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1457 The Canadian Mineralogist Vol. 38, pp. 1457-1466 (2000) ADAMSITE-(Y), A NEW SODIUM–YTTRIUM CARBONATE MINERAL SPECIES FROM MONT SAINT-HILAIRE, QUEBEC JOEL D. GRICE § and ROBERT A. GAULT Research Division, Canadian Museum of Nature, P.O. Box 3443, Station D, Ottawa, Ontario K1P 6P4, Canada ANDREW C. ROBERTS Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada MARK A. COOPER Department of Geological Sciences, University of Manitoba, Winnipeg, Manitoba R3T 2N2, Canada A BSTRACT Adamsite-(Y), ideally NaY(CO 3 ) 2 •6H
2 O, is a newly identified mineral from the Poudrette quarry, Mont Saint-Hilaire, Quebec. It occurs as groups of colorless to white and pale pink, rarely pale purple, flat, acicular to fibrous crystals. These crystals are up to 2.5 cm in length and form spherical radiating aggregates. Associated minerals include aegirine, albite, analcime, ancylite-(Ce), calcite, catapleiite, dawsonite, donnayite-(Y), elpidite, epididymite, eudialyte, eudidymite, fluorite, franconite, gaidonnayite, galena, genthelvite, gmelinite, gonnardite, horváthite-(Y), kupletskite, leifite, microcline, molybdenite, narsarsukite, natrolite, nenadkevichite, petersenite-(Ce), polylithionite, pyrochlore, quartz, rhodochrosite, rutile, sabinaite, sérandite, siderite, sphalerite, thomasclarkite-(Y), zircon and an unidentified Na–REE carbonate (UK 91). The transparent to translucent mineral has a vitreous to pearly luster and a white streak. It is soft (Mohs hardness 3) and brittle with perfect {001} and good {100} and {010} cleav- ages. Adamsite-(Y) is biaxial positive, ␣ = 1.480(4),  = 1.498(2), ␥ = 1.571(4), 2V meas.
= 53(3) ° , 2V calc. = 55
° and is nonpleochroic. Optical orientation: X = [001], Y = b, Z a = 14 ° (in
 obtuse). It is triclinic, space group P¯1, with unit-cell parameters refined from powder data: a 6.262(2), b 13.047(6), c 13.220(5) Å, ␣ 91.17(4),  103.70(4), ␥ 89.99(4) ° , V 1049.1(5) Å 3 and Z = 4. The strongest six X-ray powder-diffraction lines [d in Å(I)(hkl)] are: 12.81(100)(001), 6.45(70)(002), 4.456(60)(¯1¯21,¯120), 4.291(60)(003), 2.571(60)(005, 043) and 2.050(50)(125,¯126). Electron-microprobe and thermogravimetric analyses, supported by crystal-structure analysis and infrared-absorption spectroscopy, yield Na 2 O 8.64, CaO 0.05, Y 2 O 3 22.88, Ce 2 O 3 0.37, Nd 2 O
1.41, Sm 2 O 3 1.02, Gd 2 O
1.92, Tb 2 O 3 0.56, Dy 2 O
3.28, Ho 2 O 3 0.90, Er 2 O
2.83, Tm 2 O 3 0.27, Yb 2 O
1.04, CO 2 25.10, H 2 O 29.90, total 100.17 wt.%. The empirical formula, based on 12 oxygen atoms, is Na 1.00 (Y
0.72 Dy 0.06 Er 0.05
Gd 0.04
Nd 0.03
Yb 0.02
Sm 0.02
Ho 0.02
Ce 0.01
Tb 0.01
Tm 0.01
) ⌺0.99
C 2.04
H 11.87
O 12 . The calculated density (from the empirical formula) is 2.27 g/cm 3 , and the meas- ured density is 2.27(2) g/cm 3 . The structure has been refined to R = 0.046. The structure is layered, with two different carbonate groups, one parallel and one perpendicular to the layering. Slabs of [NaY(CO 3 )] are separated by [H 2 O] layers. Adjacent [H 2 O]
Adams (1859–1942), geologist and professor at McGill University, Montreal. Keywords: adamsite-(Y), new mineral species, sodium yttrium dicarbonate hexahydrate, crystal structure, Mont Saint-Hilaire, Quebec.
