GRE阅读真题200篇系列——Diamonds, an occasional component-西泽读留学

Diamonds, an occasional component of rare igneous rocks called lamproites and kimberlites, have never been dated satisfactorily. However, some diamonds contain minute inclusions of silicate minerals, commonly olivine, pyroxene, and garnet. These minerals can be dated by radioactive decay techniques because of the very small quantities of radioactive trace elements they, in turn, contain. Usually, it is possible to conclude that the inclusions are older than their diamond hosts, but with little indication of the time interval involved. Sometimes, however, the crystal form of the silicate inclusions is observed to resemble more closely the internal structure of diamond than that of other silicate minerals. It is not known how rare this resemblance is, or whether it is most often seen in inclusions of silicates such as garnet, whose crystallography is generally somewhat similar to that of diamond; but when present, the resemblance is regarded as compelling evidence that the diamonds and inclusions are truly cogenetic.

17. The author implies that silicate inclusions were most often formed

(A) with small diamonds inside of them

(B) with trace elements derived from their host minerals

(D) at an earlier period than were their host minerals（D）

(E) from the crystallization of rare igneous material

18. According to the passage, the age of silicate minerals included in diamonds can be determined due to a feature of the

(A) trace elements in the diamond hosts

(B) trace elements in the rock surrounding the diamonds

(D) silicate minerals’ crystal structure（C）

(E) host diamonds’ crystal structure

19. The author states that which of the following generally has a crystal structure similar to that of diamond?

(A) Lamproite