Показано, что корунды представляют собой единственную природную модификацию с корундовым мотивом упаковки Al и О и что определяет физические и химические особенности благородных корундов. На основании авторских представлений о генезисе благородных корундов с приложением оригинальных аналитических данных, уточняющих сводные литературные данные, выделены корунды магматогенной, метаморфогенной и детритовой серий. Дана схема размещения основных месторождений благородных корундов мира. Приведены методики исследований, позволяющие определить характеристики и широкий диапазон областей использования корундов. Месторождения корундов докембрия относятся к превалирующему генетическому типу.
благородные корунды, генезис, магматизм, метаморфизм, докембрий
Introduction
In the introductory part of the article, the authors give a brief overview of corundum.
Corundum is the only natural modification with corundum packing motif of Al and O atoms. Its crystals are distinguishable by their hardness, colour and shade.
All varieties of corundum, including gem varieties (ruby and sapphire), crystallize in the ditrigonal-scalenohedral form of trigonal syngony symmetry. The refractive indices of corundum are constant with Ne values of 1.760 – Ne 1.768. B. W.Andersen and C. J. Paune [1] proposed to use the specific gravity of gem corundum as a reference.
The density of most gem varieties is 3.96–3.99 g/cm3, reaching 4-4.1 for green and blue-green sapphires from Australia. The mineral has a hardness of 9 on the Mohs scale and is the second only to diamond.
Corundum has the chemical composition of Al2O3 typically containing trace amounts of iron, titanium, chromium, and vanadium. Inclusions of rutile, hematite, ilmenite, garnet and spinel can be present.
The idiochromatic colouring of gemstones is caused by transition metal ions: Fe, Co, Ni, Mn, C, Cr, V, and Ti. They are capable of absorbing a certain wavelength in the visible spectrum, thus determining colour. Ruby and sapphire contain 5 of these elements — Cr, V, Ti, Fe, Mn — as impurities.
The goal of this paper is to present the author's view on the genesis of noble corundums supported by the original analytical data and data already existing in the literature.
Corundum-bearing formations of the world
Geological and petrological conditions of corundum crystallization are diverse [2, 3, 6, 21, 23, 25–27, 29, 33, 36, 37, 40]. Deposits of noble corundums (rubies and sapphires) have been identified in the places, where their formation is associated with specific growth conditions of transparent crystals.
Rubies and sapphires are extracted mainly from the placers formed on the slopes and valleys of rivers, where the primary bedrock deposits were destroyed and corundum, as a mineral extremely resistant to mechanical and chemical attack, moved into sand/gravel river (more rarely lake and marine) sediments. Such deposits are economically viable owing to the possibility of mineral extraction through simple operations of sieving or washing [7–9].
Alluvial, deluvial-alluvial placers and eluvial, eluvial-deluvial residual deposits of ancient weathering are of primary importance in the total balance of gem corundums production [10–15]. The primary ruby- and sapphire-bearing bodies, due to which the secondary deposits are formed, have been insufficiently studied.
The layout below (Fig. 1) shows the main deposits of noble corundums in the world.
Based on the international genetic classification of world noble corundums [6], we divide corundums into types and subtypes. The magmatic series (type I) includes alkaline basalts (IA subtype) and kimberlites (IB subtype). These subtypes have no industrial significance and are considered by us as necessary taxa in the general taxonomy. The rocks have undergone metamorphism of amphibolite, granulite or eclogite facies conditions.
The metamorphosed series includes two types — type IIA metamorphic sensu stricto and type IIB metamorphic-metasomatic. Type IIA has two subtypes: subtype II A1 and subtype II A2. Subtype II A1 is represented by metabasite-ultrabasites and gneisses (Winza, Longido (Tanzania); Andriba, Anavoha, Gogogogo. Ianapera; Fotadrevo, Maniry, Vohitany (Madagascar), Kibre mengist (Ethiopia); Kitui, Rockland-former, John Saul mine; Penny Lane (Kenya), M'sawize; Ruambeze (Mozambique); Karnataka, Orissa (India); Brittany (France), Froland (Norway); Hokitika (Australia); Makar-Ruz (Russia) and others). Subtype II A2 — marbles and calcium-silicate rocks — Jegdalek (Afghanistan); Chumar; Ruyil (Nepal), Snezhnoe, Turakuloma, Badakhshan (Tajikistan); Hunza valley, Batakundi; Nangimali (Pakistan); Mogok; Mong Hsu (Myanmar), Luc Yen-Yen Bai; Quy Chau (Vietnam), Morogoro, Mahenge (Tanzania); West Pokot (Kenya); Revelstoke (Canada), Ilmen Mountains and others).
