The Na-rich zeolites from Boron, California

Studies in Surface Science and Catalysis 155 A. Gamba, C. Colella and S. Coluccia (Editors) 9 2005 ElsevierB.V. All rights reserved

The Na-rich zeolites from Boron, California William S. Wise Department of Geological Sciences, University of California Santa Barbara, California 93106 USA Unusually Na-rich zeolites have been found in cavities in blocks of basalt on the waste dump at the US Borax Mine at Boron, 135 km northeast of Los Angeles, California. The basalt underlies Miocene basin fill deposits, consisting of sand, shale, tuff, and Na-borate deposits. The zeolites show two generations of growth. Early gmelinite-Na is overgrown by later chabazite-Na, and early clinoptilolite-Na is overgrown by heulandite-Na. Other zeolites in these cavities are phillipsite-Na, analcime, Na-rich mordenite, and mazzite-Na. The occurrence of searlesite and borax, attests to the saturation of the basalt with a sodium borate solution at least at some stage. The extreme Na-rich compositions of all the zeolites strongly indicate cation exchange in Na-borate brine, while the overgrowth crystals may have grown in the brine. 1. INTRODUCTION When compiling zeolite compositions, one often wonders if the analyzed composition is same as when that particular zeolite crystallized. To be sure, there are instances where natural postcrystallization cation exchange probably occurred. For example, the occurrences of barrerite near the seashore suggest the Na probably came from the seawater, but evidence of crystallization in a Na-rich environment is lacking. There is evidence at the zeolite occurrence at Boron, California, that not only were the early formed crystals cation-exchanged, but growth continued in Na-rich solutions. An unusual suite ofNa-rich zeolites was found in cavities in blocks of basalt, sampled on the waste dump at the U.S. Borax Mine at Boron, 135 km northeast of Los Angeles, California. The blocks were exposed and removed by the ongoing quarrying operation. The basalt underlies basin fill deposits, consisting of silt, claystone, tuff, and borate deposits, all comprising the Kramer Beds.

1.1. Geologic Setting The Mojave Desert region of southeastem California contains many basins that developed as early as the Miocene. These basins were formed by crustal extension, which was accompanied by scattered volcanic activity. Because most of the basins were hydrologically closed, alkaline and saline playa lakes were common. The westem most was the Kramer basin, which was floored by two olivine basalt flows, each about 5 m thick. These flows were soon covered by a shallow, playa lake, from which several meters of silt and clay were deposited before the waters became enriched in sodium borate. The origin of the borate is uncertain, but is generally believed to have come from nearby thermal springs. Over the next several million years at least 100 m of playa deposits of claystone and sodium borate with a few interbeds of rhyolitic tuff were deposited in the basin. Fifty to 75 m of post-Miocene

14 alluvial gravel covers the lake beds. Interestingly, only 55 km to the east is the contemporaneous Barstow basin that was only saline and alkaline. Rhyolite tuff that fell into that lake was altered to clinoptilolite and analcime [ 1]. Similar tuff beds in the Kramer boraterich lake were converted to analcime and searlesite, NaBSi2Os(OH)2. All of the zeolites, including the cavity minerals in the basalt, mostly likely are a result of diagenetic alteration [2], in which minerals formed by reactions of volcanic glass with playa lake water. 2. THE CAVITY MINERALS Cavities in the basalt are all lined with saponite, which is overgrown by the iron sulfides, pyrrhotite and greigite; zeolites; borax and searlesite; and late Ca-bearing minerals that include calcite and Ca-borates. The zeolites show two generations of growth. An early set of crystals show the effects of alteration, e.g. replacement or coating by brown clay. The crystals of the later set are clear and transparent and tend to overgrow the early set. The early zeolites are phillipsite-Na, gmelinite-Na, clinoptilolite-Na, mazzite-Na, and Na-rich mordenite, while the later ones are analcime, chabazite-Na, heulandite-Na, and more phillipsite-Na.

2.1. Ferroan saponite All fracture and cavity surfaces are coated with a layer of clay, which varies in appearance from pale green and waxy to green, ball-like masses. Upon exposure to the atmosphere the colors rapidly change to brown from oxidation of the ferrous iron. Electron microprobe analysis of the green form gives the following chemical composition: (Cao.loNao.32Ko.21)(Mg2.6oFe2+a.65Alo.o7Tio.41)(Si6.9oAll.lo)Oao(OH)4.nH20. The unusual aspect of this composition is the high amount of Na in the interlayer cation sites. 2.2. Phiilipsite-Na Phillipsite-Na, the first zeolite to form, commonly occurs as blocky groups of complexly twinned crystals up to 4 mm across. Many of these groups are partially to completely coated with saponite. Commonly the phillipsite has been replaced by saponite, leaving a solid pseudomorph or a shell of saponite. A few of the saponite coated complex groups have overgrowths of colorless, transparent phillipsite. These overgrowths replicate the twinning of the host crystal, even though there is a substantial saponite layer between the host and growth crystals. All of the phillipsite at Boron has Na as the dominant exchangeable cation with only minor amounts of K, Ca and Ba. The early formed crystals have slightly higher amount of cations other than Na, than do the later ones. Electron microprobe analyses give the following typical compositions: Na3.07Ko.20Cao.08Bao.ll[A13.64Si12.35032](H20)n for the early crystals, and Na4.27K0.08Cao.01[A14.36Sill.63032](H20)nfor the late overgrowths. Note that the Si content of the framework is slightly lower in the later crystals. 2.3. Gmelinite-Na Crystals of gmelinite-Na occur as single and twinned dipyramids 1 to 2 mm across, and in spherical clusters up to 6 mm in diameter. Commonly the gmelinite crystals are coated with a thin layer of clay, causing them to have a brownish, opaque appearance. Nearly half the crystals observed exhibit dissolution, ranging from very slight to nearly complete. The solution appears to start on the prism faces and proceeds to the crystal center. Many crystal remnants are merely shells consisting of pyramid faces.

15 Many of the gmelinite crystals with a clay coating are overgrown with uncoated, hexagonal plates of chabazite (herschelite habit). The two minerals are joined on their respective {0001 } faces, commonly on both ends of the gmelinite crystal. Electron microprobe analyses of these gmelinite crystals gives the compositions similar to Na6.60Ko.loBao.05Mgo.16[A16.79Si17.20Ons](H20)n.Although the crystals were sectioned before analysis, the Mg may be from the clay coating rather than exchangeable cations. 2.4. Chabazite-Na Most commonly chabazite-Na crystals occur as colorless, epitaxial overgrowths on gmelinite-Na. These overgrowths have the herschelite habit, that is twinned, pseudohexagonal plates 1-2 mm across. Some chabazite-Na occurs as complex clusters of crystals that are transparent and colorless. These clusters are typically 2-4 mm across. A typical composition of a platy overgrowth of chabazite-Na is Na2.96K0.24[A13.21Sis.79024](H20)9.3 [3]. An analysis of a clear, crystal cluster yielded Na3.49Ko.16 [AI3.29Sis.62024](H20)n with slightly less S i in the framework. 2.5. Clinoptilolite-Na The most common zeolite in the basalt cavities is clinoptilolite-Na, which occurs in the typical heulandite-group habit of coffm-shaped, platy crystals. Lengths up to 3 mm are

