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"Skunk" calcite: mineral paragenesis in an amethyst geode from Ametista, Rio Grande do Sul, Brazil.

The famous amethyst geodes of Brazil and Uruguay rarely contain crystals of "skunk" calcite, wherein microcrystals of goethite form sharp, narrow black bands on rhombohedral calcite faces. Close study reveals the probable paragenetic relationships among goethite, lepidocrocite, calcite and two generations of amethystine quartz in a "skunk" calcite specimen; important uncertainties, however, remain.

INTRODUCTION

Amethyst-lined geodes occur in great numbers at certain horizons in Lower Cretaceous basalts in the Parana Basin. Such geodes, which reach weights of several hundred kilograms, are mined at numerous localities in Brazil's southernmost state, Rio Grande do Sul (Juchem, 1999; Mossman and Juchem, 2000), and in neighboring Uruguay. These geodes occasionally are found to contain crystals of what has been called "skunk calcite," so named because of the striped appearance. The geode discussed here, recovered near Ametista do Sul, weighs 23 kg. It is distinguished from the vast majority of other geodes in that it contains seven large (to 5 X 10 cm), distinctly terminated, honey-colored calcite crystals set amid numerous amethyst crystals (Fig. 1). Two generations of amethyst crystals are present, the earlier crystals--"quartz (I)"--reaching more than 1 cm, the later and slightly smaller crystals--"quartz (II)"--never exceeding that size. The geode also contains black goethite crystals confined within narrow stripes on the calcite crystals. Between the rhombohedral faces of the calcite crystals, paired scalenohedral faces occur, making a total of nine pyramidal faces on each of the calcite crystals: this combination of trigonal scalenohedral and rhombohedral forms is not uncommon (Gunter, 2003). It is, however, the presence of quartz (II) crystals and minute bundles of goethite crystals that lend interest to this rare and distinctive habit of calcite from Ametista (Fig. 2).

[FIGURE OMITTED]

Occurrences of such specimens are apparently confined to southeastern Uruguay and the Brazilian border region of Rio Grande do Sul. Currier (1997) described skunk calcite from the Jaketti mine in Uruguay as scalenohedral crystals which show broad or narrow stripes of black goethite. This banding apparently " ... extends up the center of the three sides of the crystal and converges at its termination. In addition, little amethyst crystals sometimes occur in the center of the goethite stripes" (Currier, 1997). This description generally fits the Ametista specimen, except that goethite is only weakly disseminated in the latter's "skunk-like" stripes (Fig. 2). Here we document the mineralogical relationships and paragenesis associated with this specimen of skunk calcite, and we briefly examine the possibility of quartz-calcite epitaxy.

INITIAL EVALUATION; METHODOLOGY

Damage to the geode during shipment to Canada revealed that at some time one of the calcite crystals had been fractured, then rather carelessly mended with a transparent glue. Whereas the glue fluoresces brightly, the calcite fluoresces weakly dark red in longwave ultraviolet light. An attractive yellow patina on the calcite crystals was initially suspected to be an artifact meant to enhance the specimen's value, but later was shown to be natural. Small, scattered whitish patches appear on several of the small quartz crystals and on some acicular black goethite crystals composing the stripes; the calcite crystals do not show similar patches. This feature is believed to be the result of the Brazilian practice of cleaning many amethyst specimens with an industrial cleaner known as chispas, a minor component of which is hydrofluoric acid (Currier, 1997).

Goniometric measurements indicate that the rhombohedral faces are {02[bar.2]1}. Subsequent investigation of the skunk calcites relied primarily on SEM with an EDX system to analyze the calcite and to image and elucidate contact relationships between the various phases. A Debye-Scherrer X-ray powder camera was used to identify crushed samples of the smaller crystals. Computer models were developed which would allow the dimensions of crystal lattices of quartz and calcite to be superimposed, and percentage differences in fit to be measured.

MINERALOGICAL RELATIONSHIPS

Judging from the edge of the broken geode (see Fig. 1), it is apparent that the quartz (1) and calcite crystals grew upon the outermost 3 to 4-mm-thick layer of banded agate which lines the cavity. Evidently the quartz (I) and calcite crystals grew simultaneously. However, understanding the textural and paragenetic relationships between calcite and several other phases poses a somewhat greater challenge.

