The Kola Peninsula as a unique alkaline mineralogical province

Khomyakov A.P.

Institute of mineralogy, geochemistry and crystal chemistry of rare elements, Moscow, Russia

The second half of the 20^th and the beginning of the 21^st century have witnessed an unprecedented increase in the rate of new mineral discoveries, as a result of which the overall mineral system has grown from 2000 to more than 4000 species in a relatively short time. By far the largest contribution to this achievement has come from the Kola region, where the first 80 years of detailed research (1890 to 1970) yielded 45 mineral species new to science, whereas the subsequent 35-year period (1970 to 2005) yielded about 200 new species. The 15-year period from 1991 to 2005 was particularly fruitful, producing 113 new IMA-approved minerals (Khomyakov, 2006). An unusually high rate of these discoveries is indicated by figures relating to three equal-time intervals (1991–1995, 1996–2000, and 2001–2005), during which 20, 33, and 60 new minerals, respectively, were discovered in this region. In all, over 250 new minerals have been described to date from the Kola region, which is significantly higher than from any other mineralogical province, including such unique provinces as Urals in Russia, Gardar in Greenland, or Monteregian in Canada. Almost all of the new minerals from the Kola region were discovered in ore deposits related to three types of alkaline rock associations: agpaitic nepheline syenite massifs, carbonatite-bearing complexes, and alkaline granites. On the other hand, copper–nickel deposits associated with ultrabasic rocks in the same region have yielded only five new minerals over the many decades of their comprehensive research.

A virtually inexhaustible source of new mineral discoveries on the Kola Peninsula, both in the recent period and in previous years, are the world’s largest deposits of rare metals and phosphates, spatially and genetically associated with the alkaline massifs of the Khibiny–Lovozero complex, which by far surpasses the other unique mineral localities of our planet both in the total number of minerals (over 600) and in the number of newly described species (about 200), representing all major classes of chemical compounds. As is well known, the first important discoveries of minerals with unusual compositions and properties (lomonosovite–murmanite group, etc.) were made by Finnish researchers led by W. Ramsay (1890s) and later by some Academy of Sciences institutions headed by A.E. Fersman, K.A. Vlasov, and other prominent scientists. Thanks to their successful mineralogical studies, most notably discoveries of unique apatite, loparite, and eudialyte deposits, the Khibiny and Lovozero alkaline massifs gained both worldwide renown and national importance as a unique mineral treasure-trove. Their students and followers continued to make spectacular achievements along these lines. This is evidenced by the following statistics: the first period of mineralogical discoveries in these two massifs, from 1890 to 1970, yielded 33 mineral species unknown to science, whereas the period from 1971 to 2005 yielded about 150 new minerals, the greater part of which (over 80 species) were discovered with the author’s participation.

The author’s extensive research on the mineralogy of the Khibiny and Lovozero massifs (Khomyakov, 1995) revealed that their deep unweathered horizons contain abundant pegmatoid rocks of an essentially new, hyperagpaitic type, supersaturated with alkali, volatile, and rare elements. These rocks are characterized by the presence of a whole series of highly unusual minerals, such as natrosilite Na₂Si₂O₅, natrite Na₂CO₃, and olympite Na₅Li(PO₄)₂, which are, respectively, the most alkaline water-soluble silicate, carbonate, and phosphate ever found in nature. The existence of such salts as crystalline phases of rocks could not be predicted from traditional theories. Hence the discovery of their abundant accumulations in Khibiny and Lovozero rocks actually signified the discovery of a previously unknown phenomenon: the peralkaline state of natural substance. In this state, all electropositive elements that are less basic than Na exhibit amphoteric properties, promoting their passage from the cationic part of the structure of agpaitic magma into the considerably more capacious anionic part. As a result, agpaitic magma turns into a universal solvent, thus accumulating huge amounts of P, Nb, Òà, Ti, Zr, Hf, REE, U, Th, Al, Ga, Sr, Ba, Be, Na, K, F, and other useful components. This factor is responsible both for the supergiant sizes of mineral deposits associated with agpaitic magmatism and for their pronounced multi-element character. It also explains why hyperagpaitic rocks are so rich in minerals with unique compositions, structures, and properties.

The results of these studies, widely publicized in the scientific and popular literature and presentations in various forums, inspired many Russian and foreign researchers to further investigate the mineralogy of these rocks. In particular, very impressive progress in this area has been achieved by the research teams led by I.V. Pekov, N.V. Chukanov, and V.N. Yakovenchuk (Pekov, 2005; Yakovenchuk et al., 2005), which have discovered many dozens of new minerals. It should be emphasized that the bulk of these minerals have been identified, as in the author’s case, within Khibiny and Lovozero, in hyperagpaitic pegmatite rocks. Following their discoveries on the Kola Peninsula, many minerals characteristic of the region’s hyperagpaitic rocks were soon identified by foreign researchers in similar rocks in the alkaline massifs of Greenland and Canada. At present, this line of research continues to grow both in Russia and elsewhere, increasingly expanding our understanding of the structural and chemical diversity of the mineral kingdom and confirming, with each new series of discoveries, the validity of our (Khomyakov, 1997, 2007) principle of an unlimited number of mineral species in nature.

This study was supported by RFBR grant 07-05-00084-a.

References:

Khomyakov A.P. Mineralogy of hyperagpaitic alkaline rocks. Oxford, U.K.: Clarendon Press, 1995. 224 p. (Translated from the Russian edition. Moscow: Nauka, 1990. 196 p.).

Khomyakov A.P. Recent mineral discoveries and the number of mineral species: a reconsideration // Structure and evolution of the mineral world. Int’l mineral. seminar. Syktyvkar: Geoprint, 1997. P. 98-99 (in Russian).

Khomyakov A.P. The record contribution of the Kola region to the overall system of mineral species // Proc. III Fersman scientific session, Kola Division, Russian Mineral. Soc. Apatity: K & M, 2006. P. 96-98 (in Russian).

Khomyakov A.P. Principle of an unlimited number of mineral species in structural and cultural aspects // III Int’l symposium 'Mineral diversity. Research and preservation'. Working papers. Sofia: Zemyata i horata, 2007. P. 265-271 (in Russian).

Pekov I.V. Genetic mineralogy and crystal chemistry of rare elements in highly alkaline postmagmatic systems. Doctoral (geol.–mineral.) dissertation. Moscow State Univ., 2005. 652 p. (in Russian).

Yakovenchuk V.N., Ivanyuk G.Yu., Pakhomovskiy Ya. A., Men'shikov Yu. P. Khibiny. Apatity: Laplandia Minerals, 2005. 468 p.