Type IIB consists of two subtypes: subtype II B1 and subtype II B2.
Subtype II B1 includes plumasites and metasomatites (Transvaal, O'Breins (Zimbabwe); Aqua; Penny lane, Rockland-John, Saul mine; Hard Rock (Kenya), Karnataka (India), Kitwalo (Tanzania); Bekily, Anavoha, Vohitany, Andilamena (Madagascar); Makar-Ruz, Hitostrov (Russia); Poona, Harts Range (Australia), Aappaluttoq; Nuuk-Stove, Kangerdluarssuk (Greenland); Corundum Hill (USA), etc.). Subtype II B2 includes metasomatized gneisses, shales, ultrabasite-basites of shire zones (Zazafotsy, Ambatomena, Ambositra (Madagascar); Nangimali formation (Pakistan), etc.). Complexes II B1 and II B2 are subjected to metamorphogenic-metasomatic processes of greenschist, amphibolite, or granulite facies conditions.
The detrital series is the most important for ruby mining. Two types belong to this series — Type IIIA and Type IIIB. Type IIIA is represented by placer deposits in the areas of alkaline basalts and kimberlites (Lava Plains, Anakie fields, New England fields; Macquarie-Cudgegong, Barrington Tops; Tumbarumba, Western Melbourne fields (Australia); Chantaburi-Trat (Thailand); Pailin (Cambodia); Ankaratra, Vatomandry (Madagascar) and others). Type IIIB comprises placers in metamorphic complexes (Ratnapura; Elahera, Polonnaruwa (Sri Lanka); Mogok; Mong Hsu (Myanmar); Ilakaka, Andilamena; Didy; Zahamena (Madagascar); Luc Yen, Yen Bai, Quy Chau (Vietnam); Tunduru, Songea, Winza, Umba valley, Morogoro; Mahenge (Tanzania); Montepuez; M'sawize; Ruambeze (Mozambique); Cowee Valley (USA) and others). Complexes IIIA and IIIB are unmetamorphosed.
Thus, the magmatic, metamorphogenic and detrital series of corundummbearing formations have been identified on the basis of previous studies [4, 6, 10, 15, 16, 17, 18, 19, 20, 23, 30] and our own data [34–36].
The technological mineralogy of corundum deposits is related to the geological and tectonic position, and the specific features of regional metamorphism as applied to specific ruby-sapphire mineralization. All properties of noble corundums are determined by their primary origin. In contrast to the international classification, we propose to separate metasomatic deposits of noble corundums into two groups for their genetic typification: 1) metasomatic, which owe their origin to the action of postmagmatic hydrothermal solutions, and 2) metamorphogenic-metasomatic, genesis of which is a logical completion in local zones [34]. Identified genetic groups of ruby deposits in their geological position determine the features of structure, composition and properties of minerals more accurately, i. e., typomorphism [36]. Studies by N. G. Barnov et al. [34] place less emphasis on the position that the magmatic series includes deposits of magmatic, pegmatitic, skarn, metasomatic (postmagmatic) genesis.
As a rule, processes of allochemical metasomatism, rock desalinization and alumina enrichment of mineral-forming solutions develop on the contact of rocks of contrast composition (ultramafic and granitoids or marble and mafic), exchange of components between them, additional inflow of substance from deep Earth zones and frequent variations of chemical composition of mineral-forming solutions circulating in metasomatism zones.
Not all genetic types of ruby and sapphire deposits are equally significant for gem mining. The noble corundums of pegmatite origin have little industrial importance. They have been mined in small amounts from formations of this genetic type in India (Tamil Nadu), Brazil (Goias), Canada (Bancroft ore field, Ontario), Russia (Murzinskaya-Aduyskaya strip), but have never been considered as a promising source for industry supply.
Noble corundums are formed in the favorable environment for growth of transparent crystals, but such conditions are extremely rare in nature. For example, corundums of alkaline basic lamprophyres are crystallized from aluminum-rich magmas at deep (mantle) levels [4]. Ruby-bearing marbles and associated ruby placers are spread over a vast area of the Mogok mining district in Northern Myanmar and in several areas of Thailand. Large sapphire placers are developed in eastern Australia, Sri Lanka, Thailand and Cambodia, where their primary sources — sapphire-bearing basalts — occupy a large area.
Megacrystals of corundum in situ are rare; they are known only in basaltoids, which break through the continental crust, and confine to the areas of extensive continental uplift.
