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Solid Solution in the Celadonite Family: the New Minerals Ferroceladonite, 2؉ 3؉ 2؉ K2fe22 Fe Si8o20(OH)4, and Ferroaluminoceladonite, K2fe2 Al2si8o20(OH)4

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Solid Solution in the Celadonite Family: the New Minerals Ferroceladonite, 2؉ 3؉ 2؉ K2fe22 Fe Si8o20(OH)4, and Ferroaluminoceladonite, K2fe2 Al2si8o20(OH)4 American Mineralogist, Volume 82, pages 503±511, 1997 Solid solution in the celadonite family: The new minerals ferroceladonite, 21 31 21 K2Fe22 Fe Si8O20(OH)4, and ferroaluminoceladonite, K2Fe2 Al2Si8O20(OH)4 GEJING LI,1* DONALD R. PEACOR,1 DOUGLAS S. COOMBS,2 AND YOSUKE KAWACHI2 1Department of Geological Sciences, The University of Michigan, Ann Arbor, Michigan 48109, U.S.A. 2Geology Department, University of Otago, P.O. Box 56, Dunedin, New Zealand ABSTRACT 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 inti- mately 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 de®ned by octahedral ex- change involving MgFe31,Fe21Al, Fe21Fe31, and probably MgAl, and show that EMPA analyses are commonly contaminated by mixtures. Two new minerals of the celadonite 2113 family are de®ned: ferroceladonite, K2Fe 22Fe Si8O20(OH)4, and ferroaluminoceladonite, 21 K2Fe 2 Al2Si8O20(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 sec- tion and occur as 1M 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, 112Å ); 3.358 (86, 022); 3.321 (100, 003); 3.090 (60, 112); 2.584 (50, 131Å ). A composite sample composed of ferroceladonite and ferroaluminoceladonite gives the following unit-cell data: space group C2/m, Z 5 2, with re®ned average lattice parameters a 5 5.270(5), b 5 9.106(7), c 5 10.125(8) AÊ , b 5 100.27 (14)8, V 5 478.1(4) AÊ 3. The calculated densities are 3.045 (3) and 2.928 (2) g/cm3 for ferroceladonite and ferroaluminoceladonite, respectively. Cel- adonite mineral-aluminous clay mineral and celadonite mineral-Ca-rich zeolite assem- blages of the zeolite facies are related to illite-chlorite 6 pumpellyite assemblages of higher grade by dehydration reactions, not necessarily under closed-system conditions. INTRODUCTION Mexico, geothermal system at a temperature inferred Celadonite is a dioctahedral mica with the ideal end- from ¯uid inclusion studies to be 240 6 10 8C. Celadonite also occurs in sedimentary rocks, especially member composition K Mg Fe31 Si O (OH) . Invariably 2 2 2 8 20 4 those with a tuffaceous or other signi®cant volcanogenic ®ne-grained and blue-green in color on crushing (celadon component, as in many Late Paleozoic to Tertiary accre- green), it has been mined for many centuries as a pigment for porcelains and other purposes from quarries near Ve- tionary complexes. In these it formed under diagenetic or rona in Italy (terre verte de VeÂrone) and in the Troodos very low-grade metamorphic conditions (Coombs 1954; massif of Cyprus (Odin et al. 1988). Most recorded cel- Wise and Eugster 1964; Boles and Coombs 1975) cor- adonites occur in hydrothermally altered ma®c volcanic responding to the zeolite facies and less commonly to the rocks including those from the oceanic crust (e.g., An- prehnite-pumpellyite and lawsonite-albite-chlorite facies drews 1980). Honda and Muf¯er (1970) described cela- (Landis 1974), but it is unknown in the greenschist facies donite in altered rhyolitic detritus in an active hydrother- (e.g., Wise and Eugster 1964). mal system spanning temperatures from 80 to 170 8Cat In thin section, celadonite is conspicuous as bright Yellowstone National Park, and Cathelineau and Izquier- blue-green, ®ne-grained aggregates in veinlets and vesi- do (1988) reported it in andesites in the Los Azufres, cles, as botryoids in intergranular void space, and as re- placements of other minerals such as hypersthene, olivine, pyroxene, and plagioclase (Hendricks and Ross * Present address: Department of Geology, Arizona State Uni- 1941; Wise and Eugster 1964; Foster 1969; Honda and versity, P.O. Box 871404, Tempe, Arizona 85287-1404, U.S.A. Muf¯er 1970; Andrews 1980; Cathelineau and Izquierdo 0003±004X/97/0506±0503$05.00 503 504 LI ET AL.: FERROCELADONITE AND FERROALUMINOCELADONITE 1988; Odin et al. 1988). Transmission electron micros- posited in the National Museum of Natural History, the copy (TEM) and analytical electron microscope (AEM) University of Michigan, and the Geology Department, data demonstrate that submicrometer mixed layering is University of Otago (OU 26049). common in ®ne-grained phyllosilicates formed at low grades (Peacor 1992), and such mixed layering is ex- OCCURRENCE AND ASSOCIATED MINERALS pected to occur with members of the celadonite family. The sample used in this study (OU 26049) is an al- The celadonite X-ray powder diffraction (XRD) pattern tered crystal-vitric tuff from the Gavenwood Tuffs (early consistently displays re¯ections corresponding to the 1M Triassic marine, Murihiku