common, and most crystals occur with the forms {010}, { 111 }, {001 }, { 100 }, {20 i }, and { 110} [2]. All clinoptilolite-Na grew during the earlier stage, because all crystals are lightly to heavily coated with a layer of saponite. There are some samples showing dissolution, leaving a partial shell consisting of saponite. The optical properties of the Boron clinoptilolite-Na crystals are similar to other clinoptilolite-Na, such as those from Agoura, California [4]. In these and several other examples of clinoptilolite the optic axial plane is parallel to (010). See Fig. 1. Electron microprobe analyses show that crystals with a saponite coating are indeed clinoptilolite, i.e. the Si/A1 is greater than 4.0, the lower limit allowed by the IMA zeolite nomenclature [5]. There is some range in composition from Na6.48K0.56Ba0.06Mg0.13 [AI7.~sSi28.77072](H20)n [1], where Si/A1 = 4.02, to Nas.86KI.06[A16.92Si29.08072](H20)n and Si/A1 = 4.20. 2.6. Heulandite-Na In several blocks of basalt clear crystals of heulandite-Na occur as epitaxial overgrowths on the brownish clinoptilolite-Na. These overgrowths have the same crystal forms as the substrate crystal. Heulandite-Na also occurs as clear, isolated crystals with a distinctive morphology, elongated parallel to [ 102]. Crystals from both types of occurrences have distinctive optical properties, and are easily distinguished from clinoptilolite-Na (Fig. 1). Like all heulandite-Ca these crystals have the optic plane normal to (010), although here the optic axial angle is negative and large. These optical properties are close to but not identical with a Na-exchanged heulandite reported by Yang et al. [6]. Electron microprobe analyses of heulandite-Na show a slight difference between the two types of occurrence. An example of the overgrowths has the following composition: Na6.68K0.66Ba0.27Mg0.41[Als.06Si27.69072](H20)n, in which Si/AI- 3.43, and an isolated crystal has the composition, Na6.57Ko.48Mg0.30[A17.59Si28.26072](H20)n, where Si/AI = 3.72. Clearly both of these examples conform to the definition of heulandite with respect to clinoptilolite.

16

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Fig. 1. The optical properties of the clinoptilolite/heulandite crystals. In general the refractive indices of the heulandite group minerals vary with the Si/AI ratio of the framework with a change in the orientation of the optic axial plane at about Si/Al = 4.0. The same change is observed in the Boron crystals, except both minerals are optically negative, possibly a result of the dominantly Na exchangeable cations (compare with [6]). 2.7. A n a l e i m e Throughout the cavities analcime occurs as single crystals and rarely as complex balls o f radial crystals. The single crystals have the common trapezohedron form, and are typically 2 mm across. The complex balls up to 3 mm in diameter consist of radial prisms terminated by a few trapezohedron faces. All analcime is clear and transparent, apparently having grown during the second stage. All o f the analcime that has been analyzed is unusually silica-rich. A typical analysis yields the composition, Nal4.02Ko.09 [AII3.33Sia4.44096](H20)n, where Si/AI = 2.58. 2.8. M a z z i t e - N a A very few cavities contain mats of very thin (10 ~tm), colorless, flexible fibers up to 2 mm. X-ray powder difl?action data show that this material has the structure o f mazzite. Subsequent electron microprobe analysis has yielded a composition close to Na8[AlsSi28072](H20)30 [7]. The crystal structure determination of this mazzite reveals some extra-framework sites with low occupancy, which supports the possibility o f postcrystallization cation exchange [7].

2.9. Na-rieh mordenite Mordenite occurs as sprays o f white fibers up to 1 mm in length. These fibers are not as thin as those o f mazzite-Na, and are brittle, rather than flexible. Some of the mordenite is

17 brownish from a thin coating of saponite indicating at least some crystallization occurred during the early stage. Energy dispersive analysis shows only the presence ofNa, A1, and Si.

