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Articles |
1 Laboratorium für chemische und mineralogische Kristallographie, Universität Bern, Freiestr. 3, CH–3012 Bern, Switzerland
2 National Center for Water Quality Research, Heidelberg College, Tiffin, Ohio 44883, USA
3 Institut für Geologie, Universität Bern, Balzerstr. 1, CH–3012 Bern, Switzerland
E-mail address: thomas.armbruster{at}krist.unibe.ch
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ABSTRACT |
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 Top Abstract IntroductionGeological Setting and Sample...ExperimentalResultsDiscussionAknowledgementsReferences |
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2.92(Si3.28Al0.72)4O10[(OH)1.14F0.86]2 has the highest Si/Al value of natural samples of phlogopite known to us. The epitactic fibers of tainiolite, of composition K1.01(Mg2.00Fe0.01Li0.59Na0.38)2.98(Si3.99Al0.01)4O10[F1.79Cl0.01 (OH)0.20]2, represent to our knowledge the first example of an extended solid-solution, established by combined electron-microprobe and laser-ablation mass-spectrometry analyses, between tainiolite, K(LiMg2)Si4O10F2, and the recently described mineral shirokshinite, K(NaMg2)Si4O10F2. The latest mineral formed in the observed paragenesis is the zeolite mesolite, accompanied by an unidentified aluminosilicate with Ca/K 
1.
Keywords: phlogopite, tainiolite, shirokshinite, mica, fibers, epitactic growth, Mont Saint-Hilaire, Quebec.
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INTRODUCTION |
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TopAbstractIntroductionGeological Setting and Sample...ExperimentalResultsDiscussionAknowledgementsReferences |
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In this paper, we report on the epitactic growth of tainiolite, a trioctahedral mica of ideal composition K(LiMg2)Si4O10F2, on phlogopite, another trioctahedral mica of simplified end-member composition KMg3(AlSi3)O10(OH)2. The sample originates from marble xenoliths of the alkaline igneous rocks at Mont Saint-Hilaire, Quebec, Canada. At first glance, one may expect rather isotropic overgrowth of one sheet silicate on the other. However, in the example investigated, epitactic tainiolite crystallizes with a fibrous habit, and all fibers exhibit a uniform orientation relative to the phlogopite host.
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GEOLOGICAL SETTING AND SAMPLE DESCRIPTION |
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TopAbstractIntroductionGeological Setting and Sample...ExperimentalResultsDiscussionAknowledgementsReferences |
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The Poudrette Quarry is developed in rocks of the East Hill suite, which is a product of the last intrusive event. Among the products of this event is a pipe of nepheline and sodalite syenites, phonolites, and intrusive breccias containing many xenoliths of older igneous and sedimentary rocks, including a massive block of Ordovician sediments that was engulfed by the magma and transformed to hornfels (Van Velthuisen 1990). Well-formed crystals of a wide variety of minerals have been recovered from pockets in these rocks. The complex geochemistry and petrogenesis have produced many paragenetically complex associations involving one mineral replacing or overgrowing another, generally epitactically oriented.
The samples for this study were collected by Richards on October 24, 2004 from a pectolite zone in a marble xenolith. This xenolith was not seen in place. Consequently, we do not know the identity of the host rock. Whereas marble xenoliths at Mont Saint-Hilaire have been interpreted as clasts of Precambrian basement incorporated into the rising magmas, at least some xenoliths must be younger because they contain fossils (McDonald & Chao 2004). In 1994, Richards found a brachiopod replaced by pectolite and sodic amphibole. Although the sample was found loose in the quarry, the mineral composition is characteristic of the marble xenoliths. The specimen, now in the Canadian Museum of Nature, has been identified as a spiriferid, probably of early Silurian age (McDonald & Chao 2004). The part of the xenolith from which the samples were collected is characterized by a loose, sugary texture, and includes well-formed crystals of dolomite and pectolite to 2 cm and mm-sized crystals of albite, which in turn are coated with small crystals of quartz and locally with the epitactic association and other minerals reported here. Some of the dolomite has an epitactic overgrowth of calcite.
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EXPERIMENTAL |
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TopAbstractIntroductionGeological Setting and Sample...ExperimentalResultsDiscussionAknowledgementsReferences |
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(
Z) algorithm (Armstrong 1995). Detection limits for K, Na, Ca and Cl are 0.01 wt.%, 0.02% for Si, Ti, Cr, Al, Mg and Mn, 0.06% for Fe and 0.10% for F. The micas embedded in epoxy (the same crystals as investigated by EMP and SEM) were analyzed for Li using laser-ablation inductively coupled plasma – mass spectrometry (LA–ICP–MS) at the Institute of Geological Sciences in Bern. The instrument consists of a pulsed 193 nm ArF Excimer laser (Lambda Physik, Germany) with an energy-homogenized Geolas Pro optical system (Microlas, Germany), coupled with an ELAN DRC–e ICP quadrupole mass spectrometer (Perkin Elmer, Canada) operated in standard mode. The laser energy used was ca. 6 J cm–1. Operating conditions were similar to those reported in Pettke et al.(2004). A laser spot 30–50 µm in diameter was used for sample ablation. Five spots for tainiolite and three spots for phlogopite were sampled. Data were reduced with the Lamtrace program, using the electron-microprobe data on K2O for tainiolite and phlogopite as internal standard. The SRM 610 glass from NIST was used as the external standard to calibrate analyte sensitivities.
