INTRODUCTION TO CERAMIC MINERALS

1. INTRODUCTION TO CERAMIC MINERALS UNIT 2.0 PROPERTIES OF MINERAL COLOUR HARDNESS LUSTRE

2. LEARNING OUTCOME At the end of the lesson, students should be able to : • Explain why mineral colour is different between each other. • Explain how atomic bonding is responsible for absorbing or transmitting the certain wavelengths in minerals. • Write the definition of chromophores, idiochromatic and allochromatic. • Write the definition of hardness. • Explain the correlation between hardness and mineral structure. • Explain the correlation between atomic size and hardness. • Write the definition of luster. • Explain the types of luster. • Explain the types of non-metallic luster.

3. INTRODUCTION • This topic provides fundamental knowledge in basic properties of minerals, uses and simple techniques of mineral identification. • The scope of the subject covers physical property of minerals such as colour, hardness, luster, streak, fracture, cleavage, tenacity, acid solubility, magnetic, and etc. • A close connection exists between the physical properties of a mineral, its crystal structure, atomic bonding and its chemical compositions.

4. INTRODUCTION • Physical properties can also be a great technical significance, since mineral may have important industrial uses that depend on its physical properties alone; for example the extreme hardness of diamond makes it a highly efficient abrasive; and piezoelectric nature of quartz is the basis of its use in electronics equipment. • Many minerals are readily recognized on the basis of their physical properties. • However, minerals can only be identified absolutely by x-ray analysis that determines the mineral’s structure and its mineral compositions.

5. INTRODUCTION Following are the common and important physical properties of mineral:

6. COLOUR

8. The Many Colors of Fluorite

9. COLOUR • The color of a mineral is one of its most obvious attributes, and is one of the properties that is always given in any description. • Color results from a mineral’s chemical composition, impurities that may be present, and flaws or damage in the internal structure. • Unfortunately, even though color is the easiest physical property to determine, it is not the most useful in helping to characterize a particular mineral. • The problem is shown to the left, in which the mineral fluorite (CaF2) displays a rainbow of colors.

10. COLOUR • When white light strikes the surface of a mineral crystal, some of the wavelengths are transmitted and reflected while other are absorbed. • Colours are produced in minerals due to the selective absorption of some of the wavelengths constituting incident white light. • The resultant colour is the white light minus the absorbed wavelength. • Incolourlessor white minerals all the wavelength are transmitted.

11. COLOUR • Atomic bonding is responsible for absorbing or transmitting the certain wavelengths in minerals. • Some elements have electron that absorb certain wavelength or colours. • These wavelengths provide energy to the elements that will emit another wavelength to get rid the extra energy.

12. COLOUR • The wavelength (colours) absorbed by the electrons depends on their energy state. • The nature of bonding in these elements affects the energy state of these electrons. • Therefore bonds to different elements produced different colour.

13. COLOUR • Elements that produce colours through the selective absorption and emittance of wavelengths are usually transition metals. • If a mineral has certain transition elements as essential part of its chemistry, it will always display a characteristic (inherent) colour.

14. COLOUR

15. COLOUR • Most minerals are usually white or colourless in a pure state. • Many impurities can colour these minerals and make their colour variable. • In addition to the colouring elements (transition elements) other impurities or factors are also linked to the colour of minerals.

16. COLOUR • Impurities like elemental fluorine, sulfur and chlorine, traces of carbonate, structural defects etc. can cause colouration in minerals. • Radiation from rare earth elements can damage a crystal structure (colouring in smoky quartz?).

17. COLOUR • The classic example of colouring by traces of transition elements is corundum which is colourless when pure Al2O3 but also occurs as red ruby, and pink, yellow, green and blue sapphire. • Minerals that have a constant and characteristic colour are generally termed idochromatic or their colour is known as inherent. • or they are due to an intrinsic ingredient of the material

18. COLOUR • Minerals whose colour may differ from specimen to specimen are called allochromatic, or their colour is referred to as exotic. • Due to trace impurities in their composition or defects in their structure. • The ions of certain elements are strongly light-absorbing and their presence in small, even trace, amounts may cause the mineral to be deeply coloured.

