Canadian Mineralogist, Vol. 12, pp. 26-32 (1973)

THE STABILITY OF GASPEITE IN INERT ATMOSPHERES AND IN AIR

M. K. SEGUIN Department of Geology, Universite Laval, Quebec 10

Abstract siliceous Silurian dolomite intruded by serpenti- Ths rate of dissociation and the degree of stability nized plugs. Associated with the gaspeite are of magnesian gaspeite in air and in inert gases (nitro (Cr,Al) spinel, serpentine, , , gen and argon) were studied over the temperature gersdorffite, polydymite, heazlewoodite, magne- range 20 to 600°C by TGA and DTA. In the tem site, and annabergite. perature range under consideration, the solid end- products are NiO and MgO. The rate of dissociation A detailed study of the rate of dissociation of gaspeite is practically independent of the initial (i.e. the kinetics of decomposition) and the de amount of material and is mainly a function of the gree of thermal stabilityof gaspeite in air and in temperature, of the composition of the gaseous medium inert gases (nitrogen and argon) was undertaken in which the reaction takes place, and of the grain over the temperature range 450-610°C in order size. Variations of gaspeite concentration are of minor to gain knowledge about the relative importance importance. The reaction is approximately of the of grain size, temperature, degree of packing, first order with respect to the amount of undecomposed gaseous medium used, and concentration of the carbonate. The activation energies, calculated from 46 reactant (gaspeite), and to explain certain charac low. to high-temperature experiments and extrapolated down to 110°C, vary between 23.1 and 34.2 kcal teristics that occur in the more general system mole-1. The variation of the activation energies with Ni-O-C-S. temperature in the low temperature range was assumed to be linear. The instability of magnesian gaspeite in Experimental Method air at 20°C (room temperature) is established. Starting material The metastability of gaspeite at room temperature The material used in all experiments was can be explained by the formation of a fi'm (thin obtained from Dr. R. Ledoux, Department of layer) of NiO-MgO in the same way as AI is meta- Geology, Universite Laval, and consisted of na stable in air due to a coating of A1203 ; it is more probable, however, that the persistence of gaspeite in tural gaspeite from the Mount Albert area, Gaspe a geological setting is due to its enclosure in sur Peninsula. Cimon (1966) observed the co-ex rounding rocks in which the local development of istence of two nickeliferous carbonate phases in C02 pressure in the very small free space available the Mount Albert area. One phase is a is sufficient to suppress decomposition. iferous whereas the other is a mag nesian nickel carbonate. Both carbonates belong Introduction to the group and have similar d-spacings. Gaspeite is an unusual mineral in space and Treatment of these two carbonates in HC1 indi time. As recently as 1965, Goldsmith &Northrop cated that the magnesium carbonate is soluble, stated that normal nickel carbonate was unknown whereas the nickeliferous carbonate is insoluble. in nature. Isaacs (1963) found that natural nickel After treatment with HC1, important changes carbonate occurs as a hexahydrate (, were detected in the x-ray diffraction patterns NiC03.6H20) and as a hydroxyhydrate (zara- (114 mm diam. Debye-Scherrer, filtered Cu ra tite, ~NiC03.2Ni(OH)2.4H20). Calcites con diation). For the untreated nickel carbonates, taining0.83 and 0.65% Ni were reported byMak- d = 4.16, 3.75, 3.525, 2.995, 2.717, 2.491, 2.303, simovic (1952) and Maksimovic &Stupar (1953). 2.233, 1.930, 1.761, 1.688, 1.503, 1.487, and 1.401A. Chu-siang etal. (1964) identified a natural mag When the carbonates were treated with HC1 nesium-nickel carbonate, called hoshiite (now (IN), the d-spacings at 4.16, 3.75, 2.995, 2.491 discredited), having properties very similar to and 2.233A disappeared. The d-spacings given by the insoluble residual nickeliferous carbonate gaspeite from the Mount Albert area. lie between those of NiC03 and MgC03 and are The Mount Albert gaspeite has been described very close to data given by Langles (1952) and in detail by Kohls & Rodda (1966) and Cimon Isaacs (1963). For comparison, the d-spacings (1966). The mineral occurs near Mount Albert, of the nickel carbonates identified by Chu-siang Lemieux Township, Quebec, as light emerald, et al (1964), by Kohls & Rodda (1966) and zoned, crystalline rhombs (^0.6 mm in length) those of MgCOa are given in Table 1. As pointed in a vein, 2.5 feet wide, in varicoloured to buff out by Kohls & Rodda (1966) : "The similarity 26

Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/12/1/26/3445355/26.pdf by guest on 26 September 2021 27 THE STABILITY OF GASPEITE of the crystallographic data for magnesian gas area, before and after treatment with HC1, are peite, for MgCOa, and for NiCOa, and the similar shown in Figure 1. Before treatment with HC1, ionic size of nickel, magnesium, and iron suggest absorption bands were observed at 5.55, 6.99 (v3), that all three have the same calcite-type struc 9.90, 11.49 (v2), 13.33 and 13.70 (v4) microns, ture." Wet chemical analysis of the insoluble whereas, after treatment with HC1, they were at nickel carbonate treated with HC1yielded36.82% 5.55, 6.99 (v3), 9.90, 11.49 (v2) and 13.70 (v4) NiO, 14.83 MgO, 0.54 CaO, 40.25 C02, 0.17 microns. Typical spectra of the calcite depict MnO, 4.33 FeO, 0.98 Si02, 0.11 H20+ and three main absorption bands for the C03 mole 0.34 H20~ ; semi-quantitative optical spectrogra cule; these modes of vibration are designated phs analysis revealed the following impurities: as v2, v3 and v4. One of the absorption bands 0.01-0.1% Cr, Na, Co; 0.001-0.01% Al, Ba, Li, of the doublet (13.33 and 13.70^) disappears when the nickel carbonate is treated with HC1. Sr; 0.001% Cu. Adler & Kerr (1963a, 1963b) have published This result indicates that one of the two nickel many infrared spectra of various carbonates but carbonate phases went intosolution in HC1. The the spectrum of nickel carbonate is not included. increase in intensity of the absorption band at Kohls & Rodda (1966) reported that infrared 9.90n, corresponding to the Si-O mode of vibra absorption occurs at 7.00, 11.42 and 13.32 microns tion, represents an enrichment of the Si02 im for the gaspeite (Ni0.49Mgo.43Feo.o8)C03 stud purity in the insoluble nickel carbonate phase ied. An infrared spectrum of nickel carbonate after treatment in HC1. from Mount Albert was obtained with a Perkin- A semi-quantitative microprobe analysis was Elmer double-grating spectrometer Model 621, conducted ; many lines oriented in various direc using a high-resolution monochromator. KBr tions were traversed on polished sections of the pellets were prepared using 1.0 mg of finely- nickel carbonate of Mount Albert. The relative powdered mineral in 200 mgof KBr. The experi intensity distribution (//Jo, expressed in %) was mental method was similar to that described by obtained, for Ni, Co, Mn, Mg and Ca. Concen Adler & Kerr (1963a, 1963b). The infrared spec trations of Co, Mn and Ca are, without excep tra of the nickel carbonate of the Mount Albert tion, directly proportional to that of Ni, whereas that of Mg is always inversely proportional to

Wavelength (microns) that of Ni. The presence of a zoned texture, as observed under the microscope and in the microprobe, the enrichment in nickel and im poverishment in magnesium detected by chemical analyses after treatment with HC1, and the dis appearance of one mode of vibration in infrared spectrometry after such treatment, are evidence supporting the co-existence of two phases of nick eliferous carbonates as indicated by Goldsmith & Northrop (1965). They observed the co-exist ence of two phases, one a nickeliferous magnesite and the other a magnesian nickel carbonate. If one considers that only Ni2+, Mg2+ and Ca2 +

05 x10s enter into the chemical formula of the mineral, Frequency (cm"') the composition of the insoluble nickel-rich Fic. 1. Infrared spectra of magnesian nickel carbonate phase is Nio.sco Mgo.423 Cao.011 C03 ; it is this from the Mount Albert area. Absorbanca increases phase that will be dealt with in detail in this downward. paper.

