SIMS Laboratory CNR-IGG Pavia

Complex matrixes 

A procedure has been developed for accurate quantification of REE, actinides, light and volatile elements in complex matrixes of interest in earth and material sciences. These minerals show a peculiar chemical composition due to the coexistence of high amounts of LREE, U, Th, with variable quantities of H, Li, Be, B and F. Molecular interferences have been resolved and problems arising from unpredictable matrix effects and lack of reference materials have been overcome. X-ray single-crystal structure refinement (SREF), which is not affected by matrix effects and does not require analytical standardisation, has been used to precisely estimate (by means of both site scattering and site geometry) in a wholly independent way the amounts and the distribution of the various groups of elements. The combined SIMS-SREF approach allowed us to obtain results of impact in mineralogy and also allowed the accuracy of SIMS investigation to be fixed (Ottolini & Oberti, 2000).

Such a procedure was fundamental in the re-definition, nomenclature and crystal chemistry of hellandite (REE mineral) (Oberti et al., 1999; 2002).

Eight root end-member compositions were identified of which two reported for the first time (ciprianiite and mottanaite-Ce) (Fig. 2).

We have stressed the capability of SIMS in the quantification of light (Z < 6) and heavy (Z > 57) elements as both minor and major constituents in two samples (named LOS and CAPR) of britholite, the REE analogue of apatite, but with REE and actinides, and Si replacing Ca and P, respectively (SREE_(ox) up to ~ 70 wt% in LOS) (Fig. 3)

Similar procedures were used to characterise other complex silicates, such as vesuvianite, and peprossiite-(Ce) (an anhydrous REE and Al mica-like borate). In the latter case, a recent investigation of a Th-rich peprossiite-(Ce) with more complete chemical analyses and advances in analytical techniques allowed for a resolution of existing problems related to the interpretation of the observed structural disorder at atomic scale. The re-definition of peprossiite-(Ce) has been approved by the IMA-Commission of New Minerals and Mineral Names (99V) (Callegari et al., 2000). We dedicated this work to the memory of Giuseppe (Pep) Rossi, whom this mineral was named after, in the decennial of his passing away.

A systematic investigation is in progress to characterise chemical (by EMPA and SIMS) and structural variations (by SREF) in minerals of the hellandite and gadolinite group. It is aimed at identifying crystal-chemical constraints, which may be used to tailor new materials for radioactive waste disposal.

Analytical methods based on SIMS were developed for the characterisation of a complex layered silicate REE-mineral, named sazhinite (Ottolini et al., 2004), for which a number of issues were still open regarding its chemistry and structure: the species and quantity of alkali cations, and coordinating water molecules within the channels; accurate quantification of REE and possible actinides; the presence and location of other minor additional chemical species such as fluorine and sulphur. Such procedures involved the analysis and quantification of light, volatile, alkaline, medium-Z, rare earth and actinide elements. 
The accuracy of the SIMS data resulted to be within the assigned precision of the concentration values assumed as reference in the calibration standards employed. REE and actinide data yield a good agreement in terms of calculated site scattering at the M site: 58.42 electrons per formula unit (epfu) vs. 60.39 epfu obtained by Single Crystal Structure- Refinement (SREF). Accuracy is estimated on the order of 5% rel. for H, Li, Be and B, and 10% rel. for F. 
Na analysis was crucial to solve the open questions about the structure. The presence of Na, which is weakly bonded within the channels and bonded to water molecules, may give rise to diffusion processes within the matrix under the electron beam, yielding underestimation (up to 52% for crystal 1) of the actual contents of this element. This effect can be even enhanced by channelling phenomenon depending, in this matrix, on the crystallographic orientation of the structure channels relative to the e-beam (Fig. 4). Interestingly, Na content by SIMS in crystal 2 is higher by ~ 10% rel. than that in crystal 1. It is very probable that we are in the presence of SIMS matrix effects related to the crystal structure, similar to those affecting H/Si ionisation in micas.

SIMS procedures were used for the analysis of H, F, Li, Be, B, REE, Y, actinides (U, Th, Pb), and other trace elements (Sr, Ba, Cs) in okanoganite-(Y) (Boiocchi et al., 2004). An excellent agreement was obtained by comparing EMPA+SIMS with  SREF data. On the basis of 38 O atoms, the resulting unit formula is (Y_4.52 REE_6.82 Ca_2.65 Na_1.63 Th_0.19 Sr_0.02 Ba_0.01U_0.01) _Σ 15.85  (Fe³⁺_0.74Ti_0.19Li_0.04) _Σ 0.97  (Si_6.71P_0.32B_2.94 Be_0.01) _Σ 9.98 (O_34.02 OH_3.98) _Σ 38 F_10.04.  

In Fig. 5 our SIMS data for REE, normalized to C1-chondrite, are reported for the crystal studied here, and compared with literature data.

