Dear Qin,

Crystal faces and crystal forms can only develop in any rock type when the crystal growth is not disturbed by adjacent obstacles. In this case euhedral crystals or subhedral crystals can grow. If no cavities for unhindered crystal growth are available, then a dense grain structure forms with unhedral crystal grains in polygonal grain shapes (granoblastic rock microstructure).

The grain size in a coarse-grained rock salt can be reduced significantly during a foliation by recrystallization.

Best regards,

Guenter

Dear Qin,

I fully agree with the answer of Guenter Grundmann, and/additionaly in other words: the development of crystals depends on factors such as available growth space, temperature which controls element mobility, available time and many more.

If I understood you correctly, you're talking about a salt "zone" which shows evidence that had been playing a role of "lubricant" possibly of a decollement zone or during salt diapirism (both explain plasticity and flow appearance) then I would recommend you the following: destruction of the previous structures within the salt due to major movements, plus relatively elevated temperatures during this event, plus insufficient time(?)-temperature to a possible recrystallization to euhedral or subhedral  salt crystalls. The other concept you should consider is the size, for example you might find crystals at microscale under a microscope.  

Regards

Ioannis A.

Dear Qin,

Guenter and Ioannis have both given good explanations as to why you cannot find well formed salt crystals.  In fact well formed crystals are rare; the norm is for anhedral crystals. All minerals are crystalline (apart from a couple of amorphous ones e.g. opal), but, when we say that, it means that the atomic structure is crystalline.  What you are looking for is but the outward expression of the internal atomic structure of the mineral, i.e. its crystal lattice.  I have attached a file which goes well beyond what you need, but it does give a simple introduction to crystal formation.  Hope that it helps.

With best wishes

George S.

Dear Atul,

I take it that you are referring to the deformation occurring during regional metamorphism and that the cleavage or fracture refers to slaty cleavage or to the development of a foliation produced by planar minerals such as mica in producing a schist.  Generally one set of cleavage is produced by any one phase of deformation.  If you have a rock displaying two distinct sets of cleavages, then a second phase of deformation has followed.  The result could produce a foliation known as crenulation cleavage, where one set of cleavages crosses obliquely the first set.  I have attached a power point which includes a section on foliation which hopefully helps.

With best wishes

George S.

Qin,

Please ignore my second answer. I was thinking I was answering Atul's question.  Sorry about that.

George S.

 Dear Guenter,

Thank you very much for your answer.From your answer, I know the factor of "no cavities" will lead to a dense grain structure with unhedral crystal grains. Will the situation happen in the process of recrystallization or at the beginning of crystallization of salt ? Thanks very much !

With best wishes

Qin

Dear Ioannis A,

Thank you very much for your answer.  You understood very correctly. The study area has happened breather geological movement in Cenozoic. Some researchers think the rock salt (like glacier tongue tiling on the ground ) is extruded by geological movement. So I agree with your viewpoint of recrystallization of salt. But, how and what methods should I use to reconstruct the process of recrystallization of salt. Could you give me more suggestions ? Thanks very much.

With best wishes

Qin

Dear George S,

Thank you very much for your good answer. From your answer, I know the crystal lattice may effect the surface of crystal. If the process of recrystallization of salt(temperature, pressure and so on  ) changed the crystal lattice ? Thans very much.

With best wishes

Qin

Dear Kenneth M Towe,

Thank you very much for your suggestion. It`s a good angle of view to think my question. Could you give me more relevant information about "stress ellipsoid" to research the process of recrysallization of salt ? Thans very much.

With best wishes

Qin