MINING GEOLOGY, 36(3), 219•`230, 1986 The Stability Relationships of Zunyite Under Hydrothermal Conditions L. C. HSU* Abstract: The stability relationships of zunyite, Al13Si5O20F2(OH, F)16Cl , were determined at Pi=1kb and T=300-600•Ž in solutions with constant HCl=1M but different HF=0.1 to 20M. The products of stoichiometric oxide mix seeded with natural zunyite in respective solutions at 400•Ž were used as starting materials. The stability field of zunyite is rather limited; it is stable in a solution with HF concentrations no more than 2M. In 1M HF solu- tion, zunyite is stable up to 450(5)•Ž and is replaced by topaz at higher temperatures. It appears to remain stable at the same temperature range in solutions with HF as low as 0.1M. In solution with HF concentration of 3M, topaz is stable at temperature above 470(5)•Ž. At lower temperatures topaz is accompanied by AlF3 phase. This assemblage is also stable at higher temperatures in solutions with HF concentrations up to 10M. In solutions with high HF con- centrations, quartz+AlF3 become stable. The rare occurrence of zunyite compared to topaz is the consequnce of stringent chemical conditions aside from temperature. Both F and Cl are indispensable for the mineral and yet higher F activities destabilize it. The occurrence of zunyite as a massive body in altered felsic volcanics reported elsewhere is thus indicative of unusual solution behavior and physico-chemical condition during the alteration. Introduction timate intergrowth of zunyite and quartz,hav- ing as much as 75 percent zunyite by volume Zunyite was originally described from the with local solid pure zunyite. The texture sug- Zuni mine, Colorado (HILLEBRAND,1885). It gests that the lode is a replacement product of was subsequently found in adjacent mining andesitic country rock. districts in Colorado (PENFIELD,1893; BUR- The significance of zunyite may well be a BANK, 1932) and in Utah (LOVERINGand double one. In its own right zunyite is signifi- STRINGHAM,1945). In these places the mineral cant as a mineral resources, e.g. used for is commonly found in altered and silicified refractory material (MASUIand SHIMIZU,1944), igneous rocks as alteration product of feld- used as catalytic cracking to improve gasoline spars and/or as gangue mineral in yield (ENGEBRETSON,1965), used as aluminum metalliferous vein. Similar occurrence of source along with alunite in future (HALL, zunyite was reported in other parts of the 1978), and possible source of fluoride. Zunyite world, e.g. Algiers (TURCO, 1962a), Burma is a mineral characteristic of advanced argillic (BARIC and ZALOKAR, 1965), Morocco alteration (MEYER and HEMLEY, 1967) and (LABLANC,1970), Puerto Rico (HILDEBRAND, occurs almost exclusively in altered and 1961) and Ukraine (KLITCHENKOet al., 1976). mineralized zones in association with fluorite, Recently a huge zunyite lode with an outcrop topaz, Be-minerals, and many sulfide ore of about 1200 ft. long and 10 to 60 ft. wide, minerals (e.g. DIETRICH,1965; HILDEBRAND, forming a cliff as much as 10 ft. high was 1961; KASHIKAIand ALIEX, 1960). Thus the oc- discovered in Elko County, Nevada (COATS,et currence of zunyite in certain assemblage may al., 1979). The zunyite lode is a fine-grained in- serve as a clue to discovery of certain economic mineral deposits. Received on February 3, 1986, Accepted on March 23, Zunyite is one of the few minerals to con- 1986. * Nevada Bureau of Mines/Geology and Department of tain as many as three different volatile com- Geological Sciences, Mackay School of Mines, Universi- ponents, viz, H2O, F and Cl. All these three ty of Nevada, Reno, Nevada 89557, USA. components are the essential constituents in Keywords: Zunyite, Topaz, A1F3, Hydrothermal altera- hydrothermal solutions capable of carrying tion, Hydrothermal synthesis, Stability relation. and depositing minerals of economic value 219 220 L. C. HSU MINING GEOLOGY Table 1 Materials used in experiments and significance. Experimental investigation with an aim to synthesize zunyite and to deter- mine its thermal stability were almost ex- clusively conducted by the European scholars, particularly in France. There were conflicting results obtained in different laboratories. ALTHAUS (1966) claimed a success in synthesiz- ing F-free zunyite from a hydrothermal system containing quartz, kaolinite, pyrophyllite and HCl solution. BAUMER and TURCO (1968) after reproducing the experimental conditions of ALTHAUS, however, concluded that the syn- thetic product was in fact Al24O11(OH)44Cl6 rather than zunyite. On the other hand, TURCO (1962a, b) stressed that the presence of F was essential for the formation of zunyite but Cl was not so. The region of stability lies between * Prepared from: 470-600•Ž under unspecified Pf (fluid pres- Fisher Scientific conc. HCl solution, 37.5%, sp. gr. 1.18. Mallinckrodt Chemical conc. HF solution, 48%, Sp, gr. 1.2. sures). In all these experiments, either