Минералогия (стр. 8 )

Ряды Au–Ag–Se, Au–Ag–S. Низкотемпературные минералы вулканогенных гидротермальных руд — фишессерит AuAg3Se2, айтенбогардтит AuAg3S2, петровскаит AuAgS–AuAg(S, Se), пенжинит AuAg4(S, Se)4. Известны и в корах выветривания. Сульфиды золота устойчивы при крайне высокой f S, нередко ассоциируют с серой.

Сложные халькогениды Au. Типичные образования вулканогенных гидротермальных руд от убогосульфидных до колчеданных — нагиагит AuPb5Te4-x(Sb, As, Bi)xS6, букхорнит AuPb2BiTe2S3, криддлеит Au3Ag2TlSb10S10, минерал AuBi5S4. Характерные образования кор выветривания (зон цементации) гипогенных золото-тeллуридных месторождений — билибинскит Au5Cu3(Te, Pb)5, билибинскит-(Sb) Au6Cu2(Te, Pb, Sb)5, богдановит-(Cu) Au5Cu3(Te, Pb)2, богдановит-(Fe) Au5CuFe2(Te, Pb)2, безсмертновит Au4Cu(Te, Pb). Макроскопически напоминающие лимонит, низкосимметричные билибинскит и богдановит в отраженном свете характеризуются яркими цветными эффектами двуотражения и анизотропии. Безсмертновит в отраженном свете имеет цвет “апельсинов из Марокко”. По оптическим свойствам к билибинскиту близка гипергенная фаза AuTeO3.

Золотоносные халькогениды. Описанные ранее золотосодержащие блеклые руды — это тонкие полиминеральные срастания. Золотосодержащие пирит и арсенопирит помимо механических вростков минералов золота на собственных дефектах содержат отдельные атомы или группировки атомов Au (облака Коттрела и т. п.). Наиболее дефектны As-пирит и S-арсенопирит в вулканогенных месторождениях типа Карлин-Лухуми. Такие пирит и арсенопирит максимально золотоносны; содержания Au в них от n100 г/т до 10 кг/т сульфида и более. В глубоко метаморфизованных рудах сульфоарсениды и арсениды Ni-Co-Fe, а также теллуриды Bi иногда содержат до n100 г Au на тонну халькогенида.

Modular mineralogy and mineral classification: the heterophyllosilicates

Giovanni Ferraris and Angela Gula.

Dipartimento di Scienze Mineralogiche e Petrologiche, Università di Torino, and (GF) Istituto di Geoscienze e Georisorse, CNR - Via Valperga Caluso, 35, 10125, Torino, Italy. *****@***unito. it

A classification of minerals can be based on different principles, depending on its purposes. Thus, ore minerals can be usefully classified according to the chemical elements of interest and a classification could be subdivided in iron, copper, uranium, etc. minerals. For the purposes of general mineralogy, usually minerals are classified in groups where the members share a common anion (silicates, phosphates, borates, etc.). In turn, within each “anionic” group, subgroups that include the members with the same or close crystal structure are usually individuated. Among modern structure-based classifications those by Bokij [1,2], Lima-de-Faria [10], [13] are worth to be mentioned here.

In recent years the modular description of the mineral structures is becoming more and more important [12]. For example, it has been proved that the comparison of not yet fully characterized phases with other phases related to them according to the principles of modularity can give a key to acquire unknown crystal data and even to model crystal structures [see examples in Ferraris [4] and Ferraris et al. [5,7]. Therefore, to obtain information useful to correlate unknown and known structures it is important to gather together those minerals which are based (at least in part) on common structural modules. These families or groups of minerals are known as polysomatic and homologous series; several series of this type are now known [12] and form a first nucleus of a mineral classification based on modular principles. An early example of this type of mineral classification has been published by Ferraris, Mellini and Merlino [9].

The polysomatic family of the heterophyllosilicates [7] is particularly suitable to show classification and structure modelling principles based on modularity. This series includes layer titanosilicates which are based on three different types of HOH layers; these are comparable to the TOT layers of the layer silicates. In the HOH layers, the H (hetero) is a sheet which may be derived from the tetrahedral sheet T of the layer silicates by inserting 6- or 5-coordinated Ti (or other vicariant cations); O is an octahedral sheet like those occurring in the layer silicates.

