Minerals and Gems

Abstract

Mining and mineral engineering are not always popular topics today. Many gemstones and mineral specimens are found during mining operations. The bulk of the minerals are then processed by physical or chemical means. Ceramists should have some knowledge of mineral processing because it can be the clue to understanding why certain impurities are present in powders used to produce high-tech ceramics (hence our discussion of raw materials in Chapter 19), but minerals and gemstones have many commercial, in addition to decorative (ornamental), uses.

1 People and history

Cullinan, Sir Thomas owned the mine where, in 1905, the world’s largest diamond was found.

De Beer, Johannes Nicholas and Diederik Arnoldus are brothers who owned the farm that became the Kimberley “Big Hole.”

Mohs, Fredrich (1773–1839) introduced the term “scratch hardness” in 1826. He was born in Gernrode/Harz Germany, studied at the University of Halle and at Freiberg. He later worked in Austria.

Moisson, Ferdinand Frederic Henri (1852–1907) discovered naturally occurring SiC in 1905 in a meteorite from the Diablo Canyon in Arizona (USA). He developed the electric furnace, which he used to make carbides and prepare pure metals. He received the Nobel Prize in 1906 for successfully isolating fluorine (1886).

Winston, Harry (1896–1978) A key figure in the diamond trade, he opened his business in New York City in 1932. In 1958, he donated the Hope Diamond to the Smithsonian.

2 Exercises

• 36.1 What are the lines in Figure 36.9?

• 36.2 In Figure 36.11 the stones are immersed in a liquid. Why is this liquid chosen? Show that the observations are what you would expect.

• 36.3 What can you deduce regarding the size, shape, and alignment of the particles causing the stars in Figure 36.15?

• 36.4 Explain the phenomenon of labradorescence seen in Figure 36.17.

• 36.5 What is the common flaw found in natural emeralds? Explain your answer from a crystallographic point of view.

• 36.6 If you had a good means for measuring thermal conductivity, would you prefer such a test to Mohs’ scratch test? How sensitive would your apparatus need to be? We can use a handheld tester to distinguish diamond and moissanite. How is this fact connected to the electronics industry?

• 36.7 Why must you be particularly careful when polishing opal? How is opal related to today’s electronics industry?

• 36.8 What do the stabilization of turquoise, the treatment of emerald, and ZnO varistors have in common?

• 36.9 Diamond has a high ${n}$ and is also a very hard material. (a) Are these two features linked? (b) If so, explain why SrTiO₃ has a higher ${n}$ but is not as hard.

• 36.10 Explain, using your knowledge of ceramic processing, how you might take turquoise powder and turn it into a gemstone.

• 36.11 We answered the question: is this turquoise natural? Clarify the answer.

• 36.12 When asked how to wash an opal, we answered: “No.” Why?

• 36.13 Diamonds have now been found in Zambia. Where else are they found worldwide?

• 36.14 Diamond is known for its sparkle. Which gemstones sparkle more? Why are they less valuable than diamond?

• 36.15 Watermelon tourmaline ranges in color from green to pink. Did the color affect its use as a piezoelectric? Explore and discuss.

• 36.16 You can make green tanzanite purple, white turquoise “blue,” and colorless topaz blue. What are you doing in each case, and how does each one relate to more technologically relevant ceramics?

• 36.17 Beryllium can be diffused into sapphire to produce a yellow color that is quite rare in nature. How would you, as a ceramist, show that it had been doped?

• 36.18 Talc has been used for many years. Discuss how its crystal structure influences its use.

• 36.19 Natural garnet occurs in many different colors. So does synthetic cubic zirconia. Discuss what features these two materials have in common to facilitate this coloration.

• 36.20 Why can we say that opal was the first photonic material?