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An interesting find of thorosteenstrupine and steenstruvine-(Ce) at Mt. Koashva, Khibinv massif (Kola Peninsula. Russia) |
Introduction
During observations and analyses of all mineral species collected by the authors in the differentiated alkaline pegmatites of Mt. Koashva, Khibiny massif (Kola Peninsula, Russia), in order to prepare a comprehensive and up-to-date article on the locality for RMl, some unusual, black-brown, radioactive crystals and aggregates have been observed. Microprobe analyses have allowed to identify such material as thorosteenstrupine and steenstrupine-(Ce), two species found for the first time at Mt. Koashva and in the Khibiny alkaline massif.
Thorosteenstrupine is a rare mineral species, whose occurrences to date are known only in Russia. Till the new find at Mt. Koashva, idiomorphic crystals of this species were unknown. Furthermore, although thorosteenstrupine was described in 1962, the status of this species remained for long time questionable. Only a recent find (1997) of a mineral with variable ratios of Th, REE, P, and Si and the holotype re-examination have permitted to establish the existence of isomorphism with steenstrupine-(Ce) and to define more thoroughly the species. Steenstrupine-(Ce), is an enigmatic mineral, that, despite the tenths of studies carried out during a 120-year period, remained not well-defined till the eighties. Despite the structure solution, due to its extremely complex crystal chemistry, steenstrupine has continued to show up to today some "dark sides". Crystal chemical considerations induce to consider the above mentioned minerals as members of a mineral group comprising various species in the modern point of view.
Historical outline
Steenstrupine-(Ce). The mineral known today as steenstrupine-(Ce) was early found in the llimaussaq complex, Greenland, by H. J. Rink during his visit in 1853-54 and the sample by him collected was misidentifiedfor long time with wolframite. In 1876 the Danish geologist K.J. V. Steenstrup collected in the llimaussaq alkaline complex many specimens of such mineral from pegmatoid veins located at some occurrences on the southern side of Tunugdliarfik and in particular at Kangerdluarssuk. These specimens were sent to Copenhagen and investigated by J. Lorenzen, who described the mineral as a new species in 1881, naming it after the finder. The suffix -(Ce) has been added to the mineral later, applying the Levinson's nomenclature rules, as stated by the 1MA CNMMN. After its first description, during a period one century long, the llimaussaq mineral (in particular specimens from Tugtup agtakorfia, Igdlunguaq, Naujakasik, Tuperssuatssiaq, Nunarssuatsiaq, Qeqertaussaq, and the head of Kangerdluarssuk to the north of Lilleelv) periodically became the object of studies on its crystallographic and optical properties, composition, and X-ray diffraction patterns by various scholars, including G. Flink, J. Moberg, O. V. Boggild, J. Thomsen, F. Zambonini, F. Machatschki, H. Strunz, H. S0rensen, and E.I. Semenov.
In 1937, steenstrupine was discovered in the ussingite pegmatites at Mt. Malyi Punkaruaiv, Lovozero massif, Kola Peninsula and later in other occurrences located in the same massif, as at Alluaiv, Bol'shoi Punkaruaiv, Karnasurt, Kedykverpakhk, Lepkhe-Nel'm, and Sengischorr mounts, at the Second Raslak Cirque and at the headstream of Kitkuai River.
Furthermore, from the middle of the 50's, the increasing interest in uranium, thorium, and rare-earth elements led to a number of specific studies on some peculiarities of steenstrupine. In particular, a REE-U deposit with steenstrupine-(Ce) in the state of ore mineral has been prospected at Kvanefjeld, northern part of the llimaussaq complex.