The structure of okanoganite-(Y) resembles that of vicanite-(Ce). They are the only two borosilicates showing a structural unit of threefold rings of BO₄ tetrahedra. The main differences between the two minerals lie in the different chemical composition: absence of As and low amount of Ca and Th in okanoganite-(Y); absence of Y and low amount of Na in vicanite-(Ce) and in the lack, in okanoganite-(Y), of a B atom that is at the center of a triangular BO₃ coordination in vicanite-(Ce).  

A new occurrence of the rare mineral wiluite, the B-rich equivalent of vesuvianite, was found at Ariccia, Alban Hills volcano, Rome (Italy). This sample was fully characterized with our ion microprobe in Pavia in terms of light, volatile and heavy elements, i.e.,  H, Li, Be, B, F, Y, REE, Th, U and Sr (Bellatreccia et al., 2005b). Wiluite from Ariccia has a composition very similar to wiluite from Sakha Republic (Russia) but is characterized by higher B (3.35 B₂O₃ wt%) and F (0.774 wt%). Its chondrite normalized (REEcn) REE pattern (Fig. 6) is broadly similar to the patterns displayed by most minerals from the volcanic ejecta of Latium.

A Titanium-rich vesuvianite from skarnoids near Nedvedice in the Western Moravia, investigated by our microprobe, shows qualitatively a similar trend (Fig. 7) with the higher Ce concentration among the Light Rare Earth Elements by 1095 ppm. Note the negative Eu anomaly and the Ce/Yb normalized concentration ratio by a factor of 100 (Filip et al., 2005).

This behaviour suggests that vesuvianite has a defined preference for LREE over HREE,  even if  this point cannot be definitely evaluated because in the mineralogical literature REE data on vesuvianite group minerals are to date rare and incomplete.

Piergorite-(Ce), a new borosilicate with a modified hellandite-type chain, has been found in myarolitic cavities inside a syenitic ejectum from the Vico volcanic complex at locality Tre Croci, Vetralla (Latium, Italy) (Boiocchi et al., 2006). The name piergorite is an acronym from the name of two Italian collectors who have provided us with the sample in study: Giancarlo Pierini and Pietro Gorini. Both are well known, appreciated and keen collectors of minerals, and are always available to provide the scientific community with specimens for several mineralogical researches. The new mineral name was approved by the Commission on New Minerals and Mineral names, I.M.A. 
The C1-chondrite-normalized REE pattern of piergorite is reported in Fig. 8, together with the C/C1 patterns of several hellandites from different localities, for comparison. It shows an enrichment in the LREE region and a linear decrease toward MREE region, with a negative Eu anomaly (Eu/Eu* = 0.06).

The pattern decreases slowly in the HREE region to increase again in Th and U. Previous studies show that REE-pattern of hellandites are very variable and mainly influenced by the environment of formation. The Eu anomaly is always present in all the samples studied and is presumably related with the presence of the coexisting feldspar (sanidine). The REE (Th,U)-pattern for piergorite is similar in shape (albeit with lower concentrations) to that of the hellandites of the same district, i.e., samples D22 and Hel-Ce (Oberti et al., 2002) both formed in a syenitic ejectum at Viterbo province (Vetralla and Capranica localities, respectively). Note that a very complete list of constituents (H, Li, Be, B, F, Na, Sc, V, Cr, Zr, Rb, Nb, Cs, Hf, Ba, Th, U, Y, REE) has been obtained for the first time by means of the SIMS technique, for a phase that strongly resembles the hellandite-type matrix (see Boiocchi et al., 2006, for details). 


Sazhinite-(La), Na₃LaSi₆O₁₅(H₂O)₂  from the Aris phonolite, Namibia, is a new mineral (IMA-CNMMN 2002-42a). Major constituents such as Na, La, Ce, together with minor and trace constituents, e.g., REE, actinides, Y, Zr, Sr, Ba, light and volatile elements (H, Li, B, F) were successfully measured with the ion microprobe. Its REE chondrite-normalized pattern is reported in Fig. 9 and compared with those from literature of the whole rock and sazhinite-(Ce) Lovozero. Sazhinite-(La) shows a linear decrease in REE from La toward the HREEs together with a depletion of Zr, Ti and, to a lesser extent, Nb. Such a depletion, more significant than in the whole-rock pattern, is caused by competition from the concomitant grow of other minerals such as eudialyte and korobitsynite, which will deplete the fluid in small-radius cations (< 0.8 Å) (Camara et al., 2006). 