S OMMAIRE
La nouvelle espèce minérale adamsite-(Y), de composition idéale NaY(CO 3 ) 2 •6H
2 O, a été découverte à la carrière Poudrette, au mont Saint-Hilaire, Québec. Elle se présente en groupes de cristaux incolores, blancs ou rose pâle, et plus rarement violacés, plats et aciculaires ou fibreux. Ces cristaux atteignent 2.5 cm en longueur et forment des agrégats fibroradiés. Lui sont associés aegyrine, albite, analcime, ancylite-(Ce), calcite, catapleiite, dawsonite, donnayite-(Y), elpidite, epididymite, eudialyte, eudidymite, fluorite, franconite, gaidonnayite, galène, genthelvite, gmelinite, gonnardite, horváthite-(Y), kupletskite, leifite, microcline, molybdénite, narsarsukite, natrolite, nenadkevichite, petersenite-(Ce), polylithionite, pyrochlore, quartz, rhodochrosite, rutile, sabinaïte, sérandite, sidérite, sphalérite, thomasclarkite-(Y), zircon et un carbonate à Na–REE non encore identifié (UK 91). Le minéral est transparent à translucide et possède un éclat vitreux à nacré et une rayure blanche. Il s’agit d’un minéral mou (dureté de Mohs de 3) et cassant, ayant un clivage {001} parfait et des clivages {100} et {010} assez bons. §
V 1457 38#6-déc.00-2203-13 27/02/01, 13:29 1457
1458 THE
CANADIAN
MINERALOGIST I NTRODUCTION The new mineral species described herein, adamsite- (Y), is the latest in a series of new carbonate minerals recently described from the Poudrette quarry, Mont Saint-Hilaire, Rouville County, Quebec. Fifty-seven carbonate species are known to occur at Mont Saint- Hilaire, many of them containing essential Na and rare- earth elements (REE) (Grice & Gault 1998). To date, a total of 333 mineral species have been identified from this locality, a well-known source of rare and exotic minerals (Horváth & Gault 1990). Adamsite-(Y) was initially recognized as being a new species in 1992 by Dr. G.Y. Chao, Carleton Uni- versity, on material submitted by a private collector, Gilles Haineault, and was designated as UK 96. In 1998, more material was found, but the chemical and X-ray data were not compared with those of the initial find, and the mineral from this second find was designated UK 106. It has since been determined that these two minerals are identical. Another mineral with similar chemical composition, similar appearance, and found in the same environment as adamsite-(Y), has been desig- nated as UK 91. This material is more finely fibrous than adamsite-(Y), however, and has a significantly dif- ferent X-ray powder-diffraction pattern. Adamsite-(Y) is named in honor of Frank Dawson Adams (1859–1942), geologist and professor at McGill University, Montreal. After graduating in chemistry and mineralogy from McGill University, he joined the Geo- logical Survey of Canada in 1879 as “lithologist” and chemist. He studied the new science of petrography at Heidelberg, Germany in 1881 and went on to become the Survey’s leading specialist in the study of crystal- line rocks. In 1889, he was appointed Logan Professor of Geology at McGill University, where he continued his studies on Precambrian rocks, particularly those of the Laurentian and Grenville regions of Quebec and Ontario. It was during this period that he studied the Cretaceous igneous rocks of the Monteregian Hills, of which Mont Saint-Hilaire is a member, and was the first to describe this group of hills as the Monteregian Hills petrographic province (Adams 1903). As well, he served as president of the Geological Society of America and the Canadian Mining Institute. The new mineral and mineral name were approved by the Commission on New Minerals and Mineral Names, IMA. Cotype material is housed in the collec- tion of the Canadian Museum of Nature under catalogue numbers CMNMC 82939 and CMNMC 82940. O CCURRENCE Adamsite-(Y) occurs as a late-stage, low-tempera- ture hydrothermal phase within cavities in a large alka- line pegmatite dike, informally designated the Poudrette dike, exposed in the southern corner of the Poudrette quarry. The quarry, in turn, is situated on the northeast- ern flank of Mont Saint-Hilaire, one of a series of plu- tons aligned along the St. Lawrence Valley for almost 150 km eastward from Oka to Megantic, in the prov- ince of Quebec. The pegmatite dike, which pinches and swells from 2 to 4 m across, has been exposed on levels 6, 7, 8 and 9 of the quarry. Five new Na- and REE-bear- ing carbonate mineral species have been described from this dike: lukechangite-(Ce), horváthite-(Y), reederite- (Y), thomasclarkite-(Y) and adamsite-(Y), the subject of this paper. Minerals found in that portion of the dike in which adamsite-(Y) was discovered include aegirine, albite, analcime, ancylite-(Ce), calcite, catapleiite, daw- sonite, donnayite-(Y), elpidite, epididymite, eudialyte, eudidymite, fluorite, franconite, gaidonnayite, galena, genthelvite, gmelinite, gonnardite, horváthite-(Y), kupletskite, leifite, microcline, molybdenite, narsarsuk- L’adamsite-(Y) est biaxe positive, ␣ = 1.480(4),  = 1.498(2), ␥ = 1.571(4), 2V mes.
