Occurrence of ruby and sapphire in Afghanistan

Occurrence of ruby and sapphire in Afghanistan

Ruby and sapphire are mined from two separate zones of mineralized marble – north and south – which are separated from each other in a distance of 600-800 m, and joined in the west. The vertical extent of the corundum mineralization is more than 400 m. Ruby occurs in irregularly shaped lenses, rarely more than 2-3 cm wide, that are oriented lengthwise within individual horizons and beds of marble.

Corundum deposits related to alkali basalts are common in many regions – especially in eastern Australia and in Southeast Asia, also in Nigeria. The origin of these rubies and sapphires has been widely debated among geologists and mineralogists. The alkali basalts are thought to carry the corundum crystals to the surface from the earth’s interior, where they formed. Note, however, that different sources of gem corundum may be entrained by the basaltic magmas.

New studies have shown that some basaltic fields yield two types of corundum: the magmatic sapphires are found together with metamorphic pastel-colored sapphires and rubies. The latter gems are thought to be derived from metamorphic or metasomatic source rocks in the earth’s interior. Two types of basalt-hosted sapphires can be identified by trace-element chemistry and/or absorption spectroscopy.

Sapphire-bearing alluvial material derived from the eroded alkali basalts was deposited in voids and crevices of the weathered Jurassic Ankarana limestone that lies south of the Massif d’Ambre. Occasionally, these sediments are cemented by secondary carbonates.

Precambrian (more than 570 million years old) emerald and beryl deposits in southern Egypt are associated with two different geologic units. The Nugrus Thrust, a regional ductile shear zone, hosts emerald deposits in biotite schists. Beryl associated with granitoids is found either in greisen bodies or in pegmatitic lenses and veins. Thirteen samples of beryl and emerald from the region were analyzed petrographically and geochemically to aid in exploration for additional deposits and to examine the generally held assumption that all Egyptian emeralds are of metamorphic origin.

Fluid flow processes associated with the emerald deposits were contemporaneous with the tectonic development of the Nugrus Thrust and the emplacement of the granitic bodies, which postdate regional metamorphic events. The chemical zoning of the emerald crystals reflects typical magmatic fractionation patterns. The data suggest that the emerald mineralization depends on syntectonic intrusive events and the chemistry of magmatic fluids, rather than on host-rock chemistry or regional metamorphism.

Topaz from Brazil

The topaz deposits occur in southern Minas Gerais. The topaz mineralization falls within an east-west trending zone that extends from Antonio Pereira village on the east, to Miguel Burnier village on the west, both in the Ouro Preto district. Pires identified four main topaz belts in the region, each of which trends east-west.

The formation of the topaz has been the subject of much debate over the last century. The mineralized zone is characterized by intensively weathered (to depths of at least 50 m) rocks underlained by unweathered granitic gneisses, granites, and three series of Precambrian metasedimentary rocks. The Minas series of Precambrian metasediments was subjected to two major intrusive events: about 2,700 million years ago, by a batholith that fractured the sedimentary rocks; and about 1,300 million years ago, by acid intrusions (high-silica igneous rocks). It is believed by Keller and others that one or both of these intrusions provided the mechanism for the fluorine-rich solutions that entered the rocks through fractures and generated the topaz mineralization.

Pires, Oliveira, and Hoover support the formation of strata-bound topaz deposits from a predominantly hydrothermal process that occurred during or shortly after a period of intense metamorphism.

Topaz crystals are found within the kaolinite - together with quartz, mica, and specular hematite. They sometimes are associated with rutile and, rarely, with green and blue euclase.
Дополнительный словарь:

marble – мрамор	alkali – щелочь
gneiss – гнейс	interior – внутренний
impurity – примесь	to entrain – доставляться
clay – глина	pastel – блеклая (краска)
irregularly - неправильно	to derive – устанавливать связь
lengthwise – продольный, вдоль	predominantly – преимущественно
bed – залежь	to erode – размывать, выветривать
precambrian – докембрий	to relate – соотноситься
schist – сланец	flow – течение, поток
contemporaneous – одновременный	emplacement - внедрение
postdate – датированный позже	belt – узкая зона, пояс
event – событие	to underlain – подстилать
to fall – понижаться, падать	to fracture – ломаться, раздроблять
to trend – наклоняться	to support – поддерживать
sapphire-bearing – сапфир-содержащий	crevice – трещина, содержащая жилу

2.6. Answer the questions:

1. What type of corundum deposit is in Afghanistan?

2. Where do rubies occur within marble horizons?

3. What origin of sapphires can be within basalt rock?

4. Where are sapphire-bearing alluvial deposits?

5. What genesis has beryl from Egypt?

6. What host-rocks were metamorphosed to form emeralds?

7. What processes provided the crystallization of topaz in Brazil?

First, we can define the terms rocks and minerals. Minerals are naturally occurring crystalline solid with a definite, but not necessarily fixed, chemical composition, inorganic (lifeless) substances. Coal, oil and natural gas are not minerals, because they are mixtures of organic chemicals formed from once-living matter. They are usually called fossil fuels. Each mineral has a definite composition and any part of a mineral is much the same as any other part. Rocks are composed of mineral grains, but the proportions of minerals vary from one sample to another. Sometimes, as in limestone, a rock is composed mostly by one mineral, but most rocks consist of a number of different minerals.