Ideally the members of the heterophyllosilicate series have formula A2+nY4+3n[Ti2(O')2+pSi4+4nO14+10n](O'')2+2n and cell parameters a ~ 5,4, b ~ (6,8 + n x 4,7), c ~ k x 11 Å (n and k integers). The chemical elements belonging, even in part, to the H sheet are shown in square brackets in the formula; A=interlayer cations; Y=octahedral cations; O' (bonded to Ti) and O'' (belonging to O only) can be oxygen, OH, F or H2O; the 14+10n oxygen atoms are bonded to Si. The value of p (0, 1, 2) depends on the coordination number and edge/corner sharing of the Ti coordination polyhedron.

The heterophyllosilicate series branches in three subseries: for n = 0, 1, and 2, bafertisite-like, astrophyllite-like, and nafertisite-like structures are known respectively. The most consistent group forms the bafertisite series. Taking into account also concepts introduced by Makovicky [11] to define plesiotype and merotype series, recently the bafertisite series has been redefined as mero-plesiotype series [8]. The series belonging to the heterophyllosilicate family can be described as based on mica and Ti-bearing modules chosen in various ways [3,4]. The presence of a mica module links the heterophyllosilicate series with the layer silicates and other polysomatic series as that of the palysepioles [6]. The latter series so far includes the well known minerals palygorskite and sepiolite together with the recently discovered kalifersite.

References: 1. Bokij G. B. Systematics of natural silicates. Moscow: VINITI, 1998. 2. Bokij G. B. Systematics of natural oxides. Moscow: VINITI, 2000. 3. Christiansen C. C., Makovicky E., Johnsen O. N. Homology and typism in heterophyllosilicates: An alternative approach // N. Jb. Min. Abh., 1999.V. 175. P. 153-189. 4. Ferraris G. Polysomatism as a tool for correlating properties and structure // EMU Notes in Mineralogy, 1997. V. 1. P. 275-295. 5. Ferraris G., e. a. A structural model of the layer titanosilicate bornemanite based on seidozerite and lomonosovite modules // Can. Min., 2001. V. 39. P. 1667-1675. 6. Ferraris G., e. a. Kalifersite, a new alkaline silicate from Kola Peninsula (Russia) based on a palygorskite-sepiolite polysomatic series // Eur. J. Min., 1998. V. 10. P. 865-874. 7. Ferraris G., e. a. Nafertisite, a layer titanosilicate member of a polysomatic series including mica // Eur. J. Min., 1996. V. 8. P. 241-249. 8. Ferraris G., e. a. The crystal structure of delindeite, Ba2{(Na, K,)3(Ti, Fe)[Ti2(O, OH)4Si4O14](H2O, OH)2}, a member of the mero-plesiotype bafertisite series. Can. Min., 2001. V. 39. P. 1306-1316. 9. Ferraris G., Mellini M. & Merlino S. Polysomatism and the classification of minerals // Rend. Soc. Ital. Miner. Petr., 1986. V. 41. P. 181-192. 10. Lima-de-Faria. Structural classification of minerals. Dordrecht: Kluwer, 2001. 11. Makovicky E. Modularity – different types and approaches // EMU notes in mineralogy, 1997. V. 1. P. 315-344. 12. Merlino S. (Ed.) Modular aspects of minerals. Budapest: Eötvös University press, 1997. 13. Strunz H. & Nickel E. H. Strunz mineralogical tables. Stuttgart: Schweizerbart’sche Verlag, 2001.

Авторский указатель:

A

Angela G., 15

F

Ferraris G., 15

Б

, 2

, 1, 2

В

, 2

, 10

Г

, 4

З

А, 2

К

, 5, 6

Л

11

М

, 11

6

Н

, 7

, 2

П

, 7

, 2, 9

Р

, 5, 10

С

, 11

, 11

, 12, 13

, 13

Т

Г, 9