= 53(3) ° , 2V calc. = 55
° , et non pléochroïque. Son orientation optique est: X = [001], Y = b, Z a = 14 ° (dans l’angle  obtus). Le minéral est triclinique, groupe spatial P¯1, avec une maille élémentaire affinée à partir des données obtenues sur poudre: a 6.262(2), b 13.047(6), c 13.220(5) Å, ␣ 91.17(4),  103.70(4), ␥ 89.99(4) ° , V 1049.1(5) Å 3 et Z = 4. Les six raies les plus intenses du spectre de diffraction X [d en Å(I)(hkl)] sont: 12.81(100)(001), 6.45(70)(002), 4.456(60)(¯1¯21,¯120), 4.291(60)(003), 2.571(60)(005, 043) et 2.050(50)(125, ¯126). Les analyses obtenues par microsonde électronique et par thermogravimétrie, supplémentées par une ébauche de la structure cristalline et un spectre d’absorption dans l’infra-rouge, mène à la composition Na 2 O 8.64, CaO 0.05, Y 2 O 3 22.88, Ce 2 O 3 0.37, Nd 2 O
1.41, Sm 2 O 3 1.02, Gd 2 O 3 1.92, Tb 2 O 3 0.56, Dy 2 O
3.28, Ho 2 O 3 0.90, Er 2 O
2.83, Tm 2 O 3 0.27, Yb 2 O
1.04, CO 2 25.10, H 2 O 29.90,
pour un total de 100.17% (poids). La formule empirique, fondée sur une base de 12 atomes d’oxygène, est Na 1.00
(Y 0.72
Dy 0.06
Er 0.05
Gd 0.04
Nd 0.03
Yb 0.02
Sm 0.02
Ho 0.02
Ce 0.01
Tb 0.01
Tm 0.01
) ⌺0.99
C 2.04
H 11.87
O 12 . La densité calculée (à partir de la formule empirique) est 2.27 g/cm 3 , et la densité mesurée est 2.27(2) g/cm 3 . La structure a été affinée jusqu’à un résidu R de 0.046. L’adamsite-(Y) est un minéral en couches ayant deux groupes de carbonate distincts, un parallèle aux couches et l’autre perpendiculaire. Les couches de composition [NaY(CO 3 )] sont séparées par des couches de [H 2 O]. Les couches adjacentes de [H 2 O] ne sont interliées que par liaisons hydrogènes, ce qui rend compte du clivage {001} parfait. Le minéral honore Frank Dawson Adams (1859–1942), géologue et professeur à l’Université McGill. (Traduit par la Rédaction)
Saint-Hilaire, Québec. V 1457 38#6-déc.00-2203-13 27/02/01, 13:29 1458
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1459 F IG . 1. Photograph of adamsite-(Y). Photograph by Harry Taylor. The National History Museum, London, specimen number BM 1998, 172. Scale bar: 1 cm. 1457 38#6-déc.00-2203-13 27/02/01, 13:29 1459
1460 THE
CANADIAN
MINERALOGIST ite, natrolite, nenadkevichite, petersenite-(Ce), poly- lithionite, pyrochlore, quartz, rhodochrosite, rutile, sabinaite, sérandite, siderite, sphalerite, thomasclarkite– (Y), zircon and an unidentified Na–REE–carbonate (UK 91). We discuss the genesis of adamsite-(Y) later in this paper.