A material that is naturally occurring is formed without the benefit of human action or intervention. It must be possible to find samples formed in the natural environment. Many crystalline solids with the same chemical and physical properties as their natural mineral counterparts may be synthesized in the laboratory. These materials are synthetic materials.

Minerals must be crystalline solids. The atoms and/or ions that comprise crystalline materials are arranged and chemically bonded in a regular and repeating long-range pattern. The beautiful, symmetrically arranged crystal faces that adorn many mineral samples are a consequence of this internally ordered atomic structure. Solids such as glass lacking long-range atomic order are considered amorphous. To be considered crystalline a material must be a solid, although crystalline materials may deform in a ductile manner under appropriate temperature – pressure conditions.

1. Does a mineral form with benefit of human action?

2. How named is material crystallized in the laboratory?

3. Are the coal and oil minerals?

4. What is solid presented as amorphous?

5. What is rock? What is rock composed of?

1) кристаллическое твердое тело;

2) синтетические материалы;

3) аморфное вещество;

4) условия температуры и давления;

5) деятельность человека;

6) соотношение минералов в породе.
Text 2.

Mineralogy

Mineralogy is the study of minerals. The beginning of this particular branch of science extends well back to prehistoric times, for our ancestors surely knew about and used many minerals. Evidence of mining and smelting minerals to extract useful metals such as copper, lead, and zinc is found in many ancient civilizations.

The modern study of mineralogy can be traced back to Theophrastus (387-272BC) who wrote the earliest preserved book dealing with minerals and rocks, titled On Stones. Some 400 years later, Pliny the Elder, who met his death at Pompeii, provided us with an encyclopedic review of mineralogy as it applied to the metallic ores, gemstones, and pigments in use in the Roman empire circa 77 AD. Some 1500 years later (1556) German physician and mining engineer Georg Bauer, known to us by his Latinized name Georgious Agricola, provided detailed descriptions and defined physical properties such as hardness and cleavage that continue to provide the basis for hand-sample identification of minerals. Through the seventeenth, eighteenth, and nineteenth centuries a number of notable scholars provided significant advances to the science.

During the twentieth century a wide variety of instrumentation has been developed to improve our ability to determine the chemical composition of minerals and refine our understanding of their crystal structures. In addition, petrologists and chemists have immensely expanded our knowledge of the chemical and petrologic behavior of minerals in a wide range of geologic environments.

1. What is the name of science which is study of minerals?

2. How long ago did people begin to study minerals?

3. Who described minerals and rocks in the early times?

4. Why is knowledge of minerals refined during the twentieth century?

5. What properties of minerals are recently best understood?

Mineral is …

Rock is …

Mineralogy is …

About 4000 minerals have been identified, described, and named, although less than a hundred mineral species are common. Fleischer and Mandarino (1995) and Nickel and Nichols (1991) provided authoritative lists of mineral names approved by the Commission on New Minerals and New Mineral Names of the International Mineralogical Association. New minerals are regularly discovered and a summary of new minerals is included in each issue of the American Mineralogist, published by the Mineralogical Society of America and in the Zapisky Rossiyskogo Mineralogicheskogo Obshchestva, published in S.-Petersburg.

A number of criteria must be satisfied before a new mineral name is approved. The candidate material must be a mineral as defined above and must not previously have been described and named. In addition, the crystal structure and composition must be accurately determined, and a type sample must be preserved in an appropriate repository such as a scientific museum or in the collection of a research institute.

A mineral may be named after an individual, a place where it is found, or in allusion to its chemical composition or a significant physical property. Some minerals have names whose origins are lost in mists of antiquity.

1. What organization provides authoritative list of mineral names?

2. What material can be the candidate for a new by named mineral?

3. What type of names is taken for new minerals?

1. 
   Read and learn necessary words:
   

2. 
   Find Russian equivalents of following words:
   

atom ion

calcite mineral

dolomite molecule

element olivine

formula quartz
Text 1.