concen- trated acids or solid compounds (AlF3 and after 2 to 5 months run depending on AlCl3¥6H2O) were used for source of F and Cl temperature. In this way the fugacities of the and the duration of runs was only several days volatile components may be controlled by the at most. Thus, it is no surprise to have conflict- initial solution compositions and the reactions ing results, because either the experimental may approach as closely as possible to conditions deviated too far from the natural equilibrium. For synthesis of F-free zunyite, ones and/or the equilibrium of reactions was pure HCl solutions of different concentrations seldom attained. In this investigation, it is with products of seeded oxide mix were used hoped that the stability relations and chemical in repeated runs. Seeded solid mix containing composition of zunyite can be determined une- AlCl3, Al2O3 and SiO2 was also used but not quivocally by adopting different experimental for repeated run. Synthesis of Cl-free zunyite approaches. Such information will, in turn, was conducted in a similar way, except that serve as a key to the understanding of natural seeded solid mixes used allowed different F/ hydrothermal environment under which eco- OH proportions. nomic minerals along with zunyite are depos- The possible crystalline phases in the system ited. Al2O3-SiO2-H2O with HF and HCl as addi- Experimental tional components are shown in Fig. 1 where abbreviation of each phase is also indicated. The stability relationships of zunyite were determined at Pf=1 kb and T=300 to 600•Ž Results in solutions with constant HCl=1M but Description of Synthetic Phases different HF=0.1 to 20M (Table 1). The pro- Zunyite: The structure of zunyite (Fig. 2) is ducts of stoichiometric oxide mix of ƒ¿- built up of unique Si5O16groups of linked Si- cristobalite and Al2O3¥XH2O seeded with tetrahedra combined with Al12O16(OH)30 natural zunyite in respective solutions at groups of linked Al-octahedra. The arrange- 400•Ž were used as starting materials. Each ment requires the inclusion of at least two F run contains 50 mg charge and 50 ƒÊl solution atoms per stoichiometric molecule resulting in of respective composition sealed in a Ag30Pd70 the formula for the mineral. The 13th Al atom capsule 20 mm•~3 mm size. The respective is in tetrahedral coordination, the AlO4 solution was replenished 2 to 3 times each tetrahedra being isolated from SiO4 tetrahedra 36(3), 1986 Stability of zunyite under hydrothermal conditions 221 Fig. 1 Crystalline phases in the system Al2O3-SiO2- H2O-HF-HCl. Open circles and rectangles are phases containing volatile components. Fig. 3 Synthetic zunyite: (a) photomicrograph as grain mounting in n=1.480 immerion oil, plane light; (b) scanning electron photomicrograph. Run products ZNE5a and ZNE3b, respectively. it defies detailed study under petrographic microscope (Fig. 3a). Its characteristic tetra- hedral form is, however, well-revealed under scanning electron microscope using secondary electron detector (Fig. 3b). With calibrated Co powders as internal standard (2ƒÆ CuKƒ¿ for Co 104=35.183•‹), the synthetic zunyite has a =13.870-13.899(3)•ð based on 333/511 reflec- Fig. 2 Crystal structure of zunyite after PAULING tion. Thus the synthtic zunyite show minor (1933), KAMB (1960), and LOUISN.ATHANand GIBBS compositional variation but to a less extent (1972); (a) group of five silicon tetrahedra, [Si5O16] than that by TURCO (1962a, b) which has a= , (b) group of twelve aluminum octahedra [Al12O16 13.85 to 13.92 •ð. BAUMER (1975) shows that (OH)30], (c) a portion of the zunyite structure: one his synthetic hydrozunyite (OH- replacing AlO4 tetrahedron is marked with Al; smaller SiO4- 4) has a=13.904(5)•ð. Natural zunyite spheres represent O2-, larger Cl- ions. from the Zuni Mine has a=13.820 (PAULING, 1933) and that from Nevada has a=13.850 •ð. and other AlO6 octahedra and the Cl atom oc- It appears that the synthetic zunyite may be in- cupying cavities in the framework. variably more hydrous than the natural one. The synthetic zunyite is so fine-grained that The Cl-free zunyite was metastably synthe- 222 L. C. HSU MINING GEOLOGY Fig. 4 X-ray powder diffractograms of zunyites: (a) natural zunyite from Nevada, (b) synthetic zunyite ZNE3b; (c) synthetic Cl-free zunvite ZNM7. sized from seeded solid mix M; such zunyite thoroughly mixed KBr and 0.2% zunyite has a=13.858-13.872(3)•ð. The X-ray powder powders were peletized into a clear disk under diffractograms of natural zunyite, synthetic 65 tons per square inch pressure. The IR ab- zunyite and synthetic Cl-free zunyite are sorption spectra of these zunyites are shown in shown in Fig. 4. Fig. 5. The synthetic zunyite shows a strong ab- The infrared absorption spectra of these sorption band at 3330 cm-1, a weaker one at zunyites were obtained on a Perkin-Elmer 3630 cm-1, and a still weaker but sharp one at Model 599 double-beam spectrometer in the 3678 cm-1.