WJ-, «HU \n 1 ,1-19, TABLE 1 N d-5PAClNlib IA ) ur n ilu3» ng "'w3

0006 1121 1113 2022 0224 1126 2131 hkii 10T2 10T4 1.676 1.477 « 2.299 2.089 1.922 1.752 NiCO, (Langles 1952) 3.504 2.704 1.678 1.4746 NiC03 (Isaacs 1963) 3.5077 2.7040 — 2.3001 2.0828 1.9224 1.7511 2.309 2.093 1.9305 1.7597 1.687 1.481 Ni carbonate, Mt. Albert (insol. 3.525 2.730 - in HC1 ) 1.692 1.485 Gaspeite (Kohls & Rodda 1966) 3.543 2.741 2.317 2.098 1.932 1.766 1.6979 1.4849 Ni carbonate (Chu-siang et al_. 1964) 2.7393 — — 2.103 — - 1.769 1.700 1.488 MgC03 (PDF 8-479) ~ 2.742 2.503 2.318 2.102 1.939

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Under the polarizing microscope, the zoned ments using another method were also conducted equigranular crystals show imperfect uniaxial but the results are not included in this paper. negative extinction figures. The refractive indices are ne = 1.58 and nw= 1.81. A comparison of Identification of the end products the optical and crystallographic data for the mag nesian nickel carbonate with those for MgC03 All crystalline products were identified by and NiC03 (Graf 1961 ; Langles 1952; Isaacs x-ray diffraction using either a Guinier focusing 1963) shows the remarkable similarity of mag camera with FeKai radiation (30 kv, 10 ma) or nesian gaspeite to these two compounds (Kohls a Debye-Scherrer camera with CuKa or FeKa &Rodda 1966). A comparison of the data obtain radiation. In cases of uncertainty, infra-red scan ed for nickeliferous (Chu-siang et al. ning was performed on the solid end-products. 1964) and for the magnesian nickel carbonate suggests that they are a solid-solution series, the Procedure end members of which are MgCOa and NiC03. Except for the DTA experiments, the experi mental procedure in use has its foundation in Instrumentation mass-difference analysis. The calculations are The decomposition of gaspeite was studied made using the ratio of the material dissociated using thermogravimetric (TGA) and differential at time t to the total amount dissociated at thermal analyses (DTA). With the first method infinite time (t = «>). In practice, the time for (TGA), two types of experiments were per complete dissociation can be considered as finite formed ; one type consisted of measuring the (e.g., time at 99.3% or at 99.8% dissociation). variation of mass of the reactant (gaspeite) as a Then the percentage of gaspeite dissociated was function of temperature at a rate of 160°C/min. plotted against log time (Fig. 2). and obtaining a thermogram, whereas the other The order n was then obtained as shown in type was a method of isothermal analysis. The Table 2. From equation (3) in Table 2, the half thermobalance used was a Perkin-Elmer (Can life of the reacting species is a function of the ada) Model TGS-1. Ancillary equipment con order of the reaction and is inversely propor sisted of a Perkin-Elmer (Canada) temperature tional to the power (n —1) of the initial con controller, a Philips Model PM 8100 dual-pen centration. This expression holds for any value flatback recorder, and an inert-gas circulator. of the order n = 1; n may be an integer or a Among other advantages, this system permits an fraction, positive, negative or zero. exact measurement of the temperature, a great An order n was assumed for the reaction and sciisitivity, a rapid cooling rate, and the attain the rate constant Kn calculated directly from the ment of an almost instantaneous isothermal curve log t against the percentage dissociated. equilibrium. The accuracy of the measurements Using individual values of Kn in some cases and of the mass differences is of the order of 0.2% the averaged value of K„ in others, the experi and the temperature is controlled with a precision mental curve was reproduced from theory. If the of ± 1°C. theoretical time obtained compared well with the The differential thermal analyses were per experimental time obtained, the order was re- formed with a conventional Stanton (Model Standata — 658) DTA instrument consisting of 2 3 4 9 two cells; one containing a reference material I I I ' (u-Al203) and the other, the solid reactant site < 60 mesh (sample). Ancillary equipment included a tem in Nitrogen) perature programmer and a Fisher Scientific Model PSOIWGA Servowriter II two-pen re corder. Platinum vs. 87% platinum : 13% rho dium thermocouples were used for these experi ments. The uncertainty in temperature is ± 2°C. The programmed rate of temperature rise was 10°C/min.; the mass of the sample varied be tween 200 and 400 mg. Control of P(C02) and (Os) The experiments were conducted in free air