P HYSICAL
AND
O PTICAL
P ROPERTIES Adamsite-(Y) occurs as flat, acicular to fibrous crys- tals, up to 2.5 cm in length. They typically form spheri- cal groups of radiating crystals, and rarely form reticulated groups (Fig. 1). The mineral is elongate on [001] and flat on (001). Forms observed are the pinacoids {010} and {001}. It is colorless to white, oc- casionally pale pink, and rarely pale purple. It is trans- parent to translucent with a vitreous and occasionally pearly luster, and a white streak. It is brittle with perfect {001} and good {100} and {010} cleavages. Adamsite- (Y) is relatively soft (Mohs hardness 3) and does not fluoresce in either long- or short-wave ultraviolet light. The density, measured by suspension in a solution of bromoform and butyl alcohol, is 2.27(2) g/cm 3 , which
compares very well to the density of 2.27 g/cm 3 calcu- lated for the empirical formula. Adamsite-(Y) is biaxial positive, ␣ 1.480(4),  1.498(2), ␥ 1.571(4); 2V meas.
= 53(3) ° , 2V calc. = 55
° , (for
= 590 nm). The optical orientation is X = [001], Y = b, Z a = 14 ° (in
 obtuse). It is nonpleochroic. A Gladstone–Dale calculation gives a compatibility index of –0.009, which is regarded as superior (Mandarino 1981).
C HEMICAL
C OMPOSITION Chemical analyses were done in wavelength-disper- sion (WD) mode on a JEOL 733 electron microprobe using Tracor Northern 5500 and 5600 automation. Data reduction was done with a PAP routine in XMAQNT (pers. commun., C. Davidson, CSIRO). The analysis of adamsite-(Y) by electron microprobe was problematic because of decrepitation of the sample and the lifting of the carbon coating due to dehydration under prolonged vacuum. The decrepitation results from the weak H- bonding linking adjacent H 2 O groups in the structure. To minimize this effect, calibration of the standards was performed first, and the samples were later introduced into the specimen chamber and analyzed immediately. This method, along with adoption of a rather large di- ameter of the beam, 50 m, limited Na migration, vola- tilization and general burn-up of the sample. The operating voltage of the electron probe was 15 kV, and the beam current was 20 nA. Data for all elements in the samples were collected for 25 s or 0.50% precision, whichever was attained first. A 100 s energy-dispersion scan indicated no elements with Z > 8 other than those reported here. Seven analyses were performed on three crystals. The presence of CO 2 and H
2 O was confirmed by infrared-absorption spectroscopy, and their concen- trations were established by thermogravimetric analy- sis (TGA). As only one-half of the CO 2 was evolved during TGA, the amount given below consists of two summed portions, one measured (the gas evolved, 12.9 wt.%) and the other, the ideal amount determined by crystal-structure analysis (non-evolved during heating, 12.2 wt.%). The following standards were used in the electron- microprobe analyses: albite (NaK ␣), calcite (CaK␣), synthetic yttrium iron garnet (YIG) (YK ␣), and a set of synthetic REE phosphates (CeL ␣, NdL␣, SmL␣, GdL␣, TbL ␣, DyL, HoL␣, ErL␣, TmL␣, YbL␣). La, Pr and Eu were sought but not detected. Lu was detected in trace amounts. Data for standards were collected for 50 s or 0.25% precision, whichever was attained first. The
cal composition is Na 2 O 8.64, CaO 0.05, Y 2 O 3 22.88, Ce 2 O 3 0.37, Nd 2 O 3 1.41, Sm 2 O 3 1.02, Gd 2 O