19. COLOUR • These elements known as chromophores (Transition Elements) mainly are : Fe, Mn, Cu, Cr, Co, Ni, and V. Ex; -deep green of emerald -chromium(Cr) -green in beryl -vanadium (V) -purple of amethyst (quartz) - iron /Fe

20. The Colors of Mineral 1. Metal ions cause the color of many common and uncommon minerals.In the mineral magnetite, Fe is present as both Fe2+ and Fe3+ which results in colors from deep blue to black. 2. Valence Charge Transfer, involving metal ions in mixed oxidation states is another important factor in the coloration of minerals. Most commonly, we encounter minerals with the Fe2+ - Fe3+ interaction and with the Fe2+ - Ti4+ interaction.

21. The Colors of Mineral 3. Colors from natural ionizing radiation are frequently encountered in nature. Most common minerals have had a long history of exposure to ionizing radiation from natural radiation sources in rocks.  A variety of minerals can also be colored by artificial irradiation which enter the commercial market in the form of colored gemstones 4. Color can also be caused by structural defects in minerals. For example, an excess electron that is unattached to any single atom may be trapped in a structural defect such as a void due to a missing ion. A hole, or the absence of an electron, can have the same effect. Crystals get their color from growth imperfections.

22. The Colors of Mineral 5. Physical Impurities Impurities may produce color in minerals. Normally colorless calcite can be colored black by MnO2 or carbon. Tiny specks of red or green minerals can impart their color to minerals. For example, chlorite (green) in quartz , and hematite (red) in feldspar, calcite, and jasper 6. Semiconducting minerals have band gaps which often result in intense colors.   Numerous sulfides are examples of this

23. Why are Minerals Colored? • The color of minerals depends on the presence of certain atoms, such as iron or chromium which strongly absorb portions of the light spectrum. The mineral olivine, containing iron, absorbs all colors except green, which it reflects, so we see olivine as green. • All natural minerals also contain minute impurities. Some minerals such as corundum get their colors from these these impurities. Blue corundum (sapphire) is formed when small amounts of iron and titanium are dissolved in the solid crystal. • Finally some crystals get their color from growth imperfections. Smoky (black) quartz is a good example. Growth imperfections interfere with light passing through the crystal making it appear darker, or almost black.

24. It is thought that certain elements cause only certain colour. Is it true? Answer: Yes, to certain extent. However, any element can be responsible for any colour. Examples • Cobalt produces the violet-red colour in erythrite. • Chromium produces orange-red colour in crocoite. • Copper produces the azure-blue colour of azurite. • Iron produces the yellow-red colour of limonite. • Manganese produces pink colour of rhodochrochite.

25. HARDNESS

26. HARDNESS • It is defined as the resistance to scratching. • This property is also affected by the atomic bonding of the mineral.

27. HARDNESS • For some minerals such as copper, hardness measures plastic deformation; for other minerals, such as quartz and feldspar, it measures stress required to initiate rupture (break). • Minerals hardness is usually expressed in terms of Mohs scale (FriederichMohs, 1824) as shown in Table 11. • Absolute hardness measured by a sclerometer, The Turner-sclerometer test consists in measuring microscopically the width of a scratch made by a diamond under a fixed load, and drawn across the face of the specimen under fixed conditions.

28. HARDNESS TABLE 11: MOHS SCALE HARDNESS

29. HARDNESS

30. HARDNESS • Hardness varies with crystallographic directions. • Hardness and chemical composition have been related according to the following generalization: • Minerals of the heavy metals, such as gold, silver, copper, mercury and lead, are soft (hardness about 3 or bit more). Exceptions are platinum (H=4-4.5), iron (H=4,5).

31. HARDNESS • Most sulfides are relatively soft (H,5). Exceptions are sulfides of Fe, Ni, Co. • Most hydrous minerals are relatively soft (H<5). • Most anhydrous oxides and silicates are hard (H>5.5). • Halides, carbonates, sulfate and phosphate minerals are relatively soft (H=5.5).

