『新东方在线托福听力』为托福考生准备了ETS托福TPO/【学术讲座】#6听力练习：What does the teacher ask the class to do?原文题目答案解析，TPO/【学术讲座】#6听力原文范文音频翻译。

托福TPO/【学术讲座】#6听力原文What does the teacher ask the class to do?【新东方在线】

What aspect of a mineral is the Mohs’ scale used to identify?

What does the teacher ask the class to do?

According to the teacher, when might the hardness of the same mineral seem to vary?

Listen to a lecture in an Earth Science class.Today I’d like to explain the Mohs’ scale, used in what is called the “scratch test.” This scale is based on the simple fact that harder minerals scratch softer ones. For example, a diamond; a quartz crystal can scratch a feldspar crystal, but not the other around. The scale is named for Friedrich Mohs, the mineralogist who devised it in 1812. His scale spans the range of minerals known at that time, from the softest to the hardest. By performing a scratch test using known minerals and a few common tools, an unidentified mineral sample can be placed between two points on the scale. By referring to the scale, the mineral can then be identified. I have here a collection of the minerals included on the Mohs’ scale, as well as the tools necessary to complete this exercise. I’d like you each to take a mineral sample from the basket at the front of the room and classify it according to its place on the Mohs’ scale. First, however, I should give you a little warning. The hardness of any mineral depends on the strength of the bonds between ions or between atoms – the stronger the bond, the harder the mineral. Because bond strength may differ in various angles of a crystal, the hardness may vary slightly depending on the direction in which the mineral sample is scratched, so be sure to scratch each sample in several different directions.

Listen to a lecture in an Earth Science class.

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