1.92, Tb 2 O 3 0.56, Dy 2 O 3 3.28, Ho 2 O 3 0.90, Er 2 O
2.83, Tm 2 O 3 0.27, Yb 2 O 3 1.04, CO 2 25.10, H 2 O 29.90, total 100.17 wt.%. The empirical formula, based on 12 at- oms of oxygen, is Na 1.00 (Y
Dy 0.06
Er 0.05
Gd 0.04
Nd 0.03
Yb 0.02
Sm 0.02
Ho 0.02
Ce 0.01
Tb 0.01
Tm 0.01
) ⌺0.99
C 2.04
H 11.87
O 12 . This simplifies to the ideal formula NaY(CO 3 ) 2 •6H
2 O. Adamsite-(Y) readily dissolves with strong efferves- cence in 10% HCl and decrepitates owing to dehydra- tion in acetone (rapid) and alcohol (slow). T HERMOGRAVIMETRIC AND
D IFFERENTIAL T HERMAL
A NALYSIS
Two differential thermogravimetric analyses (DTG) of adamsite-(Y) were done using a Mettler–Toledo TA8000 system (software version 3.0), which uses a Mettler TG50 module linked to a Mettler M3 microbal- ance. The purge gas was dry nitrogen, with a flow rate of 200 mL/min. Two samples of pure material, weigh- ing 10.196 and 10.847 mg, respectively, were each ground to a fine powder and heated from room tempera- ture to 1000 ° C at a rate of 5 ° C/min. The weight losses of 4.334 mg (42.5 wt.%) and 4.670 mg (43.1 wt.%) oc- curred in two major steps (DTG, Fig. 2): between 80 and 270 °
590 ° C, 15.8 wt.% on average. The interpretation of the DTG scan is given in Table 1. The first weight-loss step is interpreted as the loss of 10 (H 2 O) groups, which ide- ally should be 25.0 wt.%, making the measured value a V 1457 38#6-déc.00-2203-13 27/02/01, 13:29 1460
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1461 little high. The second weight-loss step is interpreted as the loss of two additional (H 2 O) groups plus the break- up of one of the (CO 3 ) 2– anions and its loss as CO 2 (Mochizuki et al. 1974). Ideally, this second weight-loss step should be 12.2 wt.%, making the observed value of 12.9 wt.% a little high. Mochizuki et al. (1974) dis- cussed their DTG and differential thermal analysis (DTA) results on synthetic double carbonates similar in composition to adamsite-(Y). The TGA and DTA curves they presented are very similar to those of adamsite-(Y). They proposed that the final product after heating is a
3 ). Although the final product after heating was X-rayed, we found no suitable match to any compound listed in the Powder Diffraction File (PDF), including NaYO 2 (PDF 32– 1203). It is well established that the production of so- dium peroxide, Na 2 O
, takes place at temperatures below 500 ° C (Cotton & Wilkinson 1980). Thus an al- ternative explanation is that Na acts as a reducing agent for the entire system and evolves CO, not CO 2 , and that the final product is a peroxide, NaYO 4 . Although two possible end-products of heating have been postulated, there is a preference for the REE oxycarbonate based on a chemical test with HCl that produced efferves- cence.
The differential thermal analysis curve (DTA, Fig. 2) reveals one smooth endothermic peak for the initial loss in H 2 O, followed by a distinct shoulder that corre- sponds to almost 25% of the weight percent loss in H 2 O. This weight loss would be the H 2 O groups more strongly bonded to both Na and Y (OW13 and OW15; see be- low), whereas the other H 2 O groups are bonded to Na only (OW17, OW18, OW19, OW20, OW21, OW22, OW23 and OW24). The second weight-loss is trimodal on the DTA curve and is due to the difference in energy required to drive off two additional H 2 O groups (OW14 and OW16) and the two less strongly bonded (CO 3 ) 2– groups.