Corundum formation is a protocrystallization from melts in deep magmatic sources under the condition of subsequent rapid solidification of the main mass of the melt. Typical corundum-bearing rocks are corundum syenites and syenite-pegmatites, which form dikes and veins among alkaline syenites or granite-gneisses and gneisses near their contacts with massifs of alkaline or nepheline syenites. Examples of such include deposits of the Ilmensky and Cherry Mountains in Russia, the provinces of Ontario and Quebec in Canada, Madras and Kashmir in India, the island of Sri Lanka and others.
Corundum is formed in magmatic melt from excessive alumina, and its crystallization occurs at great depth under the condition of rapid magma ascent to the surface [12]. The features of geochemical composition of clinopyroxene and garnet megacrystals from the alluvial sapphire deposit Dac Nong (Vietnam) and model calculations showed that they were formed as a result of crystallization of alkaline basalts in a deep intermediate chamber (pressure 14–15 kbar), which is close to MOX boundary (50 km) for this part of South-East Asia [13].
The excess of alumina required for corundum crystallization, is usually fixed at the contacts of hyperbasites with dikes of acidic or basic feldspathic rocks. In such conditions, metasomatic processes play a great role. These processes develop during the desilification of aluminosilicate rocks, which contact with ultramafic rocks; at this moment, the formation of fine-, medium- and coarse-grained rocks, containing corundum, occurs. In kyshtymites, for example, the corundum content reaches 90% (Borzovskoye corundum deposit in the Urals). Corundum-containing alkaline lamprophyres are crystallized from aluminum-rich magmas at deep (mantle) levels (Yogo Gulch deposit in Montana, USA) and Cenozoic olivine and feldspathoid alkaline basalts (Anakie deposit in Australia, Pailin in Cambodia and others); sapphire accumulations are noted in the latter.
Gem extraction from dense bedrock basalts, marble and gneisses is only possible with drilling, crushing and other technological methods, which sharply increase the cost of mining; the mining becomes unprofitable, and the extracted raw materials become defective because of the formation of numerous technogenic cracks in the crystals. Therefore, placers are generally the most important source of rubies and sapphires; gem extraction from primary, relatively soft marbles (in comparison to other rocks) is done manually and in limited quantities in Afghanistan, Pakistan, India, Myanmar, and some other countries.
Corundum is a polygenic mineral; it is widespread and found among rocks of varied composition and origin. As a rock-forming mineral, it is present in some crystalline schists and hornblendes of high degree of metamorphism, in scarny marble, in secondary quartzites, in corundum syenite-pegmatites and in plagioclasites (plumasites, kyshtymites, marundites). In the form of single occurrences, corundum is found in peridotites, alkaline and nepheline syenites, granites, granite pegmatites, basalts, andesites, trachytes and other igneous rocks.
As a resistant mineral, corundum is practically unchanged in placer deposits, except for a common weak abrasion due to corundum high hardness. For this reason, all properties of noble corundums are determined by their primary origin; the physical and chemical features of rubies and sapphires and the characteristics of their main deposit types become the basis for understanding their genesis.
Research method
High-precision trace elemental analysis of original corundum samples was carried out by a quadrupole mass spectrometer X-SERIES 2 Terhmo in IGEM RAS. The results of the analysis are illustrated with the spider diagram (Fig. 2).
Electron-microscopic analysis of corundum crystals was carried out by a scanning electron microscope Сarl Zeiss EVO-LS-10 equipped with energy dispersive spectrometer Oxford MAX 50. As a a result, qualitative EMF spectra were obtained for a number of samples and mapping of separate areas by chemical composition was carried out.
The research was carried out with Nicolet 380 FT-IR spectrometer using a THERMO Scientific Centaurus microscope. The special feature of this equipment is the use of special OMNIC computer program necessary for automated acquisition of spectra, their quality control, as well as data analysis and processing. The sample preparation is not necessary when working with an infrared microscope. The size of the sample for analysis can be limited. The optimal particle thickness ranges from 0.005 to 0.015 mm. Centaurus infrared microscope equipped with a highly sensitive MST-A detector allows to analyse particles as small as 40 microns in the range of 600–4000 cm in transmission and reflection mode. Interpretation of the obtained spectra was carried out using the database and the methodology developed at CNIGRI [39].
The mineral chemical compositions were studied by an electron-probe microanalyzer JXA-8100 by JEOL in the laboratory of mineral matter analysis of IGEM RAS. The analysis was carried out at accelerating voltage 20 kV, current on the Faraday cylinder 20 nA, probe diameter 3–5 microns. Exposure time to the main elements was 10 sec. The corrections were calculated using the ZAF correction method using JEOL software.
