By long-standing convention, all meteorites are assigned to three broad divisions on the basis of two kinds of material that they contain: metallic nickel – iron(metal) and silicates, which are compounds of other chemical elements with silicon and oxygen. As their name suggests, the iron meteorites consist almost entirely of metal. At the opposite extreme, the stony meteorites consist chiefly of silicates and contain little or no metal. A third category, stony-irons, includes those meteorites that contain similar amounts of metal and silicates. Since meteoritic metal weighs more than twice as much as the same volume of meteoritic silicates, these three kinds of meteorites can usually be distinguished by density, without more elaborate tests.
- Reading Practice Test 64 from The Collection of TOEFL Reading Comprehension
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- Reading Practice Test 66 from The Collection of TOEFL Reading Comprehension
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The stony meteorites can also be subdivided into two categories by using nothing more complicated than a magnifying glass. The great majority of such meteorites are chondrites, which take their name from tiny, rounded objects – chondrules – that occur in most of them and are among their most puzzling features. The rest of the stony meteorites lack chondritic texture and are therefore called achondrites. Achondrites vary widely in texture, composition, and history.
Irons, stony-irons, chondrites, and achondrites are by no means equally abundant among observed meteorites: chondrites are much more common than all other kinds of meteorites put together. The irons, which are usually prominent in museum displays, are really quite uncommon. Curators like to highlight iron meteorites because many of them are large and their internal structure is spectacular in polished, etched slices. A stony meteorite has a beauty of its own, but it only appears under the microscope: to the unaided eye, stony meteorites appear to be – indeed they are – rather homely black or gray rocks.
To go further with meteorite classification, it is necessary to be more specific about the minerals that make up a meteorite: which silicates are present, and what kind of metal? To answer these questions, one needs to see more detail than is visible to the unaided human eye.
1. What is the passage mainly about?
(A) The formation of meteorites
(B) Some recent meteorites
(C) The classification of meteorites
(D) How meteorites are displayed
2. The word “elaborate” in line 9 is closest in meaning to which of the following.
3. According to the passage, small, rounded objects can be found in what kind of meteorites?
4. According to the passage, the spectacular meteorites usually found in museums are
(A) gray or black
(B) generally small
(C) unimportant to science
(D) fairly uncommon
5. The word “it” in line 21 refers to
7. Where in the passage does the author suggest a means by which meteorites can be differentiated?
(A) Lines 3-4
(B) Lines 7-9
(C) Lines 18 – 19
(D) Lines 20-22