F IG . 2. Differential thermogravimetric analysis (DTG) and differential thermal analysis (DTA) of adamsite-(Y). F IG
Infrared-absorption spectrum of adamsite-(Y). 1457 38#6-déc.00-2203-13 27/02/01, 13:29 1461
1462 THE
CANADIAN
MINERALOGIST I NFRARED
S PECTROSCOPY The infrared-absorption spectrum of adamsite-(Y) (Fig. 3) was obtained using a Bomem Michelson MB– 120 Fourier-transform infrared spectrometer equipped with a diamond-anvil cell as a microsampling device. The dominant absorption band is that of the O–H stretching mode (3297 cm –1 ). The (H 2 O) bending band (1673 cm –1 ) forms a shoulder on the large, split, asym- metric stretching band for (CO 3 ) 2– . This splitting (1513 and 1377 cm –1 ) is explained by the fact that the carbon- ate groups are very distorted, with distinctly differing C–O bond lengths, as seen in the crystal-structure de- termination. For this same reason, the (CO 3 ) 2– symmet-
ric band is also split (1115 and 1070 cm –1 ). The weak intensity of this symmetric band is due to the limited ability of the (CO 3 )
group to undergo this type of stretching mode. The 876 and 849 cm –1 bands may be due to out-of-plane bending of (CO 3 ) 2– . The remainder of the spectra could not be unequivocally assigned. This infrared-absorption spectrum is very similar in appear- ance to that of thomasclarkite-(Y) (Grice & Gault 1998). In thomasclarkite-(Y), the splitting of the carbonate peaks is due to the presence of the (HCO 3 ) – anion,
whereas in adamsite-(Y), the splitting of the carbonate peaks is due to the influence of H-bonding. X-R AY
RYSTALLOGRAPHY
AND C RYSTAL
-S TRUCTURE
D ETERMINATION Precession single-crystal photographs initially showed adamsite-(Y) to be pseudo-orthorhombic, with diffraction symmetry mmm. These crystals are twinned by reflection on {001}; subsequent more precise work on single crystals using a four-circle diffractometer in- dicated triclinic symmetry, but metrically close to mono- clinic. Much of the problem with the early single-crystal work was the failure to realize that this mineral reacts with the binding agent, amyl acetate, causing dehydra- tion and subsequent almost complete decrepitation of the crystal. This decrepitation reaction was exacerbated by intense X-radiation; crystals subjected to this treat- ment developed a crazed appearance. The crystal used for the final data-collection was mounted on a glass fiber with non-reacting epoxy. X-ray powder-diffraction data, obtained with a Debye–Scherrer camera having a diam- eter of 114.6 mm and using CuK ␣ radiation, are given in Table 2. Whereas neither variation in intensity nor shifting of d-values was noted in the X-ray powder pat- terns during this study, other researchers have noted some variability in the patterns, possibly indicating that adamsite-(Y) may not be stable under minor fluctuations in humidity levels (pers. commun., A.M. McDonald, Laurentian Univ.). Indexing the powder data was diffi- cult owing to the previously mentioned pseudosym- metry, but the process was successful with the aid of powder-pattern intensities calculated from the results of the crystal-structure analysis. In the final stage of data collection, a crystal frag- ment of adamsite-(Y), measuring 160 ϫ 180 ϫ 40 m for [100] ϫ [010] ϫ [001], was mounted on a CCD- equipped Bruker P4 fully automated four-circle diffractometer operated at 40 kV and 40 mA. With the CCD detector, almost a full sphere of intensity data was collected out to 2 = 60 °
crystal-to-detector distance of 4 cm. The lowering of the incident radiation power and the high-speed data col- lection were in anticipation of potential problems with crystal decrepitation. With these operating conditions, 1457 38#6-déc.00-2203-13 27/02/01, 13:29 1462