The small sheaves of acicular black crystals were identified as goethite by X-ray diffraction. The yellow patina (already mentioned) is lepidocrocite, chemically identical with goethite, its trimorph. As measured by SEM imaging, the patina consists of a uniform, 1.6-[mu]m-thick layer of lepidocrocite (very unlikely to have been applied artificially) upon calcite (Fig. 3a). The composition of the carbonate is virtually pure [CaCO.sub.3].

[FIGURE 3 OMITTED]

In order to ascertain the true relationship between the calcite substrate, the quartz (II) crystals, the acicular goethite crystals, and the yellow patina, double-sided tape was used to peel off some of the small quartz crystals from the patina. In this way it proved possible to pluck off the quartz (II) grains, whose undersides then revealed the nature of the surface of contact between quartz and calcite. The underside view (Fig. 3b) demonstrates that this quartz grew directly on the calcite surface, for the depression in the center of an otherwise perfect crystal replicates the striations characteristic of the calcite surface. The lighter material abutted against the depression at the top of the image is iron oxide, deposited preferentially on the calcite after the quartz grains were formed. EDS spectra (Fig. 3c, 3d) confirm the identities of the quartz and the iron oxide.

The quartz (II) crystals which help highlight the "skunk" stripes on the rhombohedral calcite faces range from 3 mm to about 1 cm long, the longest crystals being crowded near the apex of each calcite crystal. These quartz crystals rest, in no apparent preferred orientation, upon the calcite rhombohedra (Fig. 4a, 4b). They occupy a band up to 1 cm wide which extends from the base of each of the three rhombohedral faces (Fig. 2b) to the apex of the crystal.

[FIGURE 4 OMITTED]

Accompanying these small quartz crystals in the stripes, and less commonly enclosed within them, are 1-mm-long, radiating sheaves of black goethite crystals. Each sheaf is about 0.5 mm in diameter and slightly greater in length. Goethite crystals do not occur in or on the large amethyst crystals within the main portion of the geode. Microscopic euhedral quartz (II) crystals also occur scattered about the surface of the calcite scalenohedrons, and tiny quartz (II) crystals commonly cluster around the sheaf-like bundles of goethite (Fig. 5c). In no observed case, however, do goethite crystals occur on the scalenohedral faces.

[FIGURE 5 OMITTED]

The surfaces of unbroken sheaves of goethite are encrusted with minute lepidocrocite crystals (Fig. 5a, 5b), nearly masking the underlying lath-like subparallel sprays of elongated (orthorhombic) goethite crystals; some of these lath-like crystals show scepter-like terminations. No iron oxide crystals occur upon quartz although some quartz (II) crystals enclose goethite sheaves.

A NOTE ON EPITAXY

According to the IMA definition and general recommendation: "Epitaxy is the phenomenon of mutual orientation of two crystals of different species, with two-dimensional lattice control (mesh in common), usually, though not necessarily, resulting in an overgrowth" (Bailey et al., 1978).

Although there is no epitactic relationship evident between quartz and calcite in the Ametista specimen, one wonders how common such a relationship may be in nature. The common occurrence together of quartz and calcite, and the tremendous richness of forms and habits, particularly in calcite (Kostov and Kostov, 1999), suggest that such epitaxy, if it is possible, could be widespread. Curiously, however, there is but one reported occurrence. In the relevant specimens, from the Madan region of Bulgaria, crystallographic c-axes and (001) (a base normally absent in quartz) are coincident in quartz and calcite (Bonev, 1983).

We tested the possibility of epitactic relations between (low) quartz and calcite, using an in-house computer program to generate crystal models. The results yielded fits along various lattice parameters, including {001}, that fall well within the generous allowance (10-15%) accorded by the Royer-Friedel rule (Turnbull and Vonnegut, 1952; Royer, 1928).

PARAGENESIS

Following its formation near the bottom of a solidifying lava flow, the amethyst geode became lined with banded agate. This provided a base for subsequent deposition of the geode's crystalline contents. From the results of fluid inclusion studies, Juchem (1999) determined that the temperature ranged from 500[degrees]C during formation of the agate to about 100[degrees]C or less during late stage (epithermal) crystallization of amethyst in the geodes of Rio Grande do Sul. According to Kostov (1968) and Moroshkin and Frishman (2001), the temperature of crystallization of various calcite forms proceeds from the {10[bar.1]1} rhombohedron at high temperature to the {[bar.1][bar.1]20} prism, {21[bar.3]1} scalenohedron, and lastly the {02[bar.1]1} steep rhombohedron at low temperature.