Chemistry of minerals

Each mineral has a distinctive chemical composition, which may be fixed, or may vary within certain limits. This composition is expressed as a chemical formula. Some minerals, known as the native elements, contain only one element. Most minerals, however, are compounds containing more than one element. They comprise two charged parts called ions. The negatively charged anions often contain oxygen.

An example of a simple mineral formula is NaCl, the formula for halite, or sodium chloride. It contains one sodium atom and one chlorine atom per molecule. More complicated formula is that for calcite, CaCO₃, or rutile, TiO₂. The numbers refer to the number of atoms of the preceding element that are present in one molecule of the mineral. Sometimes minerals form chemical series. An example of this is the olivine series, written as (Mg, Fe)SiO₄. This means that atoms of magnesium and iron can substitute for one another, to form a continuously variable series from a pure magnesium olivine (forsterite) to a pure iron olivine (fayalite).

1. 
   Answer the questions:
   

1. 
   What minerals are in the group of native elements?
   
2. 
   What does chemical formula express?
   
3. 
   What chemical element does often compound anion part of the formula?
   
4. 
   What minerals form chemical series?
   

4. 
   Pronounce chemical composition for mineral formulas:
   

Ca [WO₄] Ca [SO₄] MoS₂ SiO₂

Mg [CO₃] CaF₂ HgS (Fe,Mn) [WO₄]
Text 2.

Chemical tests

Some simple chemical tests are helpful for identifying minerals. In particular, geologists carry dilute hydrochloric acid (HCl) in the field. If a drop of it is placed on calcite, it will effervesce (fizz, giving off gas), whereas quartz does not react. This is one way in which the two similar minerals can be distinguished. Dolomite, another carbonate mineral like calcite, also reacts, but the reaction is much slower. Great care must be taken even when using dilute acids to avoid contact with skin and clothes, and to avoid breathing in fumes.

Where laboratory facilities are available, other chemical tests can be carried out, such as for solubility in acids. A powdered sample of the mineral is placed in acid in a test-tube and, if required, is gently warmed. Flame colouration tests are carried out by holding a chip of mineral in a flame with a pair of forceps. The hottest part of the flame is used, at the tip of the blue cone. The colour of the flame indicates which element presents. For example, a green flame indicates barite, which contains barium.
Дополнительный словарь:

in particular – в частности	care – внимание
to сarry – нести	to avoid – избегать
to dilute - разбавлять	to breathe – дышать
drop – капля	fume – испарение
to effervesce – выделять пузырьки	facility – приспособление
to fizz – шипеть	solubility – растворимость
to require – требовать	forceps – пинцет
gently – осторожно	cone – конус
аvailable – доступный

1. 
   Answer the questions:
   

1. 
   What acid helps to test mineral chemically?
   
2. 
   How can the carbonate mineral be casily identified?
   
3. 
   What chemical tests can be also carried out?
   
4. 
   How are flame colouration tests applied?
   

Statistically, all analyses of poudretteite were highly consistent with regard to major element: SiO₂ (78.99wt.%), B₂O₃ (10.53-11.03wt%), Na₂O (6.20-6.53wt.%) and K₂O (3.45-4.09wt.%). In addition, more than 30 other elements were measured (ranging from Li to U). Of these, 21 trace elements were detected at the ppm (parts per million) level. Two of these trace elements displayed a consistently higher concentration in the saturated part as compared to the near-colorless one: Li (14 ppm and 9-10 ppm, respectively) and Mn (49-52 ppm and 16-21 ppm). Other revealed an opposite correlation: Ca (32-55 ppm and 79-132 ppm, respectively), Rb (83-86 ppm and 112-114 ppm), and Cs (8 ppm and 9-10 ppm). Be, Mg, Al, Ti, V, Cr, Fe, Ni, Cu, Zn, Ga, St, Zr, Sn, Pb and Bi revealed a high degree of variability but no apparent correlation between the differently colored parts.

hardest – rarely –

lower – heavier –

wide – bitter –

increasing – expensive –

stronger –

Each mineral possesses certain physical properties or characteristics by which it may be recognized or identified. Although some may be identified by visual examination, other must be subjected to certain simple tests. Physical properties especially useful in mineral identification are colour, streak, lustre, transparency, hardness, cleavage, and fracture. Many of these tests do not require expensive laboratory equipment and may be carried out in the field.

Probably one of the first things that is noted about a mineral is its colour. However, the same mineral may vary greatly in colour from one specimen to another, and, with certain exceptions, colour is of limited use in mineral identification. Certain minerals have relatively constant colour, for example, azurite, which is always blue; malachite, which is green; and pyrite, which is yellow. Other minerals, such as quartz or tourmaline, occur in a wide variety of colours; hence, colour may be of little use in identifying these two minerals. Colour variations of this sort are generally due to the presence or absence of chemical impurities in the mineral.