The surface of studied samples was polished and sprayed with a thin layer of carbon to ensure electrical conductivity.
For visual observation of fluid inclusions (FI), as well as for thermometric and cryometric studies, double-sided transparent-polished plates with 0.25–0.3 mm thickness were made from corundum samples. The area of the plates was several square centimetres.
Using Olympus BX-51 optical microscope, the shape and dimensions of PVs were determined. Microthermometric studies of PV were carried out in the sector of mineralogy of IGEM RAS by a measuring complex based on THMSG-600 microthermometer of Linkam (England).
To obtain reliable microthermometric data, a research was conducted on groups of at least two fluid inclusions with the same phase relations and close temperatures of phase transitions to ensure the initial homogeneity of the trapped fluid. The temperature measurement accuracy was ±0.2 °C for the temperature range from –60 to +60 °C and ±1.5 °C out of range.
Results were interpreted by standard methods [3, 24]. The composition of the main salt components of the solutions was determined by the eutectic melting point (Tevt).
Characteristics of world noble corundums
Results of high-precision trace elemental analysis (of impurities and trace elements) are shown for several selected examples of world deposits and occurrences of noble corundums from the authors' collection (fig. 2).
Table 1 shows representative analyses of corundum by genetic type from several sources.
The infrared spectra of individual micrograins in the same sample vary (Fig. 3), indicating heterogeneous composition of corundum. The spectra of transparent and translucent grains differ significantly from each other. Most of the samples show IR absorption in three spectral intervals 4000–2500 cm–1, 2200–1900 cm–1 and 1500–700 cm–1. The last interval corresponds to natural vibrations of the crystal lattice of corundum. This conforms with the fact that only lines in the interval 1500–700 cm–1 can be traced in colourless transparent corundum, containing no visible impurities (Fig. 4).
In area 4000–2500 cm–1 show valence vibrations of OH- and NH-groups (Fig. 5)
In this case, hydroxyl groups are most likely to be included in adsorbed water (gas-liquid inclusions, intergranular water) as well as structural hydroxyl groups very typical for corundum. It can be assumed that clear lines near 3310, 3292 3080 cm and some others (Fig. 5) refer exactly to valent vibrations of structural OH-groups. These groups can be considered as typomorphic properties of corundum from different deposits. Narrow lines 2120 and 1990 cm–1 seem to be related to strain vibrations of structural OH-groups.
A number of examples are illustrated in Figs. 6, 7, 8, 9.
Minerals were studied by an electron-probe microanalyser in the Mineral Analysis Laboratory of IGEM RAS. Corundum and silicate phases were analysed using the following standards:
Si — sanidine, Na — jadeite, Fe — almandine, K — sanidine, Ni — NiO, Al — corundum, Mg — olivine, Mn — spessartine, Ca — anorthite, Cr — chromite, Ti — titanite. During the analysis of ore inclusions the standards: Ti — ilmenite, Mg — chromite, Fe — magnetite, Ca — sphene, Ni — Ni, Si — almandine, Mn — spessartine, Nb — Rb2Nb4O11, V — V, Cr — chromite, Al — chromite were used. For single zircon inclusions the following references were used: Si — zircon, P — apatite, Zr — zircon, U — UO2, Hf — HfO2, Y-Y2O3, Th- ThPO4, S — BaSO4, Nb — Rb2Nb4O11.
Mineral inclusions are represented by magnetite, zircon, amphibole, mica, chlorite, apatite, rutile, ilmenite and rare earth minerals. Study materials by electron-probe microanalyzer JXA-8100 from company JEOL are shown on Figs. 10, 11, 12, 13, 14.
In some samples the impurities of elements-chromophores, iron, chromium and titanium, were noted.
Indexes: Crn — corundum, Dsp — diaspore, Rt — rutile, Grt — garnet, Pl — plagioclase, Ap — apatite, Mc — microcline, Zrn — zircon, Chl — chlorite, Pcl — pyrochlore, Ms — muscovite, Kln — kaolinite, Mag — magnetite,
Among the FS, the following are highlighted: the first group — FS, which occur singly or form chaotic irregular clusters in the central parts and sometimes at the edges of the grains. Sometimes these fluid inclusions are confined to crystal growth zones; the second group — FS forming chains and planes, confined to cracks crossing several grains.
