite assemblages of higher grade by dehydration reactions, not necessarily under closed-system conditions.

american mineralogist

Solid solution in the celadonite family: The new minerals ferroceladonite, K 2 Fe 2+  2  Fe 3+  3  Si 8 O 20 (OH) 4 , and ferroaluminoceladonite, K 2 Fe 2+  2  Al 2 Si 8 O 20 (OH) 4