(static) and in an inert gas at the ambient Time (tee) temperature. Intentionally, no CO or C02 was Fig. 2. Dissociation of gaspeite in nitrogen for 5 dif circulated through the system. Buffered experi ferent isotherms.

Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/12/1/26/3445355/26.pdf by guest on 26 September 2021 THE STABILITY OF GASPEITE 29 tained. The rate constant, Kn, can also be ob rate of dissociation is independent of the initial tained graphically by dividing the slope of a amount of reactant and that the order of the log™ (100 — x) against t curve by logioe. It was reaction is approximately one. established experimentally for gaspeite that the Integration of the equation for a first order reaction (equation 4, Table 2), yields equation (5) in Table 2. A linear plot of log C/C0 against TABLE 2 t will show that the reaction is first order. The method of study is based on the rate of reaction of any order «, whose formula is Experimental Results ?= -de K C dt :d Figures 2 and 3 show the results of some 10 where s=rate of the reaction, ^concentration of the thermobalance experiments on —60 mesh ma reactant(s) in wt. %, t=time, K =rate constant (of terial. Seventy-five other experiments, some with order n), n=order. n — 100 and others with —200 mesh material, Integrating equation (1): were also performed in a nitrogen atmosphere and in air. Reaction temperatures ranged from 400-600°C for a nitrogen atmosphere, and 350- 600°C for air. Similar experimental results and K t = (2) good reproducibility were obtained in both atmos («-l)G1 n-1 Wf -1! pheres.

From equation (2), at c=c0/2, the half life tx is Interpretation of results given by ~ For a first order reaction, the half-life is given (3) by (6) in Table 2. Knowing Ki (which is a constant independent of the percentage of ma The equation for a first order reaction terial reacted) and the percentage of material -dC = K,C (4) reacted, the corresponding time, t, may be cal dt ' culated. That is, if the value of Ki is correct, on integration gives In C_--K->t, (5) the experimental curve can be reproduced from Co theory; this was done. Once the rate constant where c=wt. of non-reacted sample at time t, Coa wt. of original sample at t=0, c/C<,=fraction of Ki is found, the activation energy of the reaction decomposed material. can be calculated by using the Arrhenius formula given in Table 2. For a first order reaction: i = ln2 / K (6) A plot of logioKi against 103/T gives a straight where K, is a constant. line, the slope of which is E/4.576. Because the The activation energy of the reaction can be cal order, rate constants, and temperatures are known, culated using the Arrhenius formula: the activation energy can be calculated from -E/RT low-temperature runs (400° and 450°C) for the K] ~ Ko reaction (Ni0.566Mg0..i23Cao.ii)C03 ~* C02 + where ^frequency factor or rate constant at in

finite temperatures (1/Z=0), E=activation energy, 2 3 4 1 6 789 1 2 3 4 S 6 7691 #=gas constant, T= temperature in degrees Kelvin.