Supergroup), grid reference mica polytype (Yoder and Eugster 1955; Wise and Eugs- E45 649657, Hokonui Hills, Southland, New Zealand. ter 1964; Buckley et al. 1978; Andrews 1980; Odom Microprobe analyses of heulandite, plagioclase clasts, 1984; Odin et al. 1988). However, powder XRD data are and celadonite in this rock were reported by Boles commonly complicated by the presence of intimately in- (1972) and Boles and Coombs (1975). Broken surfaces tergrown minerals, some of which are also 2:1 phyllo- of the hand specimen are dark greyish-green in color silicates (Hendricks and Ross 1941; Andrews 1980; with reddish ¯ecks and show feldspar clasts up to about Loveland and Bendelow 1984; Odin et al. 1988). Optical 1 mm in length. Larger dark clasts, 1±3 mm in diameter, properties derived from aggregates that may include other prove in thin section to be micropumiceous shards and phyllosilicates are frequently only approximations to lapilli. Vitroclastic texture is well developed, with cus- those of celadonite, and electron microprobe analyses of pate shards typically 0.2±0.6 mm in length. Glass in the such material commonly include intergrown phases (Li et cuspate shards and much of that in the pumiceous lapilli al. 1996). has been replaced by ®brolamellar or platy Ca- and Si-r- End-member celadonite and the end-member Fe21Fe31 ich heulandite; the interior of the shards commonly is analogue have been synthesized by Wise and Eugster pigmented red or yellow. Heulandite also occupies some (1964), but attempts by these authors to synthesize the Al of the microvesicles. Celadonite-family materials ®ll end-members were not successful. Velde (1972) reported clusters of microvesicles and partially replace glass in syntheses with substantial solid solution between the the pumiceous shards, collectively forming dark blue- MgFe31 and Fe21Fe31 end-members and much more lim- green patches. Vesicles are commonly zoned with a ®lm ited substitution of MgAl for MgFe31. The above synthe- of celadonite-family mineral lining the rims and larger ses were conducted at temperatures of about 300±430 8C celadonitic ¯akes admixed with chlorite in the centers. and (mostly) 2 kbar ¯uid pressure and were mostly un- Some vesicles are ®lled only with chlorite, and chlorite reversed experiments. They do not provide de®nitive in- also occurs in the matrix. Color ranges from bright glau- formation on possible solid solution and stability rela- cous-green celadonite through olive-green to pale-green tionships at the lower temperatures of the zeolite and chlorite. TEM studies reveal that mixed-layer chlorite- prehnite-pumpellyite facies. berthierine and chlorite-corrensite are intimately intergrown Boles and Coombs (1975) obtained electron micropro- with the chlorite host. Other authigenic minerals include be analyses of celadonitic minerals occurring in altered titanite as a dusting of micrometer-sized granules, minor crystal-vitric tuffs from the Hokonui Hills, Southland, pyrite and quartz, localized ®ne-grained twinned albite, New Zealand, suggesting substantial solid solution but and rare prehnite. Rare siderite of composition with probable contamination from intergrown chlorite. To (Fe0.76Mn0.10Mg0.01Ca0.13)CO3 occupies solution cavities in determine the compositions of the pure minerals as dis- former glass shards and microfractures in the matrix. tinct from mixtures, we have re-analyzed one of the Ho- Abundant detrital plagioclase, zoned and ranging from konui Hills specimens using TEM and AEM. The results labradorite to oligoclase, is accompanied by lesser reveal a broad range of solid solution among the cela- amounts of augite, magnetite, and rare hornblende. 31 donite-family components K2Mg2Fe2 Si8O20(OH)4 (cela- Quartz and biotite are very sparse. There are also scat- 31 donite), K2Mg2Al2Si8O20(OH)4,K2Fe2Fe2 Si8O20(OH)4, tered lithic volcanic fragments of andesitic to rhyolitic 21 and K2Fe2 Al2Si8O20(OH)4. Schaller (1950) suggested that composition. an old name, leucophyllite, be applied to micas of end- METHODS member composition K2Mg2Al2Si8O20(OH)4. With an al- kali content of only 3.39% K2O and 1.42% Na2O, the The XRD data were obtained for , 0.2 mm separates analysis on which this suggestion was based is likely to be (obtained by settling in a water column), using a Philips from a mixture. Schaller's proposal has not achieved accep- automated diffractometer with graphite monochromator tance, and celadonite itself was the only one of four end- and CuKa radiation (35 kV and 15 mA) with quartz as members that had been accepted as a valid mineral species. an internal standard. A step size of 0.02 82u and a long The data of our current study allow characterization of the counting time of 6 s per step were used, the latter to 31 new minerals ferroceladonite, K2Fe2Fe2 Si8O20(OH)4, and increase resolution of weak peaks.
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