In spite of the large number of studies on steenstrupine, due to the complexity of the mineral and the frequent metamictization and alteration (its composition may differ from one specimen to the other and even inside sectors of the same crystal or grain), the formulae proposed till the eighties are not reliable. For instance, Boggild (1901) suggested that steenstrupine is probably a metasilicate. This approach was reconsidered by Machatschki (1931) and maintained by Soviet authors. On the contrary, Strunz (1944) and the same Boggild (1953) considered the mineral as an orthosilicate. Povarennykh (1966, 1972) is the only author treating steenstrupine explicitly as a cyclosilicate. In the eighties two fundamental papers on steenstrupine-(Ce) have been published (Makovicky & Karup-Moller, 1981; Moore & Shen, 1984) thanks to the find of crystalline, only lightly metamict steeenstrupine-(Ce) on the northern shore of Tunugdliarfik, south of the lake Taseq, llimaussaq complex. In the former study is given the empirical formula Na3,41Ca1,04(REE)6,03(Th0,48U0,05Zr0,41 Mn1,80Fe1,69AI0.07Ti0.10)4.60(Si13.39P4.61)18056.82F0.44 + H20 totale < 17.94 and, taking into account statistical variations in composition, the general formula Nal- 12H7.0CaREE6Me5[Si6018]2[(P,Si)O4I6(OH,F) - nH20;
is proposed; in the latter is determined its crystal structure: a cyclosilicate of extreme complexity in which at least 23 elements may play some role and for which is given as a possible end-member the following sempli-fied formula Na14Ce6 Mn2Fe3+2(Zr,Th)(P04)7(Si6018)2 - 3H20, that is reported on Fleischer's Glossary (Mandarino, 1999). Other proposed possible formulae are Na14Ce6 Mn2+2 Fe2+Fe3+Th(OH)(OH)2(P04)6 (Si6018)2 - 3H20 and Na2[ ]12Ce6Mn2+2 Fe2+Fe3+Th(OH)(OH)2 (P04)6 [Si6012(OH)6]2 - 3H20.
In any case, the three proposed formulae show a too high P2O5 content and, clearly, (P,Si) solid solution must be taken into account. The assumption that more closely complies with the real analytical data is (Si+P)=18.
Outside Greenland and Russia, the presence of steenstrupine-(Ce) has been reported at Mont Saint-Hilaire, Quebec, Canada. Here the mineral forms 0.5-3 mm, sharp, tabular, hexagonal crystals, rounded glassy plates and blobs in sodalite xenoliths in association with sodalite, eudialyte and aegirine.
Thorosteenstrupine. Thorosteenstrupine was described as new mineral species in 1962 from metasomatite veins at Chergilen REE-occurrence, on the SE slope of the Turana Range, left bank of the Verkhnii Mel'gin River, 60 km NW of Chekunda town, Verkhnebureinsky district, Khabarovsk Territory (only "Siberia" in the original paper). Here, the mineral forms dark brown lamellar segregations, 2-4 mm and sometimes up to 1 cm in length, usually at the contact with grains of REE-bearing miserite, microcline, quartz, and aegirine-augite. Fluorite and thorite have also been found in this association. The original crystal chemical formula, determined by microchemical analysis, is semplified as (Ca,Th,Mn)3Si4(0,OH)12F 5.3H20 and till now it is reported in Fleischer's Glossary (Mandarino, 1999).
In 1985 Kalinin et al. reported the find of a similar mineral from arfvedsonite and aegirine-albite metasomatites, at the contact with alkaline rocks andpicrite-porphyrites, located at two points W and S-SE of Mt. Vavnbed respectively, the NE side of the Lovozero alkaline massif, Kola Peninsula. Here, the mineral forms dark brown irregular grains, up to 1 mm across, associated with accessories as loparite-(Ce), eudialyte, lorenzenite, pyrochlore, and two titanoniobates, rich in Th and U respectively. The empirical formula given in this case is (Th0.98O0.01)0.99( REE0.65Ca0.23)0.88 (Mn0.80Fe0.22Ti0.05)1.07(Si3.59P0.12Be0.29)4(011.370H0.63)12(00.75F0.25)1.00 - 4H20.
Such finding seems to be doubtful not only for the anomalous content of Be in the mineral, but especially for the unusual paragenesis. Later studies on the albitized exo- and endocontact rocks in the northeastern part of the massif do not show, in fact, the presence of such a mineral, while only U-bearing pyrochlore, thorite, monazite, zircon, magnetite, etc. are put in evidence.
