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Strontiojoaquinite and Bario-Orthojoaquinite: Two New Members of the Joaquinite Group

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Strontiojoaquinite and Bario-Orthojoaquinite: Two New Members of the Joaquinite Group American Mineralogist, Volume 67, pages 809-8/6, /982 Strontiojoaquinite and bario-orthojoaquinite: two new members of the joaquinite group WILLIAM S. WISE Department of Geological Sciences University of California Santa Barbara, California 93106 Abstract Two new minerals have been found in San Benito County, California, that are structurally and chemically related to joaquinite. A combination of electron and ion microprobe analyses of strontiojoaquinite yields the formula (Na2.2sLio.IIFeI.44Do.17) D2Ba4Ti4[01.27(OHh.73]Sr4.oIREEo.oJ[Si4012kI.62H20. The virtual absence of REE indi- cates that this is the Sr analog of joaquinite. X-ray single crystal study showed the mineral to be monoclinic with the same submicroscopic twinning and one dimensional disorder exhibited by joaquinite. Several reflections in the X-ray powder pattern show that the space group must be P2, Pm, or P2/m, as compared to the C2 of joaquinite. Refined cell dimensions are a 10.516(6), b 9,764(5), c 11.87(1)A, f3 109°17(4)' Z = I. The strongest lines in the X-ray powder pattern are 2,801(100)(004,311), 2.967(72)(023), 3,001(48)(113), 2.923(45)(301), 2.611(42)(401,402), 2.432(40)(312), and 4.30(35)(121). The pseudo-orthorhombic, bipyramidal crystals have the forms {11O},{II I}, and {001}. All crystals are zoned with ajoaquinite core. Strontiojoaquinite is green, yellow-green, and less commonly yellow-brown. Hardness (Mohs) is 5Y2; cleavage {001},good. Calculated density is 3.68 glcm3. It is optically biaxial, positive, 2V 35° to 45° with strong dispersion of the optic axes r > v. Refractive indices are a 1.710(2), f3 1.718(2), '}' 1.780(3). Optical orientation and axial colors: X (colorless):a = 19°, Y (colorless) = b, Z (yellow) = c; absorption X = Y < Z. A combination of electron and ion microprobe analyses of bario-orthojoaquinite yields the formula (Nao.IOFe3.61Mno.24)D2Ba4(Ti3.ssAIo.12)(Ba2.ssSro.ssCao.osAlo.19)04[Si4012]'2H20. The absence of REE and the excess Ba indicates that this mineral is the Ba analog of joaquinite. X-ray single crystal study showed it to be orthorhombic, space group Ccmm, CC2m, or Ccm2" a 10.477(5), b 9.599(1), c 22.59(I)A, Z = 2. The strongest lines in the X-ray powder pattern are 2.997(100)(224), 2.953(95)(132), 2.824(90)(008), 5.64(70)(004), 2.935(70)(117), 4.30(62)(203),3.203(50)(223), and 2.602(50)(401). Crystals are up to 8 mm in diameter, in the form of steep bipyramids {III} truncated by the basal pinacoid, and are yellow-brown with a vitreous luster. Hardness (Mohs) is 5Y2. Cleavage {001}, good. Measured density is 3.959(2) glcm3 (calculated, 3.962 g/cm3). It is optically biaxial, positive, 2V 10° to 15°, with strong dispersion of the optic axes r > v. Refractive indices are a 1.735(2), f3 I. 737(2), '}' 1.80(1). Optic orientation and axial colors: X(very pale yellow) = a, Y (pale yellow) = b, and Z (yellow) = c, absorption X < Y« Z. These two new minerals, along with others previously described, allow names of the joaquinite group minerals to be based on the symmetry and occupancy of the REE position: REE-bearing members are joaquinite and orthojoaquinite; Sr-bearing, strontiojoaquinite and strontio-orthojoaquinite; and Ba-bearing, bario-orthojoaquinite. Introduction initial discovery and naming (Louderback, 1909). The type locality is the well known Benitoite Gem The characterization of the mineral joaquinite Mine in San Benito County, California. X-ray and was not complete for nearly 70 years following its morphological studies by Palache and Foshag 0003-004X/82/0708-0809$02.00 809 810 WISE: STRONTIOJOAQUlNITE AND BARIO-ORTHOJOAQUlNITE (1932) suggested that the mineral had orthorhombic (00l), which gives rise to a pseudotetragonal mor- symmetry, and their chemical analysis yielded the phology and the basal cleavage. The Ti-bonded formula NaBa(Ti,FehSi401s. There was little fur- Si4012 sheets are held together in two ways, one by ther work on the mineral until two new occur- a sheet of Ba and 0 ions and water molecules, and rences, Seal Lake, Quebec (Bell, 1963) and Ilimaus- the other by Na, FeH, OH, and the ions of the REE saq, Greenland (Semenov et al., 1967) lead to position. The Na, Fe, and OH occur along 2-fold renewed interest in the mineral. Semenov and his axes and have a multiplicity of only 2. There is one co-workers found that the rare earth elements site preferred by Na and another by FeH (Dowty, (REE) are an essential part of the composition of 1975, Fig. 2), but in mostjoaquinites these positions joaquinite from all three localities. Laird