Natroapophyllite, a new orthorhombic sodium analog of apophyllite; I, Description, occurrence, and nomenclature

Natroapophyllite NaCaoSirO2oF ' 8H2O, a sodium analog of apophyllite, has been found in white skarn at the Sampo Mine, Okayama, Japan. Electron microprobe analysis of the mineralgives SiO2 52.79, CaO25.4l, NarO 3.05, K2O 0.33, F 2.27, HrO (by difference) l7.ll weight percent; (-O=F' : 0.96). Optical and crystallographic parameters are: a: 1.536(2), B : 1.538(2) , y : l .5 tA(2) ,2V(meas.) :32( l ) " , r<v;a:8.875(4) , D:8.881(6) , c : 15.79( l )A, space group Pnnm, Z : 2; D obs. : 2.50, D calc. : 230 g/cm3. The crystal structure, refaed (R : 0.056) by least-squares methods from 563 ditruse X-ray reflections collected with an automatic single-crystal difractometer, has rings of SiOo tetrahedra. Caand Na-polyhedra are more distorted in natroapophyllite compared with apophyllite because of the substitution of Na for K. TGA shows discontinuous water loss of 16 weight percent, similar to apophyllite. DTA and IR data suggest hat natroapophyllite has hydrogen atoms in three di-fferent structural environments. A pseudo solid-solution series exists between natroapophyllite and fluorapophyllite which bridges the orthorhombic-tetragonal symmetry change. Crystals are shown which display continuous and discontinuous zoning. Introduction The nomenclature of the apophyllite group is complex. Fluorapophyllite and hydroxyapophyllite have recently been defined by Dunn et al. (1978) as the valid names for the end members of the tetragonal apophyllite series, in which K is the major alkali cation. Orthorhombic varieties of apophyllite have also been described. Sahama (1965) gave details of an orthorhombic fluorine-rich apophyllite KCa.AlSi, OroF'8HrO, which contained significant Al. Although Na may substitute for K in apophyllite to a small degree, samples in which Na predominates over K have only recently been described (Matsueda, 1975, 1980; Miura et al., 1916; Miura, 1977; Miva I On leave from Department of Mineralogical Sciences and Geology, Faculty of Science, Yamaguchi University, Yamaguchi 753' Japan. and Rucklidge, 1979). The Na-rich examples are invariably orthorhombic and fluorine-rich, and the name natroapophyllite has been proposed for this sodium analog of the orthorhombic apophyllite described originally by Sahama (1965) and Belsare (1969). It is not the sodium analog of ordinary tetragonal apophyllite. The name natroapophyllite, given in allusion to its compositional relation to apophyllite, was approved by the IMA Commission on New Minerals and New Mineral Names in October, 1976. Other additional prefixes for natroapophyllite would be in order, e.9., orthoand fluor-, but in the absence of non-orthorhombic or other than fluorine-rich natroapophyllite, it seems unnecessary to burden the literature with more complex names than natroapophyllite, which is defined as being the orthorhombic sodium end member of the apophyllite group minerals with the formula NaCaoSi'OroF'8HrO. 0003-004x/8 I /030+04 l0$02.00 410 MATSUEDA ET AL.: NATROAPOPHYLLITE Fig' l. Optical micrographs of natroapophyllite from the Sampo Mine. (a) Composite crystals of typical natroapophyllite (SA-lb) and fluorapophyllite (SA-la). SA-l samples are up to 0.15 x I mm. (b) Composite natro(SA-2a) fluorapophyllite (SA-2b) crystals and intermediate compositions (SA-2c, d, e). Sample descriptions, physical properties, and morphology The four samples described here have SA-numbers referring to the Sampo Mine, some l0 km west of Takahashi City in Okayama Prefecture. The Sampo Mine is one of the typical contact metasomatic ore deposits of the Chuhgoku Province and was emplaced at the contact of Paleozoic limestone and slate with Late Cretaceous granite (Matsueda, 1973, 1980). Natroapophyllite occurs in association with zeophyllite, cuspidine, apophyllite, and calcite in the 'hrhite skarn" on the 9th level, No. I Yoshiki Ore Body of the mine. Other associated minerals include andradite, xonotlite, wollastonite, a clinopyroxene in the diopside-hedenbergite series, magnetite, quartz, and native bismuth, all of which occur in banded zones between the granite and marble contacts. Natroapophyllite was found in the white skarns close to the marble contact (Matsueda, 1973, 1980). The fluorinebearing silicates zeophyllite and apophyllite crystallized during a hydrothermal alteration stage, which occurred after the thermal metamorphism and skarn generation by the granite intrusion (Matsueda, 1980). The white skarn was formed by fluorine metasomatism of ferrobustamite and wollastonite-andradite skarns (Matsueda, 1980). In transmitted light the grains of natroapophyllite are invariably rimmed by apophyllite (Fig. l), which suggests that a pseudo solid solution exists between apophyllite and natroapophyllite, in which there is a transition from tetragonal to orthorhombic symmetry. This would represent a range of chemical variation beyond the solid-solution series fluorapophyllite to hydroxyapophyllite reported by Dunn et al. (1978) and Dunn and Wilson (1978). Natroapophyllite samples SA-l to 3 in Table I occur as minute prismatic euhedral to subhedral crystals up to 0.15 x I mm (Figs. I and2), brownish-yellow to yellowish-brown (in SA-l and 3) or colorless to white (in SA-2). The luster is vitreous to pearly. A typical example of natroapophyllite in SA-l is zoned with fluorapophyllite, and the two phases are referred to in Table I as SA-lb and SA-la, respectively. Composite crystals exhibit {100} prisms truncated by the {l I l} dipyramid striated perpendicular 4t2 MATSUEDA ET AL: NATROAPOPHYLLITE Table l. Occurrences, parag€nes€s, compositions, and optie axial angles of fluorapophyllite and natroapophyllite from the Sampo Mine Spec iEen No. Occurrence Paragenes is A lka l j con tenE* 2V(neas . )