The X-ray single-crystal study on two composite phlogopite–tainiolite assemblages was performed with an Enraf Nonius CAD4 diffractometer using graphite-monochromatized MoK
X-radiation. In addition, reciprocal-space X-ray-diffraction images were displayed with a CCD camera of a Bruker SMART 1 K system. The goal of these experiments was to explore the epitactic relationship between the two sheet silicates by analyzing the overlay of the reciprocal lattices of host and guest crystals and to determine polytype and cell dimensions at high scattering-angles.
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RESULTS |
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TopAbstractIntroductionGeological Setting and Sample...ExperimentalResultsDiscussionAknowledgementsReferences |
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10], which also are expected directions of growth owing to the pseudohexagonal symmetry of mica sheets. The blocks of tainiolite (Fig. 1d) growing on (010) of phlogopite appear particularly fragile because fiber growth is parallel to the contact plane. The blocks also tend to split because they are subperpendicular to the (001) cleavage of both minerals. Some phlogopite crystals display dissolution steps on the (001) surface (Figs. 1a–c). Such holes are filled with fibrous aggregates of tainiolite without obvious relation in orientation to the host crystal (e.g., Fig. 1b).
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DISCUSSION |
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TopAbstractIntroductionGeological Setting and Sample...ExperimentalResultsDiscussionAknowledgementsReferences |
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Our analytical results on phlogopite from MSH are unusual owing to their low level of IVAl (Table 1). In fact, excluding trioctahedral micas with tetrahedral Fe3+, such as tetra-ferriphlogopite, the observed IVAl content is lower than that in all samples of natural phlogopite for which structural data are available (Brigatti & Guggenheim 2002). However, crystal structures of synthetic phlogopite-related crystals with compositions of KMg2.5Si4O10F2 (Toraya et al. 1976), KMg2.75[Si3.5Al0.5]O10F2 (Toraya et al. 1978), and KMg3[Si3Al]O10F2 (Takeda & Morosin 1975) have been determined. In the first two compounds, the deficit in IVAl is charge-balanced by vacancies at the octahedral site.
The misfit between the sheets of tetrahedra and octahedra in mica causes deformation of the sheet of tetrahedra, lowering the sheet symmetry from hexagonal to trigonal. The degree of deformation is expressed in terms of the rotation angle of the SiO4 tetrahedra (e.g., Bailey 1984). Increasing leads to a reduction of the lateral dimension of the sheet of terahedra. The structural studies on synthetic "fluoro-phlogopite" cited above have shown that increasing content of IVAl (from 0 to 1 apfu), which actually expands the lateral size of the sheet of tetrahedra, is partly balanced by an increase of from 1.42° to 6.5°. In contrast to the synthetic samples, the MSH phlogopite acting as host for tainiolite crystallization is a stoichiometric trioctahedral mica with all octahedral sites close to fully occupied (Table 1). The low IVAl level is caused by VILi+ content requiring a higher charge in the sheet of tetrahedra. The Fe/(Fe+Mg) ratio in our material is between 0.32 and 0.25, leading to a significant extension of the sheet of octahedra, compared to the synthetic samples, as indicated by increased unit-cell dimensions in the (001) plane (Table 2). The combination of high level of VIFe2+ substitution and low IVAl suggests low rotation angles for the phlogopite found as host crystals of the epitactic tainiolite.
Preliminary results of the energy-dispersion analyses of tainiolite epitactic on phlogopite from MSH indicate the presence of significant Na. An external source of Na [e.g., Na migration under the electron beam (Spray & Rae 1995)] must be excluded because all analyses were performed on isolated flakes of mica, as displayed in Figure 1. Furthermore, Na contents between 2.5 and 3.5 wt.% Na2O were subsequently confirmed by EMP and LA–ICP–MS analyses. Normalization of the tainiolite data on 22 negative charges (11 O) yields the composition (Mg2.00Fe0.01Li0.59Na0.38)2.98 for the sheet of octahedra (Table 1). Thus the epitactic tainiolite would represent a solid solution between tainiolite, K(LiMg2)Si4O10F2 (Toraya et al. 1977) and shirokshinite, K(NaMg2)Si4O10F2 (Pekov et al. 2003). Both structures show very similar cell dimensions and distortions (Table 2). In particular, both minerals have the sheet of tetrahedra fully expanded, giving rise to very low rotation angles (), as already predicted for the phlogopite host. In shirokshinite, Na–Mg order is close to complete (Pekov et al. 2003), and Na is concentrated at the larger M1 site (on the mirror plane). In contrast, Li–Mg order in tainiolite is less pronounced, and Mg occupies 71% of M1 and 66% of M2 (Toraya et al. 1977).