and Albee and the OH position are not completely filled. In the (1972) conducted a thorough study of the X-ray minerals described here Na and Fe substitute for properties and the composition of a number of each other, and more importantly the REE ions are crystals from the type locality. This work, not only completely replaced by divalent ions such as Sr and further defined the composition, but also demon- Ba. strated that the crystals are monoclinic, although parts of a few were found to be orthorhombic. Group composition Furthermore, Laird and Albee found that the ortho- A number of joaquinite group crystals were ana- rhombic-appearing crystals exhibit submicroscopic, lyzed as part of this study, and a selection of polysynthetic twinning parallel to {001}. analyses is presented in Table 1. All samples were The crystal structure of joaquinite was studied by analyzed for the major elements with an electron Cannillo et al. (1972) on relatively untwinned mate- microprobe. Crystals or groups of crystals (about 1 rial from Greenland. Although they based their mm in diameter) were embedded in epoxy and analysis on the monoclinic space group C2/m and ground to expose the center of the crystals. Such did not detect several atoms, the basic structure did grains were analyzed in at least four spots to emerge. This structure was refined by Dowty (1975) determine the extent (if any) of compositional zon- using the space group C2, and the positions for the mg. remaining atoms were located. Standards used were benitoite (Ba, Ti, and Si), Semenov et al. (1967) and Laird and Albee (1972) hematite (Fe), rhodonite (Mn), celestite (Sr), albite found such variation in the composition of the (Na and AI), andesine (Ca), and synthetic rare earth crystals they analyzed, that they proposed a num- glasses (Drake and Weill, 1972). The analyses were ber of possible end members and referred to the carried out under the following conditions: acceler- whole as the joaquinite group. Recent collecting in ating voltage 15 kV for all elements, sample current San Benito County has lead to the discovery of two 8 nA, and a spot diameter 10 to 20 JLm. Emission members of the group, one iq. which the REE have data were reduced and interelement corrections been replaced by Sr and the other by Ba. The applied with a modified version of EMPADR7 (Ruck- compositional adjustments to maintain charge bal- lidge and Gasparrini, 1969). ance in these phases provides a further understand- Because two samples represented new composi- ing of the composition and symmetries of the group. tions for the joaquinite group, mass scans with an This paper describes these new minerals, fits their ARL ion probe were performed. The ion probe compositions into a general scheme, and proposes a microanalyses were carried out with a negatively system of nomenclature for the group. The new charged primary beam of monatomic 160- at 17 kV. minerals, mineral names, and group nomenclature The spot size was approximately 20 JLm and a have been approved by the I.M.A. Commission for sample current of 6 nA. The hydrogen content was New Minerals and Mineral Names. estimated from the working curves of Hinthorne and Anderson (1975), and other element data were The joaquinite group quantitatively reduced, using sensitivity factors de- Based on his structure refinement Dowty (1975) rived from a theoretical ionization model (Anderson proposed an ideal formula for joaquinite, and Hinthorne, 1973). Na2Fe~+(OHhBa4Ti404REE4[Si40I2k2H20. The Combined results of both microprobe methods structure is characterized by Si4012 rings (see Can- are presented in Table 1. The ideal formula for nillo et al., 1972, and Dowty, 1975), connected by joaquinite has an anionic charge of -106. If the Ti ions. These bonded rings form sheets parallel to number of cations in the analyses of Table 1 are WISE: STRONTIOJOAQUINITE AND BARIO-ORTHOJOAQUINITE 811 Table 1. Microprobe analyses and cell contents of various that the total anionic charge may be somewhat members of the joaquinite group from San Benito County, variable. Therefore, the unit cell contents were California proportioned by setting the number of Si to 16. The total cationic charge, the number ofTi ions, and the S102 37.54 35.87 37.57 34.99 35.15 36.16 34.75 TiOz 12.48 12.86 12.45 12.01 11.33 12.54 10.95 total number of ions in the X position can be used to Alz03 o o o o .57 .16 .24 check the quality ofthe analysis. If the oxygen ions FeO 4.03 3.91 5.04 3.96 9.47 3.13 3.23 MnO o .14 o .09 .62 .09 .41t between the Si4016 rings can be replaced by OH, 24.52 23.22 26.39 22.51 38.56 23.81 22.40 and if the hydroxyl sites on the Na and Fe 2-fold 16.23 8.37 12.45 4.07 3.34 4.91 .31 o o o o .17 o o axes (Dowty, 1975, Fig.
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