Variation in concentration after time ty In c} /c0= -K^ (7)

after time t~. In C? lc°~ "*l*2 (8) To obtain almost complete dissociation (99.3%):

(9) = -270 in c I C ln 0.9927 In c2 ' fc» - 270 ,_/l-0.17905\ or t. " -0.00732 in[ 0.18031/ - 270 ,. /0.00126\ _ 270 , - llfinnx " -0.007321" ^OTlSOTTJ " -0.00732("5-11600J 1000 Time (sec) Fig. 3. Dissociation of gaspeite in air for 5 different = T.89 x 105 days -520 years isotherms.

Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/12/1/26/3445355/26.pdf by guest on 26 September 2021 30 THE CANADIAN MINERALOGIST (0.566 NiO + 0.423 MgO + 0.11 CaO) and t2, by formula (8). After 270 days (h) an origin extrapolated down to 100°C (approximate insta al 0.42638-g. sample of gaspeite had lost 0.00131 bility limit of gaspeite in air). Rate constants g. (0.73%) at room temperature. As a gram were found by extrapolation to lower tempera molecular weight of (Ni0.5G6Mgo.423Cao.oii)C03 tures. In the approximate temperature range which is 104.1, would lose 44 g. of C02 at com 450c to 620°C, and for a grain size smaller than plete dissociation, the loss in weight of the ori 60-mesh, the calculated activation energy was ginal 0.42638-g. sample of gaspeite would be 23.9 kcal mole-1 in a nitrogen atmosphere and 0.18021 g. at complete dissociation (=99.3%), 34.0 kcal mole-1 in air. For smaller grain sizes that is: (e.g., 200- and 300-mesh), the calculated activa tion energy was somewhat smaller. Figure 4 shows a plot of logioKi vs 103/T for some experi ln(l - |S)= (_/Cl) (270) and thus Kl = -In 0.9927/270 ments performed in air and in a nitrogen atmos phere. The activation energies were calculated Knowing K\, one can calculate the time (fe) directly from this diagram. required to obtain almost complete dissociation (99.3%) as shown in equation 9 in Table 2. At 120°C, approximately 0.4% and 0.75% of the original magnesium gaspeite dissociated after 116 and 240 hours, respectively, and the average calculated value of Kj. is equal to 3.26 X 10-5 hours-1. The calculated half life (tyt) is then 2.43 years and after 17.1 years almost complete dissociation (99.2%) is attained. This means that, for all practical purposes, gaspeite is un stable in air and at the very most metastable in an inert atmosphere at room temperature. The experimental evidence (see Fig. 5) indicates that the rate of decomposition is slower in an inert atmosphere than in air. Large ideal crystals

S ,0 v Nitrogen •i » S 30 Air \

8 30 o

10 /T (°lO f » & 80 Fig. 4. Diagram of calculation of the activation energy "• 90 too • for the decomposition of magnesian gaspeite in air 396 428 460 492 324 SS6 and in nitrogen. Temperotuie (*C) Fig. 5. Thermogram of magnesian gaspeite dissociated The dissociation of gaspeite is a direct reaction an air and nitrogen gaseous medium. involving no intermediate products in the transi tion state. NiO is encountered as an end-product of gaspeite, approximately 5 to 8 cm across, may at all temperature ranges investigated ; the other be geologically stable but such crystals have not end-products are MgO, CaO (traces) and iron yet been found in nature. Gaspeite heated in an oxides (traces). The x-ray powder diffraction oven at atmospheric pressure between 100° and patterns of NiO are particularly in evidence. The 150°C for a few hours develops a coating of dull time of nearly-complete dissociation (99.3%) pistachio yellowish-green oxide indicating a pre obtained by calculation for a temperature of ferential surface reaction. This thin coating of 20°C and in air is of the order of 520 years oxide(s) on gaspeite may act in exactly the same (t*4 = 2.6 X 104 days = 72 years) for a grain way as does AI2O3 on the surface of Al. The size smaller than 100-mesh, and at least twice degree of packing of gaspeite in the containers, as large for a size of approximately 60-mesh. as well as variations of concentration of gaspeite This result was arrived at in the following way: when mixing it with inert components such as after a time h, the variation in concentration is AI2O3, Si02, etc., does not change the order of given by formula (7) in Table 2, and after time the reaction.