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1463 no decrepitation was evident in the final analysis of the intensity standards. Information relevant to the data collection and structure determination is given in Table 3. The three-dimensional data were reduced for Lorentz, polarization, and background effects using the Bruker program SAINT. An empirical plate-absorption correction was done on the basis of 3998 reflections and reduced the merging R of this dataset from 3.45% be- fore the absorption correction to 3.10% after the absorp- tion correction with a glancing angle of 7 ° . Phasing of a set of normalized structure-factors gave a mean value | E 2 – 1 | of 0.862. A calculated sharpened Patterson function for space group P¯1 located the two Y sites, the two Na sites and six lighter-element sites. This model refined to R = 0.22. Additional O and C sites were added following a series of ⌬F synthesis maps, which, in turn, reduced the R index to 0.066. In the final least- squares refinement, all atomic positions were refined with anisotropic-displacement factors to a residual of R = 0.046. The addition of an isotropic-extinction factor did not improve the results. Although the |E 2 – 1| statis- tic is low for centrosymmetric structures, lowering the symmetry to P1 did not improve the refinement, nor was there any indication that the lower symmetry suggested a better structural model. Use of the program MISSYM (Le Page 1987) suggests the possible presence of a two- fold axis. The Laue merging for monoclinic symmetry is 6.1%. The metric deviation from monoclinic symme- try is only 0.2 ° , with a 25.72, b 6.260, c 13.085 Å, ␣ 90.19,
 91.21 and ␥ 89.87. Essentially the same struc- ture can be refined in C2/c (R = 0.06) with four of the six H 2
half-occupied sites each. We view the 0.2 ° metric devia- tion, the 6.1% Laue merging and the site splitting in C2/ c as sufficient evidence to rule out monoclinic symmetry. Table 4 contains the final positional anisotropic-dis- placement factors and equivalent isotropic-displacement parameters, and Table 5 contains selected interatomic distances and angles. Observed and calculated structure- factors have been submitted to the Depository of Un- published Data, CISTI, National Research Council of Canada, Ottawa, Ontario K1A 0S2, Canada. 1457 38#6-déc.00-2203-13 27/02/01, 13:29 1463
1464 THE
CANADIAN
MINERALOGIST D ESCRIPTION AND
D ISCUSSION
OF
THE S TRUCTURE The structure of adamsite-(Y) has four large-cation sites with two distinct polyhedra. The two Na sites, each with six-fold coordination, occupy [Na(H 2 O) 6 ] polyhe- dra that may be described as a bifurcated tetragonal pyramid with the Na atom slightly above the square base. This base consists of four H 2 O groups, whereas the bifurcated apex has two additional H 2 O groups. The nine-fold coordination (seven O atoms and two H 2 O groups) around each Y site may be described as a monocapped square antiprism. All (CO 3 ) polyhedra share edges with this (Y 9 ) polyhedron ( is an unspeci- fied ligand). The crystal structure of adamsite-(Y) is layered on (001) (Fig. 4). The layering of REE-bearing carbonates is described in detail by Grice et al. (1994). In adamsite- (Y), there are thick slabs (Fig. 4) composed of a unit of Y atoms and parallel “flat-lying” (CO 3 ) polyhedra sand- wiched between layers of Na(H 2 O) 6 polyhedra and per- pendicular “standing-on-end” (CO 3 ) polyhedra (Grice et al. 1994). These [NaY(CO 3 )] slabs are H-bonded to- gether, and it is these H-bonds that give rise to the per- fect {001} cleavage and the relatively unstable nature of adamsite-(Y). Adamsite-(Y), NaY(CO 3 )
•6H 2 O, and thomas- clarkite-(Y), NaY(HCO 3 )(OH) 3 •4H
2 O (Grice & Gault 1998), are found intimately associated with one another at Mont Saint-Hilaire. These two minerals share simi- larities in chemical composition and crystal structure. In both minerals, there is a distinctive fundamental unit that consists of a stacked, edge-sharing unit of three polyhedra centered by a C, a Y and a Na atom (Fig. 5). Although the two sets of polyhedra differ in coordina- tion number (Y polyhedron) and ligands (OH groups versus H 2 O groups in both the Na and Y polyhedron), the fundamental building block is essentially the same. In adamsite-(Y), the Na–Y–C tri-polyhedra form planes and have a strong cross-linkage in the (001) plane with the (YO
