gwd6-23,25

盈儿

25,Vote for C

BTW,the answer for 25 is C in my GWD material

we can get the answer from below:

For example, G10 garnets,

a type of garnet typically found in

 (45)    diamond-rich pipes, are lower in

calcium and higher in chrome than

garnets from barren pipes.  Geo-

chemists John Gurney showed that

garnets with this composition were

 (50)    formed only in the diamond-stability

field;

kingdoms

以下是引用pactzy在2004-9-21 12:25:00的发言：

Q23 to Q26:

      Diamonds are almost impos-
                

       sible to detect directly because they

       are so rare:  very rich kimberlite

Line       pipes, the routes through which

  (5)      diamonds rise, may contain only

three carats of diamonds per ton

of kimberlite.  Kimberlite begins as

magma in Earth’s mantle (the layer

between the crust and the core).  As

(10)      the magma smashes through layers

of rock, it rips out debris, creating

a mix of liquid and solid material.

Some of the solid material it brings

up may come from a so-called

(15)      diamond-stability field, where condi-

tions of pressure and temperature

are conducive to the formation of

diamonds.  If diamonds are to sur-

vive, though, they must shoot toward

(20)      Earth’s surface quickly.  Otherwise,

       they revert to graphite or burn.

       Explorers seeking diamonds look

for specks of “indicator minerals”

peculiar to the mantle but carried up

(25)      in greater quantities than diamonds

and eroded out of kimberlite pipes

into the surrounding land.  The stan-

dard ones are garnets, chromites,

and ilmenites.  One can spend years

(30)      searching for indicators and tracing

them back to the pipes that are their

source; however, 90 percent of

       kimberlite pipes found this way are

barren of diamonds, and the rest

(35)    are usually too sparse to mine.

      In the 1970’s the process of

locating profitable pipes was refined

by focusing on the subtle differ-

ences between the chemical

(40)     signatures of indicator minerals

found in diamond-rich pipes as

opposed to those found in barren

pipes.  For example, G10 garnets,

a type of garnet typically found in

(45)      diamond-rich pipes, are lower in

calcium and higher in chrome than

garnets from barren pipes.  Geo-

chemists John Gurney showed that

garnets with this composition were

(50)      formed only in the diamond-stability

field; more commonly found ver-

sions came from elsewhere in the

mantle.  Gurney also found that

though ilmenites did not form in the

(55)      diamond-stability field, there was a

link useful for prospectors:  when

the iron in ilmenite was highly

oxidized, its source pipe rarely

contained any diamonds.  He rea-

(60)      soned that iron took on more or less

oxygen in response to conditions in

the kimberlitic magma itself—mainly

in response to heat and the avail-

able oxygen.  When iron became

(65)      highly oxidized, so did diamonds;

that is, they vaporized into carbon

dioxide.

Q25:

The passage suggests that the presence of G10 garnet in a kimberlite pipe indicates that

              

1. the pipe in which the garnet is found has a 90% chance of containing diamonds
2. the levels of calcium and chrome in the pipe are conducive to diamond formation
3. the pipe passed through a diamond-stability field and thus may contain diamonds
4. any diamonds the pipe contains would not have come from the diamond-stability field
5. the pipe’s temperature was so high that it oxidized any diamonds the pipe might have contained

我选B. 请看我在文中划出的重点，不知有没有道理，请NN们帮忙看看

我觉得C thus 有点怪怪的。不是因为the piple passed through a diamond-stability field， 才contain diamonds.

B不对, 因为文中涂黄的部分说明calcium and chrome 对寻找钻石有指示作用,特别是calcium ,修饰语是lower!,怎么会是conducive to diamond formation?

因此, B一定是错误的.

gmat30

In the 1970’s the process of

locating profitable pipes was refined

by focusing on the subtle differ-

ences between the chemical

(40) signatures of indicator minerals

found in diamond-rich pipes as

opposed to those found in barren

pipes. For example, G10 garnets,

a type of garnet typically found in

(45) diamond-rich pipes, are lower in

calcium and higher in chrome than

garnets from barren pipes.

explain why No.25 choose B

yangjh1111

我觉得25题没什么争的，G10的作用就是发现钻石，而the levels of calcium and chrome in the pipe 是区别G10与别的garnet的手段。

我的24题选了E，NN帮忙分析一下24题吧？

加拿大帅哥

I think that the answer of Q25 should be B. B is better than C.

先看文章 作者 mentioned the indicator and chemical signature before he introduced G10 as an example.

这个for example和前面的联系更大一些。

然后我们可以推断作者的意图是介绍indicator 如何 indicating diamond.

而且，diamond stability 是在后面才提到。

B. the levels of calcium and chrome in the pipe are conducive to diamond formation

C. the pipe passed through a diamond-stability field and thus may contain diamonds

高宝拉

以下是引用tianwan在2004-6-27 20:03:00的发言：

23, E is correct. The first paragraph states that the conventional method was not feasible, and in the second paragraph, the author introduce a feasible method of exploring diamond and explain this method. B is wrong at least from the fact that the article mentions nothing of the reason for the scarcity.

恍然大悟！！

dreamalittle

support C

phoenixfr

I voted for C

ghostzpy

25应该选C.

Some of the solid material it brings

up may come from a so-called

(15)      diamond-stability field, where
            condi-

tions of pressure and temperature

are conducive to the formation of

diamonds. 

钻石是在diamond stability-field里生成的。从全文来看，Pipes are the routes through which diamonds rise.Pipes不是钻石生成的地方。同样Garnet的指示性作用是因为 G10 garnets,

a type of garnet typically found in

(45) diamond-rich pipes, are lower in

calcium and higher in chrome than

garnets from barren pipes.Geo-

chemists John Gurney showed that

garnets with this composition were

(50)      formed only in the diamond-stability

field。

所以pipes中并不生成钻石，只是在通过field的时候将Garnet 10和钻石一同带出来。

wangwei8581

文章说的是 G10 are lower in calcium and higher in chrome thangarnets ，选项B 说的是pipe 有着 合适的calcium  和 chrome 的level， 根本就是概念偷换！！！！！！！！！！！！！！！

C！