The perceived paragenetic sequence in the Ametista geode is:

(1) Formation of a thin inner wall of banded agate.

(2) Crystallization of large amethyst crystals--quartz (I)--and calcite crystals upon the agate lining of the geode.

(3) Formation of tiny sheaf-like sprays of black goethite together with small amethyst crystals--quartz (II)--in "skunk" stripes upon the calcite rhombohedrons.

(4) Continuing formation of quartz, (II) so as to enclose some goethite sprays; during this interval sparse microscopic quartz (II) also grew upon the calcite scalenohedron {21[bar.3]1}. There is no evidence to indicate that any quartz (II) grew upon quartz (I) at this time.

(5) Deposition of a patina of yellow lepidocrocite upon all exposed surfaces of calcite and upon the goethite sheaves (but evidently the surfaces of the amethyst did not accept lepidocrocite).

DISCUSSION AND CONCLUSION

The formation of skunk calcite was initiated with the earliest precipitation of iron oxides as goethite. It has been shown that quartz (II) crystals formed, contemporaneously with the goethite sheaves and in some cases enclosing them, along the "skunk" stripes. To account for the origin of the "skunk" pattern it is necessary to explain the confinement of goethite and the concentration of quartz (II) crystals along the stripes on calcite rhombohedron faces. Provided that the formation of the rhombohedron faces did indeed succeed the formation (at lower temperature) of scalenohedrons, the later conditions must have favored precipitation of quartz (II) and goethite. Unfortunately, proof for this most simple of all scenarios is lacking, as is an explanation for the relatively orderly arrangement of goethite and quartz (II) into stripes.

Clarke et al. (1985) studied experimentally the precipitation of iron oxides on calcite (Iceland spar) surfaces, at room temperature and in a solution containing [Fe.sup.2+] Precipitation rates were controlled chiefly by the rate of oxidation of [Fe.sup.2+], by pH, by concentration of [Fe.sup.2+], and by rate of aeration. Results showed that a slow growth of well-crystallized lepidocrocite ([gamma]-[Fe.sup.3+]O(OH)) as 1 X 4-[mu]m platelets, and of smaller, less well-formed crystals of goethite ([alpha]-[Fe.sup.3+]O(OH)), occurred on the calcite. Some of the grains of lepidocrocite, the dominant phase, also showed subspherical morphology (Clarke et al., 1985; compare also Welton, 1984), a morphology closely resembling the patina on Ametista calcites (see Fig. 3a). At the higher temperatures prevalent in the Brazilian geode, still other, unnamed factors doubtless enter the picture. Among them are probably some thus far elusive aspects of the surface chemistry of certain crystal faces which permit the remarkable stripes on skunk calcites to form.

ACKNOWLEDGMENTS

We thank Mickey Gunter for his constructive criticism as internal referee of an early draft of this paper; helpful comments by Emil Mckovicky, Stan Cameron, and Brian and Catherine Skinner are gratefully acknowledged. Financial support was provided by the Natural Sciences and Engineering Research Council of Canada in the form of operating grant A8295 to DJM.

BIBLIOGRAPHY

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David J. Mossman

Department of Geography, Mount Allison University

144 Main Street, Sackville, New Brunswick, E41 1A7

James M. Ehrman

Department of Biology and Digital Microscopy Facility

Mount Allison University, 63B York St., Sackville, New Brunswick, E4L 1E4

Ralf Bruiting and Lynne Semple

Department of Physics, Mount Allison University, 67 York St. Sackville

New Brunswick E41 1E6

Lee A. Groat

Department of Earth and Ocean Sciences, University of British Columbia

6339 Stores Road, Vancouver, British Columbia, V6T 1Z4
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Author:Mossman, David J.; Ehrman, James M.; Bruning, Ralf; Semple, Lynne; Groat, Lee A.
Publication:The Mineralogical Record
Geographic Code:3BRAZ
Date:Mar 1, 2009
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