Due to the lack of data, confirming composition and homogeneity of the type material, various authors, such as Semenov and Clark, considered doubtful the status of thorosteenstrupine as a single mineral species and its relationship with steenstrupine-(Ce). Recently (1997), thorosteenstrupine has been found in one specimen from the dumps of the Karnasurt underground mine, Ml. Karnasurt, Lovozero massif. This specimen shows dark brown isometric crystals with rhombohedral-pinacoidal habit and rounded grains, up to 1-2 mm across, embedded in ussingite in proximity of the selvages of a pegmatite veinlet, cutting grey foyaite. Associated minerals include aegirine, sodalite, vuonnemite, serandite, members of nordite-(Ce)-ferronordite-(Ce) series, natisite, kazakovite, and Co-bearing lollingite. The composition of the dark brown mineral varies inside the same crystal or grain, where are present statistically settled sectors of phases in which the REE/Th and Si/P ratios strongly change from those typical of steenstrupine-(Ce) to those of thorosteenstrupine. Each sector attains the size of a tenth of microns. By microprobing the Lovozero specimen and the Chergilen holotype Pekov derived the follow average formula for thorosteenstrupine: Na0.5Ca1-3(Th,REE)6(Mn,Fe,AI,Ti)4-5 [Si6018]2[(P,Si)04]6(OH,F)0-2 - nH20.
Recently, thorosteenstrupine has been found by L. N. Kogarko in the drillcore of borehole 272 at Mt. Alluaiv as thin segregations filling the interstices between grains of rock-forming minerals in foyaite. Here, the mineral is associated with steenstrupine-(Ce) and nordite-(Ce).
Crystal chemical considerations on "steemtrupines"
Large variations in chemical composition of "steenstrupines" are due to their peculiar crystal structure. The analytical data published in literature and those of the present article induce to think that exist more than one species in the modern sense of view, and, therefore, "steenstrupines" may be considered as members of a family of phosphosilicates, having the same structure motif but differing the one from the other for the array of contained cations and their mutual distribution in the structure sites. A similar situation occurs, for example, in the case of zirconosilicates of eudialyte group, as shown by the recent description of various new members and the relevant nomenclature problems. The case of "steenstrupines" is still more complicated, because their frequent metamictization is a serious obstacle to deep structure investigations. However, some interesting crystal chemical considerations may be done. We assume the general formula as [6]A(2)6[8]A(3)6[6]A(1)2[6]M(2)2 [6]M(3)2[8]REE6[6]M(1)[T04]6[Si6012(0,OH)6]2[OH,0,F]x - nH20, by means of which practically all valid published analytical data on steenstrupines can be represented. In this formula cation charge varies from 111 to 123, including all the more or less hydroxylated forms.
Then, the authors have considered the distributions of ion populations in the different structure sites, considering ionic radius, coordination number and ion-oxygen distance. T site is mainly occupied by P at Ilimaussaq, while Si prevails at Lovozero and Koashva; at Chergilen P is totally absent.
For Ilimaussaq phosphorous steenstrupines in M(2) site generally prevails Mn, in M(3) Mn or Fe, in M(1) Al, Th or Zr. For Lovozero silicious steenstrupines in M(2) site prevails Mn, in M(3) Mn or Fe, in M(l) Ti,, Zr or Th; in REE site both REE and Th can prevail. At Koashva in M(2) site prevails Mn or REE, in M(3 ) Mn or Fe, in M(l) Ti, Zr or Th; in REE site both REE and Th can prevail. At Chergilen in M(2) site prevails Mn, in M(3) Mn or Fe, in M(l) Al; in REE site Th always prevails on Y and REE. Concerning the other structure sites, A(1) is mainly occupied by Na and/or Ca, A(2) and A(3) are occupied by Na (easy exchangeable) and sometimes by da K, Sr, H,O+, and vacancies. Na content is not a significant index for such minerals, due to the easy exchangeability of this ion.
Nomenclature problems generated by such situations are evident. In absence of nomenclature rules for these minerals, distinction between steenstrupine-(Ce) and thorosteenstrupine has been done on the basis of the Pekov's assumption, i.e. considering only the substitution in the REE site and defining thorosteenstrupine if Th/REE > 3 for REE + Th = 6.
