GWD5-Q23

cjyc1

 

      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.

 

GWD5-Q23:

The primary purpose of the passage is to

             

1. discuss an objection to Gurney’s theories about the uses of indicator minerals
2. explore the formation of diamonds and the reasons for their scarcity
3. analyze the importance of kimberlite pipes in the formation of diamonds
4. define the characteristics of indicator minerals under differing conditions
5. explain a method of determining whether kimberlite pipes are likely to contain diamonds

 

答案是E，我的答案是C

 

 

GWD5-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

 

答案是B，我的答案是C。

nickynicky

for Q25, diamond是在下面形成，然后通过kimberlite管道上升。并不是C所说，kimberlite管道经过钻石形成的地方。

23题只能自己体会了

 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.

       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.

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. 

caicaichang

25题我也选了C

G10 garnets，a type of garnet typically found in  diamond-rich pipes, are lower incalcium and higher in chrome than garnets from barren pipes.

文中只是说G10 garnets are lower incalcium and higher in chrome，而G10经常能在diamond-rich pipes里被发现，却并没有直接说lower incalcium and higher in chrome对钻石形成有益。只能说两者之间有关系。B项不正确。

yukiyingying

我的25题也同样错了，不知道为什么