2.10. Boron-bearing minerals Scattered among the cavities are crystals of searlesite, NaBSi2Os(OH)2, and borax, Na2B4Os(OH)4(H20)s, each with a distinctive morphology. Searlesite crystals are colorless, glassy, tapered blades up to 4 mm in length with a recognizable monoclinic aspect. Borax crystals are glassy, blocky crystals less than 1 mm across. Upon exposure of more than a few days, borax dehydrates to opaque white tincalconite. Where these minerals occur with zeolites, both are late in the growth sequence. Ulexite, NaCaBsO6(OH)6(H20)5, and colemanite, Ca2B6011(H20)5, also occur in some cavities, but appear to have formed later during a third stage from Ca-bearing fluids. 3. ORIGIN OF THE ZEOLITE PHASES AND THE UNUSUAL COMPOSITIONS Zeolites in the basalt underlying the Na-borate lake beds at Boron are probably a result of diagenetic reactions of basaltic glass with groundwater rather than hydrothermal alteration. The rock shows no effect of pervasive hydrothermal alteration, and the flows appear to have been saturated with lake water as soon as the basin formed. 2.5

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Si/(Si+AI) Fig. 2. Compositional plot of analyses of zeolites from the basalt at Boron, California. Phiilipsite-Na, gmelinite-Na, clinoptilolite-Na, Na-rich mazzite, and Na-rich mordenite crystallized during the first stage. Some phillipsite-Na, chabazite-Na, heulandite-Na and analcime formed during the second stage. The diagenetic clay and zeolite minerals all have unusually Na-rich compositions. The compositional ranges of each zeolite are plotted in Fig. 2. Each of the zeolite species with the exception of analcime has Na-compositions at the limit of known ranges in world-wide occurrences [2]. Those zeolites formed in the first generation, phillipsite, gmelinite, and clinoptilolite, have the wider ranges of composition. Overgrowth phillipsite-Na tends to have the higher Na and lower Si contents. Because overgrowth chabazite-Na overlaps much of the

18 range of earlier gmelinite-Na, the change from one crystal structure to other is difficult to explain. Overgrowth heulandite-Na has significantly less Si than the early clinoptilolite. The slight, but persistent Si difference between the overgrowth crystals and the isolated ones, is also difficult to explain. The high Si content of the analcime crystals is similar to that in the analcime-searlesite replacement of rhyolitic tuff within the Na-borate sediment and to that replacing tuff in the nearby Barstow basin [1 ]. I conclude that the initial zeolites were phillipsite, gmelinite, clinoptilolite, and possibly mazzite and mordenite, which had compositions typical of basalt occurrences, i.e. there was substantially more Ca and some K. The influx of Na-borate waters caused exchange of the cations to the Na-rich compositions and in many instances dissolution of the pre-existing crystals. Further crystallization did occur in the Na-rich water and resulted in the transparent crystals of phillipsite-Na, chabazite-Na, heulandite-Na, and analcime. REFERENCES

[1] R.A. Sheppard and A.J. Gude, 3rd, U. S. Geol. Surv., Prof. Pap. 634 (1969). [2] W.A. Deer, R.A Howie, W.S. Wise and J. Zussman, Rock Forming Minerals, Vol. 4B, Second ed.. Framework Silicates: Silica Minerals, Feldspathoids and the Zeolites, The Geological Society, London, 2004. [3] W.S. Wise and W.D. Kleck, Clays Clay Miner., 36 (1988) 131. [4] W.S. Wise, W.J. Nokleberg, and M. Kokinos, Am. Mineral., 54 (1969) 887. [5] D.S. Coombs, A. Alberti, T. Armbruster, G. Artioli, C. Colella, E. Galli, J.D. Grice, F. Liebau, J.A. Mandarino, H. Minato, E.H. Nickel, E. Passaglia, D.R. Peacor, S. Quartieri, R. Rinaldi, M. Ross, R.A. Sheppard, E. Tillmanns and G. Vezzalini, Mineral. Mag., 62 (1998) 533. [6] P. Yang, J. Stolz, T. Armbruster and M. Gunter, Am. Mineral., 82 (1997) 517. [7] R. Arletti, E. Galli, G. Vezzalini, and W.S. Wise, Am. Mineral., (in press).