Thorosteenstrupine and steenstrupine-(Ce) at Mt. Koashva
The occurrence is a differentiated pegmatite located at the step no. 1 of the Koashva apatite open pit, where thorosteenstrupine and steenstrupine-(Ce) form dark brown to black crystals and grains. Thorosteenstrupine crystals differ from those of steenstrupine-(Ce) for the usually more coarse, not rarely rounded faces and for a dull aspect The crystals of both the minerals usually attain a few millimeters, but very rare crystals up to 1 cm or more have been found. More usually, steenstrupine-(Ce) crystals show the rhombohedral-pinacoidal habit, similar to the one described by Moberg at Kangerdluarssuk (type 1), while hexagonal crystals flattened along (0001), similar to the ones described by B0ggild at Igdlunguaq and Nunarssuatsiaq (type 2), are more rare and typical for thorosteenstrupine. Small idiomorphic crystals of these minerals, up to 1-2 mm, black to dark-brown, sometimes covered by a brownish-gray layer of fine granular alteration products, are found in the dissolution cavities of villiaumite in the sodalite-natrolite aggregates, and within the interstices at the border of pectolite spherules. In this association fine tabular crystals of umbite, up to 0.5 mm, often occur.
Furthermore, these minerals have been found in association with natrolite and pectolite, as components of blackish rounded aggregates up to 1cm across. Such aggregates consist of thorosteenstrupine and/of more rarely of steenstrupine-(Ce), metamictic thorium orthosilicate ThSiO4 and a bituminous substance. When observed in polished section under SEM, such aggregates show unusual spotted structures, often remembering a "turtle shield", in which "plates" of the minerals of steenstrupine family alternate "plates" of bitumen, while the thorium orthosilicate fills the interstices among them.
In the same pegmatite, idiomorphic crystals showing steenstrupine habit, up to I cm, altered in a mixture of a bituminous substance and a Th-dominant silicophosphate (see analyses 5 and 6), have been found in association with aggregates of nacaphite-vitusite-(Ce)- La-dominant analogue of vitusite-(Ce). Microprobe analyses of such minerals have been carried out using the CAMEBAX SX50 microprobe at the Mineralogy Dept., Moscow State University (MGU), analyst N.N. Kononkova, and the CAMEBAX Microbeam microprobe at IMGRE (Institute for mineralogy, geochemistry, and crystal chemistry of rare elements of the Russian Academy of sciences), Moscow, analyst I.M. Kulikova (Table I).
Table I
| Componente |
1 |
2 |
3 |
4 |
5 |
6 |
|
| Na2O |
1,88 | 2,05 | 4,46 | 4,43 | 1,34 | 1,21 |
| K2O |
2,89 | 2,10 | 1,98 | 2,47 | 1,10 | 1,16 |
| CaO |
1,46 | 0,70 | 0,66 | 0,64 | 2,83 | 5,34 |
| SrO |
- | - | 0,67 | - | 0,52 | 0,34 |
| BaO |
- | - | - | - | 0,02 | 0,00 |
| MnO |
5,90 | 5,26 | 4,71 | 2,82 | 4,92 | 5,63 |
| FeO |
3,03 | 0,67 | 1,08 | 1,04 | 1,27 | 52,14 |
| Y2O3 |
1,60 | 1,60 | 1,10 | 1,30 | 0,50 | 0,50 |
| La2O3 |
4,75 | 5,38 | 5,89 | 2,92 | 4,07 | 4,27 |
