अनुबाद - रूसी-अंग्रेजी - Эвлитин является одним из самых редких минералов...

Hi agent z

Someone really seems interested in crystallography!

Another great translation! I have made some minor changes below; please let me know whether you agree.

Eulytite is one of the rarest minerals in nature. It occurs as a single crystal*, an assembly** of crystals or maybe better: assemblies of crystals and radially fibrous spherulites***. Crystals usually have a tetrahedral appearance. Two sides are mainly developed, sometimes one, seldom 5. Crystal appearance of eulytite from the Shneeberg field.

*maybe better: single crystals
**Possible alternative: aggregates
*** I don't know anything about spherulites - in fact, I had never heard the word! But I think in geology, this is referred to as radially fibrous spherulites. See http://www.nps.gov/history/history/online_books/geology/publications/rpt/7-1/contents.htm for an image

In English, I think the second sentence would normally describe plural forms (crystals, aggregates etc) and not singulars. If you do prefer singulars, you will need an article (a crystal).
As for the last sentence, it looks like this may be a description or legend for an image. Do you think that might be the case? I have based my sentence on this assumption.

There is one sentence I don't understand. Do you know what is meant by this one?

Two sides are mainly developed, sometimes one, seldom 5.

The English looks fine, I just don't understand what it means - but maybe this will make perfect sense to a mineralogist!

No reply

I will edit as described above; please let me know if you don't agree! Thanks!

I believe that the meaning was that: Second side(s) is (are) mainly developed, sometimes first, rarely fifth.

Thanks Siberia, I think that makes sense.

Agent z, would you agree? In that case I'll change the sentence to

It is mainly the second side that is developed, sometimes the first, rarely the first.

Please let me know what you think and I can edit (or not) and accept

"crystal aggregate" is ok, please see the link below for proof
http://www.websters-online-dictionary.org/definitions/crystal+aggregate?cx=partner-pub-0939450753529744%3Av0qd01-tdlq&cof=FORID%3A9&ie=UTF-8&q=crystal+aggregate&sa=Search#921

As to "Преимущественно развиты грани 2, иногда 1, редко 5." I agree with Agent Z as it seems to me to be more logical. However, it is not realy clear from the context if it should be trasnlated as Cardinal numerals or Ordinal numerals.