According to the criteria of E. Roedder [22], the first group of inclusions is referred to the primary, trapped during the crystal growth, the second group is referred to the secondary, which contain the fluid penetrated through cracks after crystallization of the mineral. The size of primary inclusions is from 2 to 50 microns; inclusions have rounded elongated or irregular shape or shape of negative crystals with elements of crystallographic cut. The size of secondary inclusions does not exceed 10 µm. Primary fluid inclusions with sizes from 5 µm and, in some cases, large secondary inclusions (from 5 µm) have been selected for the study.
According to the phase composition at room temperature all inclusions are single-phase. On cooling and heating it was found that all PVs contain liquid carbonic acid List of the main samples analysed (No. 3 Musse, Kashmir, India (blue No. 29, Zapskor, Kashmir; No. 47 ruby, Mysore, Karnataka State, India; No. 4 Mura-Bora, Tanzania, No. 5 Ihosy, No. 40 pink corundum, Morogoro; No. 48 ruby in muscovite, Moro Goro, Tanzania; Madagascar (pegmatite); No. 7 Olkhon Island, Baikal, Russia; No. 10 , Rai-Iz, Polar Urals, Russia; No. 19 Ilmensky Nature Reserve, Urals, Russia; No. 20 Budun on Olkhon, Baikal; No. 32 blue corundum, Olkhon Island, Baikal, Russia; No. 23 Ihosy; No. 30 Vatondradi, No. 34 leucosapphire, Ambusitra, Madagascar; No. 36 Sandnessien, Norway; No. 38 blue corundum, Koltashi village, Middle Urals, Russia; No. 39 corundum, Muzor, India.
Two-phase PVs — gas + liquid — have been found in some corundums. According to the ratio of phases in them 2 types can be distinguished: 1 — with the gas phase occupying not more than 30 % of PV volume; 2 — with the gas phase occupying up to 50 % of PV volume. Fluid inclusions of the first type contain water-salt fluid. Fluid inclusions of the second type contain carbon dioxide-methane fluid.
The crystallization of all corundums was likely to occur in a heterogeneous medium consisting of immiscible low-salinity water-salt and high-density carbonic acid fluids. The inclusions of the first type occur in chains and planes next to chains of inclusions of the second type (carbonic acid), which suggests their syngenesis, i.e., simultaneous trapping.
High-density carbonic acid with small amounts of other gases was the predominant component of the mineral-forming system during corundum crystallization. The amount of impurities (methane) in the inclusions varies from 12 to >1 % [7].
Main applications for corundum,
rubies and sapphires
• for grinding precious stones, metals and optical glasses
• manufacturing of grinding machine wheels from cemented ground corundum sandpaper
• rubies are used as bearings and backstones in watch movements, ensuring high precision and prolonging their life. Up to 16 of these can be fitted in a single watch movement.
• sapphire is used to produce a superstrong glass known as sapphire crystal.
• sapphire crystal is popular for mobile phones
• rubies and sapphires are used in optical quantum generators (lasers)
• sapphire does not react with any acids and alkalis, resists high pressures and temperatures, harsh radioactive radiation
• sapphire windows in devices and apparatuses where vacuum, high temperatures and pressures reign, bathyscaphes etc.
• sapphire as a differential pressure sensor is used to detect emergency sections of gas pipelines
• a millimeter-long layer of corundum insulators replaces a five-centimeter-long layer of ordinary mineral wool
• corundum is a unique type of fibre-reinforced high-performance cement composite, which fulfils the highest requirements for new roads.
Conclusion
The systematisation and synthesis of available material and our own data on noble corundum deposits around the world provide an opportunity to assess the role of indigenous corundum-bearing objects in the geological history of their formation.
The first group includes primary deposits of magmatic, pegmatitic and metasomatic (postmagmatic) origin, and the second — includes polychronous and polyfacial primary metamorphic and metamorphogenic-metasomatic deposits.
Deposits of metamorphogenic type are generally older than deposits of other types. Their age is defined as Archean in Greenland, Neo-Archean-Paleoproterozoic in Russia (Karelia, etc.), South Asia (India, Sri Lanka, Myanmar, Tajikistan, etc.) and East Africa (South Africa, Madagascar), Proterozoic in the USA (North Carolina) and Upper Paleozoic, and sometimes even older in Southern Russia, Africa, Asia and Australia (Ural, Kenya, Tanzania, Mozambique, etc.).
The analysis of the geological position and genesis of noble corundum deposits allows us to interpret the properties of rubies and sapphires and to recommend their use in traditional and special, including fundamental diagnostics.
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