| Ce2O3 |
6,52 | 8,14 | 7,38 | 5,09 | 5,60 | 7,26 |
| Nd2O3 |
1,24 | 2,01 | 2,36 | - | 1,20 | 2,29 |
| Sm2O3 |
0,00 | 0,18 | 0,14 | - | - | - |
| ThO2 |
26,87 | 25,08 | 18,01 | 31,73 | 29,06 | 25,77 |
| ZrO2 |
1,50 | 0,61 | 0,48 | 0,61 | 0,50 | 0,50 |
| T1O2 |
0,60 | 0,60 | 0,55 | 0,75 | 0,80 | 0,80 |
| UO2 |
0,52 | 0,36 | 0,56 | 0,86 | 0,88 | 0,59 |
| Nb2O5 |
0,30 | 0,30 | 0,37 | 0,33 | 0,35 | 0,35 |
| SiO2 |
26,85 | 27,77 | 24,55 | 23,61 | 16,48 | 15,65 |
| P2O5 |
4,68 | 0,97 | 2,14 | 0,52 | 4,80 | 8,11 |
| H2O |
n.d | n.d | n.d | n.d | n.d | n.d |
| F |
n.d | n.d | n.d | n.d | n.d | n.d |
|
| Total |
90,59 | 83,78 | 77,09 | 79,12 | 76,24 | 81,91 |
|
The analytical data allow to determine the following formulae, calculated assuming (Si+P)=18:
analysis 1:
(K2,l5Na2,02)4,17(Ca0,91Y0,50Mn0,47Na0,11REE0,01)2,00 (Mn)2,00(Fe1,48Mn0,45U0,07)2,00 Th3,34REE2,66)6,00 (Zr0,43Ti0,26Th0,23Nb0,08)1,00 [Si6,00O12,00(0,OH)6,00]2[(Si3,39P2,31)6,00O24,00] (OH,F,O)X - nH2O - thorosteenstrupine (on albite crystal);
analysis 2:
(K1,69Na1,51)3,20(Na0,99REE0,54Ca0,47)2,00 (Mn1,22Y0,54REE0,23)1,99(Mn1,59Fe0,35U0,05)l,99 (Ti3,15REE2,85)6,00(Tn0,44Ti0,28Zr0,19Nb0,09)1,00[Si6,00O12,00(O,OH)6,00]2 [(Si5,48P0,52)6,00 024,00](OH,F,0)x - nH20- idiomorphic crystal from a vug in sodalite nest;
analysis 3:
(Na4,48K1,72Sr0,27)6,47 (Na1,42Ca0,48 REE0,10)2,00
(Mn1,43Y0,40REE0,17)2,00(Mn1,29Fe0,62U0,09)2,00 (REE3,65Th2,35)6,00(Th0,45Ti0,28Zr0,16Nb0,11)1,00[Si6,00012,00(0,OH)6,00]2 [(Si4,76P1,24)6,00024,00] (OH,F,0)x - nH20 - steenstrupine-(Ce) (association with pectolite and aegirine);
analysis 4:
(Na5,84K2,36)8,20(Na0,59Ca0,51REE0,90)2,00(REE0,90Y0,52Mn0,58)2,00(Mn1,21Fe0,65U0,14)2,00(Th5,15REE0,85)6,00(Ti0,42Th0,25Zr0,22Nb0,11)1,00[Si6,00012,00(0,OH)6,00]2[(Si5,67P0,33)6,00024,00] (OH,F,O)x - nH20 - thorosteenstrupine (associated with bitumen and thorium orthosilicate);
The alteration product of steenstrupine shows the following empirical formulae, calculated for 4 oxygen atoms;
analysis 5:
(Th0,34Mn0,21 Ca0,16Na0,13Ce0,11La0,08Nd0,02K0,07Fe0,05Y0,04Zr0,04Ti0,03Sr0,02U0,11Nb0,01)1,32[(Si0,85P0,21)1,06O4] - nH20 - crystal rim;
analysis 6:
(Th0,27Mn0,22Ca0,26Na0,11Ce0,12La0,07Nd0,04K0,07Fe0,08 Y0,04Zr0,03Ti0,03Sr0,01U0,11Nb0,01)1,37[(Si0,72P0,32)1,0404] - nH20 -crystal core.
These minerals may be reasonably referred to phases tending to assume the final composition of members of rhabdophane group with general formula (Th, Me2+,REE)[(Si,P)O4] - nH2O.
Minerals of rhabdophane group are quite diffuse in small amount in the alkaline massifs of llimaussaq, Khibiny, Lovozero and Mont Saint-Hilaire, where they formed in late hydrothermal or hypergene conditions. Further investigations on the material collected at Mt. Koashva are in progress for a thorough characterization of such phases.
Autor: Lisitsin Dmitriy; Paolo Bosio
"RIVISTA MINERALOGICA ITALIANA" №4 2001
Published: 24.01.2001
Published on sait: 20.03.2001
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