The issue of VINa in lithian micas was addressed and critically reviewed by Pekov et al.(2003). Their electron-microprobe results on alleged lithian micas with VINa did not confirm the results of previous wet-chemical analyses on corresponding samples. The authors concluded that evidence of VINa in mica obtained from wet-chemical analyses must be considered with great caution because of the possibility of contamination with impurities in the large sample required. In fact, some of the quoted results of wet-chemical analyses match stoichiometry of a mica very poorly. In all re-analyzed samples (Pekov et al. 2003) and also in other samples of tainiolite and polylithionite from the Khibiny massif, the Na content is less than 1 wt.% Na2O, and (K + Na) never is over 1.0 apfu within standard deviations.
On the basis of the critical evaluation of older analytical results by Pekov et al.(2003), the epitactic tainiolite on phlogopite from MSH would seem to represent the first example of an extended solid-solution member between tainiolite and shirokshinite determined by EMP and LA–ICP–MS techniques. Thus, the conditions of formation of this unique sodian tainiolite are of special importance.
The paragenesis (Fig. 2), based on overgrowth relationships, is pectolite, NaCa2Si3O8(OH), followed by albite and dolomite. Phlogopite formed next, and was overgrown and partially replaced by tainiolite. Quartz then crystallized, covering most exposed surfaces. Calcite then crystallized epitactically on exposed dolomite. Mesolite was the final phase to form, and occurs as isolated crystals and clusters of crystals on quartz, phlogopite, tainiolite and dolomite. The other minor phases are post-tainiolite, but their exact location in the paragenesis cannot be established with the materials available. During this paragenesis, pectolite was partially corroded and may have served as source of Na for sodian tainiolite.
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Orientation of the tainiolite fibers
At first glance, it is most puzzling that the epitactic tainiolite fiber-orientation does not follow the pseudohexagonal symmetry of the sheet of tetrahedra in micas, but instead the fibers grow only parallel to [100] of phlogopite (Figs. 1a–c). Fibrous growth of sheet silicates is not common except for chrysotile and related minerals, where the sheets are rolled up to cylinders, forming hollow fibers. Such a case is completely different from fibrous tainiolite, for which a single fiber may be envisioned as a flat ribbon-like portion of a sheet silicate structure. For this reason, tainiolite fibers are very flexible and easily bent, in contrast to chrysotile fibers, which are elastic. In this context, we like to recall the Greek origin of the name tainiolite, which was derived from "tainia", a band or ribbon, which emphasizes the general tendency to form rather elongate bands or ribbons rather than the pseudohexagonal platelets typical of most other micas. Nevertheless, the fibrous habit of tainiolite observed on the MSH samples is highly unusual.
Güven (2001) described authigenic fibers of illite grown epitactically on kaolinite and mica cores found in sandstones. He observed three epitactic relationships: (1) a single set of illite laths oriented parallel to [100] of a euhedral core of kaolinite, (2) double sets of laths oriented at an angle of 120° to each other, (3) triple sets of laths following the pseudohexagonal symmetry of the sheet of tetrahedra. Relationship (1) is an analogue to our finding. Güven (2001) also reviewed experimental studies producing fibrous illite. In particular, diagenetic conditions (low temperature) and the presence of organic acids seem to stimulate illite-fiber precipitation. It is assumed that illite laths are metastable forms that eventually recrystallize to platy equilibrium forms.
We cannot provide direct evidence for the conditions of epitactic growth of fibrous tainiolite on phlogopite, but the mechanisms seems to be in line with the arguments stated by Güven (2001). The crystal structure of a mica provides three dominant directions of growth (Fig. 3), namely [100], [10] and [0]. Growth of mica parallel to [001] requires two-dimensional nucleation or screw dislocations. In contrast, tetrahedra and octahedra can be readily attached along [100], [10] and [0] without need of nucleation. The growth fronts (010), (110) and (10) are parallel to the growth chains [100], [10] and [0], respectively. Laths and fibers develop if the growth rate on the (010) front is slower than on the (110) and (10) fronts. Development of epitactic fibers of tainiolite along [100] and parallel to [100] of phlogopite requires special conditions. Thus, there must be some mechanism retarding growth parallel to the (010) front. The atomic configurations exposed on the (010), (110) and (10) fronts are considerably different (Fig. 3). The surface of (010) exposes terminal OH groups or F as ligands of octahedra and also densely packed rows of interlayer K (Fig. 3). This growth front must be somehow "poisoned" to explain fibrous growth parallel to [100].
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10] and [0]. Güven (2001) interpreted the simultaneous occurrence of double and triple sets of illite fibers separated by 60° on a kaolinite core as a consequence of stacking faults in the kaolinite. The prevalence of the tainiolite fibers parallel to [100] of phlogopite is in accordance with the lack of evidence for twinning or stacking faults in phlogopite analyzed by us.
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AKNOWLEDGEMENTS |
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TopAbstractIntroductionGeological Setting and Sample...ExperimentalResultsDiscussionAknowledgementsReferences |
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REFERENCES |
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Received January 11, 2006 ,revised manuscript accepted October 30, 2006.
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