X-ray Studies

xraystudiesshowthattheinternalarrangementofhaliteisadefinitecubicpatterninwhichionsofsodiumalternatewiththoseofchlorineeachsodiumionisthusheldinthecenterofandatequaldistancefromsixsymmetricallyarrangedchlorinejonsandviceversaitisthisbasicatomicarrangementorcrystallinestructurethatgiveshaliteitsdistinctivecubiccrystalformanditscharacteristicphysicalproperties

Minerals are chemicals

Mineralsarechemicalstheyarechemicalelementsorcompoundsfoundnaturallyinthecrustoftheearththeyareinorganicincontrasttoorganicchemicalsmademainlyofcarbonhydrogenandoxygentypicaloflivingthingssomemineralshaveafixedchemicalcompositionothersareaseriesofrelatedcompoundsinwhichonemetallicelementmaywhollyorpartlyreplaceanothersomemineralscanbeverysimilarchemicallybutareusuallyquitedifferentincolourandphysicalpropertiesonlyrarelywillasinglephysicalorchemicalpropertyidentifyamineralusuallymorecharacteristicsmustbeused.

Exercise 7. According to the following definitions find the words in the list of vocabulary given above:

1. a fine soft white clay, resulting from the natural decomposition of other clays or feldspar;

2. an organism or inorganic object or material which takes various forms;

3. A hard sedimentary rock, composed mainly of calcium carbonate or dolomite, used as building material and in the making of cement;

4. a shiny silicate mineral with a layered structure, found as minute scales in granite and other rocks, or as crystals;

5. an abundant rock-forming mineral typically occurring as colourless or pale-coloured crystals and consisting of aluminosilicates of potassium, sodium, and calcium;

6. a shiny yellow mineral consisting of iron disulfide and typically occurring as intersecting cubic crystals;

7. the chemical element of atomic number 12, a silver-white metal of the alkaline earth series. It is used to make strong lightweight alloys, and is also used in flash bulbs and pyrotechnics, as it burns with a brilliant white flame. (Symbol: Mg);

8. an olive-green, gray-green, or brown mineral occurring widely in basalt, peridotite, and other basic igneous rocks;

9. a colourless mineral consisting of magnesium oxide, occurring chiefly in marble and limestone;

10. a white or colorless mineral consisting of calcium carbonate

Exercise 8. Match the words on the left with their definitions on the right:

Nouns:

1. grain a. the process by which a beam of light is spread out passing through

2. lattice b. the reason for which it is made or done

3. diffraction c. the action, process, or result of location or being located

4. sample d. the action of agreement with something

5. arrangement e. a seed from wheat, rice, or other cereal crop

6. purpose f. a small part or quantity intended to show what the whole is like

7. approval g. a structure consisting of strips crossed and fastened together

Adjectives:

1. rare a. occurring, appearing, or done infrequently and irregularly

2. spatial b. being worth having or doing because it is useful, necessary, or popular

3. internal c. being firm and stable in shape; not liquid or fluid

4. desired d. being not common and is therefore interesting or valuable

5. occasional e. describing things relating to areas

6. solid f. occurring, found, or done often

7. common g. happening inside

Exercise 9. Read and translate the following text “Mineral Definition and Classification”:

Mineral Definition and Classification

To be classified as a true mineral, a substance must be a solid and have a crystalline structure. It must also be a naturally occurring, homogeneous substance with a defined chemical composition. Traditional definitions excluded organically derived material. However, the International Mineralogical Association in 1995 adopted a new definition: a mineral is an element or chemical compound that is normally crystalline and that has been formed as a result of geological processes.

The modern classifications include an organic class – in both the new Dana and the Strunz classification schemes.

The chemical composition may vary between end members of a mineral system. For example the plagioclase feldspars comprise a continuous series from sodium and silicon-rich albite (NaAlSi₃O₈) to calcium and aluminium-rich anorthite (CaAl₂Si₂O₈) with four recognized intermediate compositions between. Mineral-like substances that don't strictly meet the definition are sometimes classified as mineraloids. Other natural-occurring substances are nonminerals. Industrial minerals is a market term and refers to commercially valuable mined materials.

A crystal structure is the orderly geometric spatial arrangement of atoms in the internal structure of a mineral. There are 14 basic crystal lattice arrangements of atoms in three dimensions, and these are referred to as the 14 "Bravais lattices". Each of these lattices can be classified into one of the seven crystal systems, and all crystal structures currently recognized fit in one Bravais lattice and one crystal system. This crystal structure is based on regular internal atomic or ionic arrangement that is often expressed in the geometric form that the crystal takes. Even when the mineral grains are too small to see or are irregularly shaped, the underlying crystal structure is always periodic and can be determined by X-ray diffraction. Chemistry and crystal structure together define a mineral. In fact, two or more minerals may have the same chemical composition, but differ in crystal structure (these are known as polymorphs). For example, pyrite and marcasite are both iron sulfide, but their arrangement of atoms differs. Similarly, some minerals have different chemical compositions, but the same crystal structure: for example, halite (made from sodium and chlorine), galena (made from lead and sulfur) and periclase (made from magnesium and oxygen) all share the same cubic crystal structure.

Crystal structure greatly influences a mineral's physical properties. For example, though diamond and graphite have the same composition (both are pure carbon), graphite is very soft, while diamond is the hardest of all known minerals. This happens because the carbon atoms in graphite are arranged into sheets which can slide easily past each other, while the carbon atoms in diamond form a strong, interlocking three-dimensional network.

There are currently more than 4,000 known minerals, according to the International Mineralogical Association (IMA), which is responsible for the approval of and naming of new mineral species found in nature. Of these, perhaps 100 can be called "common", 50 are "occasional", and the rest are "rare" to "extremely rare".

Exercise 10. Look through the text above once again and answer the following questions:

1. What features must substance have to be classified as a true mineral?

2. What is a mineral?

3. How many classification schemes does modern classification include?

4. How may the chemical composition of minerals vary?

5. What does the word mineraloids mean?

6. What is crystal structure?

7. What is the crystal structure based on?

8. Crystal structure can be determined by X-ray diffraction even when the mineral grains are too small, can’t it?

9. What is mineral defined by?

10. If two or more minerals have the same chemical composition how do they differ then? Do you know any examples?

11. Can some minerals have different chemical compositions, but the same crystal structure? Do you know any examples?

12. What does crystal structure of minerals influence greatly? Do you know any examples?

13. How many minerals are currently known?

14. What is the International Mineralogical Association responsible for?

Exercise 11. Look through the following text “Differences between Minerals and Rocks” and fill in the words given below: