GRAIN SHAPE IN CRYSTALLINE ROCKS

Almost every mineral species has one or several characteristic crystal shapes which, if circumstances are right, it can develop. (Visit Crystal City to review.)
The 'right circumstances' involve conditions which permit growth to be unimpeded: the growing crystal is able to push surrounding materials out of the way. For example, crystals that grow in liquid or air and are able to complete their growth without bumping into other crystals or imovable objects are frequently able to retain their crystal shape. Those that are impeded may only achieve partial crystal shape or may lack any crystal shape.

Interestingly, some minerals are able to grow perfectly formed crystals within solid rocks. Evidently, as the crystals grow, atoms from the surrounding material migrate elsewhere and create room for the crystal.
Achieving crystal shape is not necessarily 'forever'. Once formed, crystals may have their shape corroded and destroyed by later events.
Observations made on the growth of salt crystals from an evaporating salt solution may provide information that is relevant to crystal shapes in nature.

In this experiment, a drop of concentrated salt water is placed on a microscope slide and is allowed to evaporate. The sequence of crystallization events that accompany evaporation may be viewed through a microscope. Salt forms cubic crystals which here are represented as squares or rectangles. The area putlined in red will be examined.

Stage 1: A row of small square crystals forms along the edge of the drop.

Stage 2: Rows of progressively larger square crystals form along the margins of the drop. In the interior of the drop, isolated square crystals begin to form.

Stage 3: Rows of progressively larger square crystals continue to form along the margins of the drop. The isolated square crystals in the interior increase in size.

Stage 4: The square crystals in the interior grow so large that they begin to impinge on one another.

Stage 5: Myriads of small crystals form in the spaces between the large interior crystals. Few of the small crystals are perfect squares. Most are parts of squares; some have no elements of 'squareness'.
Towards the end of crystallization, radiating clusters of long, needle-like crystals form rapidly along the exterior of the drop.

Stage 6: All the grains develop a corroded, worm-eaten appearance.

The shapes of the grains of salt that crystallized from the salt solution vary in several ways:

degree of geometric perfection: well-formed crystals; partially formed crystals; crystal shape absent.
crystal shape: square; needle-like.
corrosion: present; absent.

Certain tentative generalizations may be made:

well-formed crystals are likely to have formed earlier than less well-formed crystals.
interstitial grains (those in the spaces between larger grains) formed later than the grains to which they are interstitial.
crystals that form rapidly have a different shape than those that form more slowly: needle-like vs. equidimensional.
late-stage processes may corrode grains.

Conclusion: From this experimental investigation, it appears that grain shape may provide clues as to the sequence of events that take place during a particular episode of crystallization. It does not, however, address the question of distinguishing between crystalline materials that originate from a melt, a solution, or rearrangement of atoms within a solid. To address that question, more experimental or theoretical investigation would have to be undertaken.