9 ) polyhedra, sharing corners and edges and being further reinforced by the “flat-lying” carbonate groups (Fig. 5a). In thomasclarkite-(Y), the Na–Y–C tri- polyhedra share edges, forming single chains parallel to [001] (Fig. 5b). Genesis of adamsite-(Y) Adamsite-(Y) occurs as a very late-stage, low tem- perature, hydrothermal phase within cavities in an alka- line pegmatite dike in the nepheline syenite intrusion. In these cavities, adamsite-(Y) appears to crystallize after rhodochrosite and petersenite-(Ce), is contempo- raneous with horváthite-(Y) and donnayite-(Y), and pre- cedes thomasclarkite-(Y), which has been observed as an epitactic growth on adamsite-(Y) (pers. commun., L. Horváth). This sequence of crystallization is indicative of lowering carbonate activity and Lewis basicity. Com- paring the formulae of the minerals involved shows a decrease in the ratio of carbonate groups relative to the number of cations plus the other chemical species in- volved [(H 2 O), (OH), F]. The lowering of Lewis basic- ity is effected by the distribution of the H atoms. As H bonds to O, the basicity decreases; for example, typical Lewis base strengths (valence units) (O’Keeffe & Navrotsky 1981) are: (CO 3 )
0.22, (HCO 3 ) – 0.17, O
2– 0.50, (OH) – 0.40, (H 2 O) 0.20. Thus in the transition from adamsite-(Y) to thomasclarkite-(Y), there is a shift from a carbonate to a bicarbonate. Grice & Gault (1998) and Grice (1991) discussed the typical formation of bi- carbonate minerals at low-temperature, slightly acidic conditions. Similarly, in this transition, more of the O atoms become protonated, thus lowering the Lewis base strength of thomasclarkite-(Y) relative to adamsite-(Y). 1457 38#6-déc.00-2203-13 27/02/01, 13:29 1464
ADAMSITE -( Y ) FROM
MONT
SAINT
- HILAIRE
, QUEBEC
1465 A CKNOWLEDGEMENTS The authors thank Gregory Young and Elizabeth Moffatt, Canadian Conservation Institute, Ottawa, for the thermogravimetric and infrared-absorption analyses, respectively, and Dr. F.C. Hawthorne, University of Manitoba, for use of the fully automated CCD-equipped four-circle diffractometer. Dr. Peter Tarassoff gener- ously provided the specimens that were used in this study. We also thank László and Elsa Horváth for infor- mation regarding the occurrence and associated species of adamsite-(Y), Dr. Andrew McDonald, Laurentian University, for biographical information on F.D. Adams, and Dr. Terry Williams, The Natural History F IG
The adamsite-(Y) structure projected along [010]. Red triangles represent (CO 3 ) polyhedra, the Na atoms are yellow, the Y atoms are orange, and the (H 2 O) groups are blue. The unit cell is outlined. F IG . 5. a) The adamsite-(Y) structure projected along [010] with c vertical. Red triangles represent (CO 3 ) polyhedra, the Na polyhedra are yellow, and the Y polyhedra, orange. b) The thomasclarkite-(Y) structure projected on [100] with b vertical. Same color scheme as for adamsite-(Y). 1457 38#6-déc.00-2203-13 27/02/01, 13:29 1465
1466 THE
CANADIAN
MINERALOGIST Museum, London, for providing the photograph of adamsite-(Y) shown in Figure 1. The manuscript was improved by the comments and suggestions of two ref- erees, Drs. Andrew McDonald and Carlo Gramaccioli and Editor Dr. Robert F. Martin. This research was made possible by the Canadian Museum of Nature. R EFERENCES A DAMS
, F.D. (1903): The Monteregian Hills – a Canadian petrographical province. J. Geol. 11, 239-282. C OTTON
, F.A. & W ILKINSON
, G. (1980): Advanced Inorganic Chemistry (4 th ed.). Wiley-Interscience, New York, N.Y. (497-498). G RICE , J.D. (1991): Bicarbonate minerals: crystal chemistry and geological significance. Geol. Assoc. Can. – Mineral. Assoc. Can., Program Abstr. 16, A47. ________ & G AULT , R.A. (1998): Thomasclarkite-(Y), a new sodium – rare-earth-element bicarbonate mineral from Mont Saint-Hilaire, Quebec. Can. Mineral. 36, 1293-1300. ________, V AN V ELTHUIZEN , J. & G
AULT , R.A. (1994): Petersenite-(Ce), a new mineral from Mont Saint-Hilaire, and its structural relationship to other REE carbonates. Can. Mineral. 32, 405-414. H ORVÁTH , L. & G AULT
, R.A. (1990): The mineralogy of Mont Saint-Hilaire, Quebec. Mineral. Rec. 21, 284-360. L E
AGE , Y. (1987): Computer derivation of the symmetry el- ements implied in a structure description. J. Appl.
M ANDARINO , J.A. (1981): The Gladstone–Dale relationship. IV. The compatibility concept and its application. Can. Mineral. 19, 441-450. M OCHIZUKI , A., N AGASHIMA
, K. & W AKITA
, H. (1974): The synthesis of crystalline hydrated double carbonates of rare earth elements and sodium. Bull. Chem. Soc. Japan 47, 755-756.
O’K EEFFE
, M. & N AVROTSKY
, A. (1981): Structure and Bond- ing in Crystals. Academic Press, New York, N.Y. Received June 12, 2000, revised manuscript accepted Septem- ber 30, 2000. 1457 38#6-déc.00-2203-13 27/02/01, 13:29 1466 Yüklə 142,6 Kb. Dostları ilə paylaş:
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