12/12/11

MLK MemorialThis overhead view of the Memorial shows its location near the tidal basin. The Memorial is a complex of paths and walls with the statue in the center.

MLK MemorialThis closer view shows the two main parts of the Memorial: The “Mountain of Despair” and the “Stone of Hope.”

MLK MemorialThe “Mountain of Despair” is a large granite block with a gap in the middle. This gap is what makes up the statue of Martin Luther King, Jr.

MLK MemorialThe “Stone of Hope” is a large block of granite emerging from the “Mountain of Despair,” and out of that block of granite emerges the likeness of Martin Luther King, Jr.

Sculpting with GraniteGranite is an extremely hard, durable rock that has been used sparingly throughout history. The Egyptians created some of their monuments using granite, but because it is difficult to sculpt, most cultures used different types of rocks.

Sculpting with GraniteModern-day examples of granite structures include the Empire State Building.

Sculpting with GraniteThe most famous modern example of a granite statue is Mt. Rushmore. The granite face of a mountain was chiseled into the faces of Washington, Jefferson, Lincoln, and Teddy Roosevelt.

MLK MemorialGranite makes an impressive statement. To create a sculpture in granite is to create a permanent monument that can withstand the ravages of time.

Rocks and MineralsGranite is a type of rock, and rocks are made up of combinations of different minerals. Minerals usually have a crystal structure. Rocks and minerals have different properties, among them hardness.

Rocks and MineralsThe Mohs Scale is a rough measure of the hardness of different minerals. Starting with talc, a soft mineral, and ending with diamond, the hardest substance, the Mohs scale tracks the hardness of key minerals.

Mohs scale Mineral

1 Talc

2 Gypsum

3 Calcite

4 Fluorite

5 Apatite

6 Orthoclase Feldspar

7 Quartz

8 Topaz

9 Corundum

10 Diamond

MLK MemorialThe Mohs Scale is an ordinal scale where the hardness of a rock or mineral is determined relative to two places on the scale.

MLK MemorialFor example, if an unknown rock can scratch feldspar and is, in turn, scratched by quartz, then the hardness of the rock of the is between feldspar and quartz.

Rocks and MineralsSince feldspar and quartz are at positions 6 and 7, respectively, on the Mohs scale, then the unknown rock has a hardness between 6 and 7.

Rocks and MineralsBy convention, this rock would have a hardness of 6.5. However, since the Mohs scale is an ordinal scale, such a measurement is not possible. All we can say is that the hardness is somewhere between the two values.

Rocks and Minerals

• Granite is composed mostly of quartz, feldspar, and other minerals.

• Granite should have at least 20% quartz.

Rocks and MineralsFor simplicity, let’s suppose that granite is composed of just quartz and feldspar. These minerals are adjacent to each other on the Mohs Scale. So, as with the previous example, quartz has a hardness between 6 and 7.

Rocks and MineralsDifferent types of granite have different proportions of quartz and feldspar, potentially affecting the hardness of the rock. For a more accurate measure of the hardness, we need a continuous measurement of hardness.

Rocks and MineralsThere is a more accurate hardness scale that takes into account the absolute hardness of different substances. This table shows the hardness values, and as you can see, this is a nonlinear data set.

Absolute hardness scale Mineral

1 Talc

3 Gypsum

9 Calcite

21 Fluorite

48 Apatite

72 Orthoclase Feldspar

100 Quartz

200 Topaz

400 Corundum

1600 Diamond

Rocks and MineralsThis is a graph of the data set. Note that it is roughly exponential.

Rocks and MineralsAn exponential regression shows how dramatic the growth is. The shaded region shows the part of the scale where the hardness of granite can be found.

Rocks and MineralsWith a continuous hardness scale, we can see how the hardness increases from feldspar (x = 6) to quartz (x = 7). Depending on the percentage of quartz in the granite, the hardness can nearly double.

Rocks and Minerals

• Because granite is at the higher end of hardness, to cut granite usually requires a diamond- tipped cutting tool.

Rocks and Minerals

• What adds to granite’s hardness is that it is formed under intense heat and pressure inside the Earth, so that the molecules of quartz and feldspar are intermingled and tightly knit.