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分异完全的花岗伟晶岩内部常可分为哪些带?各带的矿物组合及结构构造有何特征?

答案

参考答案:

(1)边缘带:该带与围岩接触,带内矿物结晶不好,具典型的细粒结构(<1㎝),断续不连。主要由长石、石英、云母组成,有时可见少量的石榴石、电气石、绿柱石、偶见稀有元素矿物。其中金属矿物不具工业意义。(2)外侧带:矿物组成与边缘带基本相同。这一带内绿柱石(BE.,白云母常具工业价值,矿物主要呈粗粒和文象结构;(3)中间带:岩脉的主体部分,矿物成分复杂,除主要矿物之外,还含有大量金属矿物等有用组分,如稀有、稀土、分散元素矿物等、绿柱石、锂辉石,因此此带是稀有稀土分散元素矿化最为集中的部位,具较大的工业价值。呈粗粒结构、文象结构和块状构造。(4)内核:位于伟晶岩体的中心部位,常由结晶粗大的石英和长石组成,或仅由石英以及电气石、锂辉石等组成致密块状内核。有时亦可构成晶洞,可有质量较好的水晶产出。

此外,在许多伟晶岩矿区,常有更晚期的裂隙充填-交代型石英脉叠加在上述各带之上,而往往在这种具有充填-交代作用的脉中有更具工业价值的金属组分。

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(46)"Popular art" has a number of meanings, impossible to define with any precision, which range from folklore to junk, with poles being clear enough but the middle tending to blur. The Hollywood Western of the 1930’s, for example, has elements of folklore, but is closer to junk than to high art or folk art. There can be great trash, just as there is bad high art. The musicals of George Gershwin are great popular art, never aspiring to high art. Schubert and Brahms, however, used elements of popular music--folk themes--in works clearly intended as high art. The case of Verdi is a different one : he took a popular genre-- bourgeois melodrama set to music (an accurate definition of nineteenth-century opera)- and, without altering its fundamental nature, transmuted it into high art. (47) This remains one of the greatest achievements in music, and one that cannot be fully appreciated without recognizing the essential trashiness of the genre.

As an example of such a transmutation, consider what Verdi made of the typical political elements of nineteenth-century opera. (48) Generally in the plots of these operas, a hero or heroine--usually portrayed only as an individual, unrestrained by class--is caught between the immoral corruption of the aristocracy and the doctrinaire rigidity of the leaders of the civilians. Verdi transforms this naive and unlike formulation with music of extraordinary energy and rhythmic vitality, music more subtle than it seems at first hearing. There are scenes and arias that still sound like calls to arms and were clearly understood as such when they were first performed. Such pieces lend an immediacy to the otherwise veiled political message of these operas and call up feelings beyond those of the opera itself.

Or consider Verdi’ s treatment of character. (49) Before Verdi, there were rarely any characters at all in musical drama, only a series of situations which allowed the singers to express a series of emotional state. Any attempt to find coherent psychological portrayal in these operas is misplaced ingenuity. The only coherence was the singer’ s vocal technique: when the cast changed, new arias were almost always substituted, generally adapted from other operas. Verdi’ s characters, on the other hand, have genuine consistency and integrity, even if, in many cases, the consistency is that of pasteboard melodrama. The integrity of the character is achieved through the music: (50) once he had become established, Verdi did not rewrite his music for different singers or allow alterations or substitutions of somebody else’s arias in one of his operas, as every eighteenth-century composer had done. When he revised an opera, it was only for dramatic economy and effectiveness.

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