SIMS is generally employed to measure the concentrations of trace elements by means of an empirical approach to quantifica­tion. Matrix effects, i.e., non-linear effects relating ion intensities to elemental con­centrations, increase considerably with increasing elemental contents. Hence, SIMS is rarely employed in major-element analysis. The SIMS technique capabilities of CNR-IGG, Pavia were recently extended to quantify a wide range of constituents, including major cations such as Si and Ca, to overcome the problems encountered during EMP analysis (see Cámara et al., 2008 for details). 
Secondary positive ions were detected at the following masses (in amu): 30 (Si), 44 (Ca), 89 (Y), 137 (Ba), 139 (La), 140 (Ce), 141 (Pr), 146 (Nd), 149 (Sm), 163 (Dy), 167 (Er), 174 (Yb), 232 (Th), and 238 (U). ²⁰⁸Pb⁺ was also monitored and quantified. The analysis of 1 (H), 7 (Li), 9 (Be), 11 (B), and 19 (F) was done on a different day to allow the crystal to degas under a vacuum of ~10^–7 Pa with the proper H-reference samples in the dual specimen-holder inlet-chamber. ³⁰Si⁺ was used as the internal reference for the matrix for Li, Be, B, U, Th, and Pb, whereas both ⁴⁴Ca⁺ and ³⁰Si⁺ were used for the quantification of H and F.
The chondrite-normalized REE (plus Th,U) pattern calculated from SIMS analysis (Fig. 10) is compared with that of a Th-rich hellandite-(Ce) from the same volcanic deposit.

The main result of this study is that lithium can be an important constituent of gadolinite-group minerals, and its presence should be systematically checked. Preliminary SIMS data on Alpine gadolinite shows significant quantities of Li at the ppm level [up to 250 ppm in a sample of hingganite-(Y) coming from Cuasso al Monte, Varese, Italy]. Lithium is most likely to occur when (1) the (Th + U) content is significant, or (2) the sum Si + B + Be is lower than 2 apfu. Hence, metamict samples (containing Th and U) should have significant Li contents. This work is part of a more systematic investigation of gadolinites and hingganites; so far, Li was found only at the ppm level in most of the samples examined, but we cannot exclude it might become a major component in suitable geochemical environments.

 

References cited

Ottolini L. & Oberti R.: Accurate quantification of H, Li, Be, B, F, Ba, REE, Y, Th and U in Complex Matrixes: A Combined Approach Based on SIMS and Single-Crystal Structure Refinement, Anal. Chem., 72, (2000), 3731-3738.

Oberti R., Ottolini L., Camara F., Della Ventura G.: Crystal structure of non-metamict Th-rich hellandite-(Ce) from Latium (Italy) and crystal chemistry of the hellandite-group minerals, Amer. Mineral., 84, (1999), 913-921.

Oberti R., Della Ventura G., Ottolini L., Hawthorne F.C., Bonazzi P.: Re-definition, nomenclature and crystal-chemistry of the hellandite group, Amer. Mineral., 87, (2002), 745-752.

Della Ventura G., Bonazzi P., Oberti R., Ottolini L.: Ciprianiite and mottanaite-(Ce), two new minerals of the hellandite group from Latium (Italy), Amer. Mineral., 87, (2002), 739-744.

Oberti R., Ottolini L., Della Ventura G., Parodi G.C.: On the symmetry and crystal chemistry of britholite: New structural and microanalytical data, Amer. Mineral., 86, (2001), 1066-1075.

Callegari A., Caucia F., Mazzi F., Oberti R., Ottolini L., Ungaretti L.: The crystal structure of peprossiite-(Ce), an anhydrous REE and Al mica-like borate with square-pyramidal coordination for Al, Amer. Mineral., 85, (2000), 586-593.

Ottolini L., Camara F., Devouard B.: New SIMS procedures for the Characterization of a Complex Silicate Matrix, Na₃(REE,Th,Ca,U)Si₆O₁₅·2.5H₂O (Sazhinite), and Comparison with EMPA and SREF Results, Microchim. Acta, 145, (2004), 139-146.

Boiocchi M., Callegari A., Ottolini L., Maras A.:  The chemistry and crystal structure of okanoganite-(Y) and comparison with vicanite-(Ce), Amer. Mineral. 89, (2004), 1540-1545.  

Bellatreccia F., Cámara F., Ottolini L., Della Ventura G., Cibin G., Mottana A.: Wiluite from Ariccia, Latium (Italy): occurrence and crystal-structure, Canad. Mineral., 43, (2005), 1457-1468.

Filip J., Houzar S., Ottolini L.: Titanium-rich vesuvianite from skarnoids near Nedvedice in the Western Moravia, Bull. mineral.-petrolog. Odd. Nár. Muz., 13, (2005), 125-129.

Boiocchi M., Callegari A., Ottolini L.:  The crystal structure of piergorite-(Ce), Ca₈Ce₂(Al_0.5Fe³⁺_0.5)_S1(☐,Li, Be)₂Si₆B₈O₃₆(OH,F)₂: A new borosilicate from Vetralla, Italy, with a modified hellandite-type chain, Amer. Mineral., 91, (2006), 1170-1177.

Camara F., Ottolini L., Devouard B., Garvie L.A.J., Hawthorne F.C.: Sazhnite-(La), Na₃LaSi₆O₁₅(H₂O₂)₂, a new mineral from the Aris phonolite, Namibia: Description and crystal structure, Mineral. Mag., 70(4), (2006), 407-420. 

Cámara F., Oberti R., Ottolini L., Della Ventura G., Bellatreccia F.: The crystal-chemistry of Li in gadolinite: a multi-analytical approach, Amer. Mineral., 93 (2008), 996-1004.