32. HARDNESS Question: How do you find the correlation between mineral structure and their hardness?

33. HARDNESS Correlation between hardness and mineral structure is given below; • Greater the hardness, the smaller the atoms or ions. • Greater the hardness, greater the valence or charge on the cations. • Greater the hardness, greater the packing density (F.C.C > B.C.C > S.C)

34. HARDNESS Also if the atomic size is small so the packing is also high, so: • hardness  1/atomic distant or ion. • hardness  bonded density.

35. HARDNESS The above factors can be understood as: • The effect of ionic size difference: • Calcite (CaCO3) and Magnesite (Mg CO3), where Ca2+=1.00Å and Mg2+=0.57Å. Hardness (H) Calcite is H=3 and for Magnesite is H=4.5. • Hematite (Fe2O3), H=6 compared to corundum (Al2O3), H=9. Ionic radius of Fe2+ is 0.64Å whereas for Al3+ is 0.51Å.

36. HARDNESS • The effect of valence or charge, • KNO3 (H=2) and aragonite (CaCO3), H=4. Because in KNO3, K+=1.37Å whereas in CaCO3, Ca2+=1.00Å, so hardness increase when valence become higher.

37. HARDNESS • The effect of packing density: • The effect of density of packing is well seen in relationship between density and hardness of different polymorph. • e.g. calcite (SG=2.71, H=3) and aragonite (SG=2.93, H=4, quartz (SG=2.65, H=7) and tridymite (SG=2.26, H=6.5).

38. HARDNESS • The same correlation between hardness and density exists between hardness and A.P.F. • The greater the A.P.F the greater the hardness.

39. LUSTRE

40. LUSTRE • The term luster (shine) refers to the general appearance of a mineral surface in reflected light. • There are 3 types of luster: • metallic • non metallic • submetallic.

41. LUSTRE • METALLIC LUSTER • A mineral having the brilliant appearance of a metal has a metallic luster. • It is absorb radiation very strong and are opaque. • Normally have refractive index higher than 3. • Such minerals are quite opaque to light and, as a result, give a black or very dark streak. • Galena, pyrite, and chalcopyrite are common minerals with metallic luster.

42. LUSTRE • SUBMETALLIC LUSTER • Minerals have refractive index between 2.6 to 3, normally show property of opaque or nearly opaque. • Example of minerals of submetallic luster are cuprite (n=2.85), cinnabar (n=2.9) and hematite (n= 3.0).

43. CUPRITE CINNABAR HEMATITE

44. LUSTRE • NONMETALLIC LUSTER • Minerals with nonmetallic luster are, in general, light-coloured and transmit light. • The streak of non metallic mineral is either colourless or very light in colour.

45. LUSTRE The following terms are used to describe further the luster of nonmetallic minerals: • Vitreous : • The luster of glass. • Normally found at minerals with refractive index between 1.3 and 1.9 (cover 90%). • E.g. all silicates, quartz, carbonate, phosphate, sulfate and tourmaline.

46. LUSTRE • Resinous: • Having the luster of resin. • E.g. sphalerite and sulfur. • Pearly: • An iridescent (shining) pearl-like luster. • This is usually observed on mineral surface that are parallel to cleavage planes. • E.g. basal plane of apophyllite and cleavage surface of talc.

47. LUSTRE • Greasy: • Appears as if covered with a thin layer of oil. • This luster results from light spread by a microscopically rough surface. • Eg. Nepheline

48. LUSTRE • Silky: • Silk like. • It is caused by the reflection of light from a fine fibrous parallel aggregate. • E.g. fibrous gypsum, malachite, and serpentine.

49. LUSTRE • Admantine: • A hard, brilliant luster like that of a diamond. • It is due to the mineral’s high index of refraction (1.9-2.6). • E.g. lead minerals – cerussite and anglesite.

50. ADAMANTINE LUSTRE Adamantine minerals possess a excellent lustre, which is most notably seen in diamond. Such minerals are transparent or translucent, and have a high refractive index (of 1.9 or more). Minerals with a true adamantine lustre are uncommon, with other examples being cerussite and zircon. Minerals with a lesser (but still relatively high) degree of lustre are referred to as subadamantine, with an example being garnet.