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In the low-temperature range (150° to 480°C), the rate of dissociation of gaspeite is faster in air than in nitrogen ; actually, gaspeite is relatively stable in a nitrogen atmosphere below 380°C.

This is not the case in air, where the dissociation DTA cu*«t ol magneton gaipeile

is readily observed at 40°C (0.75% dissociated K>*C / mix at 80°C after 400 hours). In the temperature range 500-580°C, the rate of dissociation is not very different in air from that in an atmosphere of nitrogen, but, above 580°C, the difference is more pronounced. This means that the gaseous medium alters the rate of the reaction and in fluences the speed of dissociation ; thus at higher o 4o»i»irozoot«oao3»M040o**o«ao93>9co(oo<4oaoTzo>*otoo temperatures (roughly 570 to 600°C), gaspeite Temperature (*C) dissociates more slowly in air than in an inert atmosphere. The slower decomposition of the Fig. 7. DTA curve of gaspeite from the Mount Albert gaspeite in air above 570°C, as indicated by the area. The direction of endothermic reaction is thermogram, should also reflect the fact that downward. almost all the gaspeite has already decomposed. One hypothesis that can explain this kinetic (1966) stated that the decomposition of magne process is that the outward diffusion of large sian gaspeite begins at 520°C and reaches an polar molecules, such as C02, is slowed by the endothermic peak at 690°C. A DTA analysis of surface layers of NiO (mainly), MgO and CaO zaratite (Fig. 6) shows two endothermic peaks, that formed during carbonate decomposition. For the first corresponding to concurrent dehydration a percentage of dissociated gaspeite smaller or and loss of water of crystallization, and the equal to that corresponding to the half-life, gas second corresponding to carbonate decomposition peite persists longer in air than in nitrogen in into NiO. The thermal stability of the Mount the high-temperature range; the opposite situa Albert gaspeite is much greater than that of tion is encountered in the low-temperature range. zaratite or synthetic NiC03. DTA curves were obtained for gaspeite at Attempts were made to calculate the variation 1 atmosphere pressure (Fig. 7). The accuracy of in enthalpy or heat of reaction in a semi-quan the temperature is ± 2°C and that of the dif titative fashion. The quantity of heat absorbed ferential e.m.f. is ± 0.2uv; the programmed rate or released during a reaction is proportional to is 10°C/min. A typical DTA curve showsa single the area under the DTA curves obtained, the endothermic peak at 740°C. The endothermic quantity of sample, the heat capacity of the reaction, which starts at 360 and is completed sample, etc.. The instrument was calibrated using at 740°C, corresponds to the decomposition of a standard FeO(OH) of trade mark Mapico, the magnesian gaspeite into C02 and the Ni and manufactured by Columbian Co. Ltd., Montreal, Mg oxides; this suggests that either MgO and Canada, and by Mapico, Cities Service Co., NiO form at the same temperature, or that the U.S.A. The DTA curve shows an endothermic two endothermic peaks are unresolved and form peak at 330°C and the temperature range of the a single fairly wide peak. peak extends from 230° to 360°C. The enthalpy Isaacs (1963) conducted DTA experiments on of reaction of this dehydration is 16.0 kcal several samples of synthetic NiC03 and observed mole-1 of H2O and the equivalent area under only one endotherm at 345°C. Kohls & Rodda the curve is 3.334 volts C° per molecular weight. The equivalent area under the gaspeite decom position curve (from 300 to 750°C) is 6.929 volts C° per molecular weight and so the en thalpy of reaction is the order of 33.2 kcal mole-1 of C02 (Fig. 7). In all these experiments, the Zorotite grain size was approximately 60 mesh. Ni C03.2Ni(OH2).4H20 25*C/min Conclusion In the temperature range 20° to 600°C, the rate of dissociation of magnesian gaspeite is 100 200 300 400 500 600 700 °C 80° practically independent of the amount of admixed Fig. 6. Typical DTA curve of zaratite. The direction inert material and the degree of packing. Con of endothermic reaction is downward. sistently similar results, obtained from experi-

Downloaded from http://pubs.geoscienceworld.org/canmin/article-pdf/12/1/26/3445355/26.pdf by guest on 26 September 2021 32 THE CANADIAN MINERALOGIST ments on systems with the gaspeite concentra Geological Survey of Canada (grant no. 30-68- tion, being varied by admixture of inert compo 1971) and to the Dept. of Physics of Universite nents and with differing degrees of packing, con Laval for special funds for financially supporting firm this conclusion. In order of importance, the this work. rate of dissociation is a function of the tempera ture of the gaseous atmosphere in which the References reaction takes place, and of the grain size. A Adler, H.H. & Kerr, P. (1963a) : Infrared absorption first-order equation was established for the re frequency trends for anhydrous normal carbonates. action, and the activation energy was calculated Amer. Mineral. 48, 124-137. for low-and medium-temperature reactions. No & (1963b) : Infrared spsctra, intermediate metastable dissociation products symmetry and structure re ation of some carbonates. were detected. Amer. Mineral. 48, 839-853. For all practical purposes, magnesian gaspeite Chu-siang, Y., Kuo-fun, F., & Chen-ea, S. (1964) : is metastable at room temperature and at 110°C Hoshiite (a new mineral). Acta Geol. Sinica 44, in air. Up to a temperature of 350°C, gaspeite 213-218. persists indefinitely in an inert atmosphere but Cimon, J. (1966) : Le Carbonate de Nickel du Mont would decompose very rapidly in air if it were Albert. Those de Baccalaurcat en Genie Geologique, Universite Laval. it not for a thin surface layer composed of a Goldsmith, J.R. & Northrop, D.A. (1965) : Sub- mixture of NiO and MgO which inhibits the solidus phase relations in the systems CaC03- diffusion of C02 out of the crystal lattice. Thus, MgCO:,-CoC03 and CaC03-MgC03-NiC03. /. Geol. the smaller the grain size and the total binding 73, 817-829. energy, the greater the surface energy per unit Graf, D.L. (1961) : Crystallographic tables for the volume favouring outward diffusion of large rhombohedral carbonates. Amer. Mineral. 46, 1283- polar molecules. 1316. Magnesian nickel carbonate is not found in Isaacs, T. (1963) : The mineralogy and chemistry of the nicke' carbonates. Min. Mag. 33, 663-678. great abundance in nature. The mineral occurs Kohls, D.W. & Rodda, J.L. (1966) : Gaspeite (Ni.Mg, as fine grains, up to 0.5 mm in diameter, in Fe)(C03), a new carbonate from the Gaspe Penin veins and fractures in the Mount Albert area, sula, Quebec. Amer. Mineral. 51, 677-684. and in an oxidized nickeliferous copper sulphide Langles, R. de S.-L. (1952) : Preparation et structure deposit in an arid region of central China (Chu- du carbonate neutre de nickel anhydre cristallisc. siang et al. 1964). Gaspeite is also known to Ann. Chimie, Serie 12e, 7, 568-583. occur at Pafuri Native Trust, Transvaal, Republic Maksimovic, Z.J. (1952) : Resultats preliminaires de of South Africa, and at Kambalda, West Austra l'examen des affleurements de nickel du village de lia. The rarity of gaspeite in nature may be an Ba, pres de Ljig, dans la Serbie occidentale. Re:ueil indication of its instability under normal atmos Travaiix, Acad. Serbe Sci. 23, Inst. Geo]. 4, 21-48 et pheric conditions. 49-52. & Stupar, J. (1953) : La calcite nickelo- magnesienne et l'aragonite de Rujevac (village de Ba, Acknowledgements Serbie occidentale). Rezueil Travaux, Acad. Serbe Thanks are due to the National Research Sci. 33, Inst. Geol. 5, 220-221. Council of Canada (grant no. 7070-110), to the Manuscript received June 1972, emended October 1972.

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