GWD6-24 没人问过

Q23 to Q26:

      Diamonds are almost impos-

            sible to detect directly because they

            are so rare:  very rich kimberlitic

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.

--------------------------------------------------------------------------------

Q23:

The primary purpose of the passage is to

chemists John Gurney showed that

garnets with this composition were

(50)     formed only in the diamond-stability

field

E如何定位啊？？？求救ING

谢谢啦！

Each of the following is mentioned in the passage as a difference between G10 garnet and other versions of
garnet EXCEPT

答案A主要錯在 level
     of oxidation超出題目定位點,是下文討論另一種indicator minerals ilmenite時用的句子

如同上面朋友提到的,我也認為這幾句話是關鍵,它們在討論不同點(BCDE都有提到)

E是說是否和DIAMOND一起出現,個人淺見請指教

G10 garnets,

a type of garnet typically found in

diamond-rich pipes, are lower in

calcium and higher in chrome than

garnets from
barren pipes.

Geochemists John Gurney
showed that

garnets with this composition were

formed only in the diamond-stability

field; more commonly found ver-

sions came from elsewhere in the

mantle.

Appearance应当理解为“出现”的意思，这样E项就没问题了，对应“Geo-chemists John Gurney showed that garnets with this composition were  formed only in the diamond-stability field; ”。

不好意思，E项也对应的应该“G10 garnets, a type of garnet typically found in diamond-rich pipes”