Sand grain inclusions: crystals within crystals
Like gemstones, sand grains often carry microscopic crystals within themselves. The sand grain at right is packed with them. In the course of examining sand grains for their surface features through a light microscope, in order to determine their history, one can often see the most beautiful crystals sealed away inside. Scanning electron microscopy is the preferred method for looking at surfaces, but a light microscope allows one to focus below the surface.
Sand grains begin as crystals that form when molten rock cools very slowly. Conditions are right for several different kinds of minerals to crystalize at the same time, and those that grow fastest may encompass others that are still small. They grow until all the magma or dissolved mineral is used up. The rock that forms is often a granite. When erosion eventually releases the crystals, they still carry within themselves these inclusions. In grains that are clear, like quartz and feldspar, the embedded crystals are visible.
Some crystals are elongated, and others rather oval, but in both cases with regular sides or facets. Both of these appear in the feldspar crystal at left. Other minerals form needle-like crystals, and these may be randomly oriented, all parallel, or in a particular cross pattern, as in the sand grain at right.
(Just above the center of the right-hand image is a series of small cresentic fractures running diagonally across the surface of the sand grain. These are chattermarks made by the glacier that brought this grain to the deposit of glacial till in which it lay. Even after 10,000 years, the surface is so unweathered that we may look right through it to the crystals within.)
Some inclusions are not crystals at all, but tiny pockets of gas or liquid trapped within the sand grain. These are a good deal more common that one might suppose. Sometimes there is a mineral grain included in the pocket, and then it constantly jiggles about in a manner characteristic of Brownian motion, proving that liquid lies within. Such an inclusion cell may be smaller than a human red blood cell, with the enclosed grain being even smaller. There is one in the upper corner of the image at left, among an array of oriented crystal needles.
The kind of inclusions that appear within sand grains are also found in what is left of the rock from which they came, and this allows geologists to trace sand back to its ultimate source.
Especially beautiful mineral crystals that form in a variety of different ways are considered gems, and while some inclusions detract from their brilliance, others enhance the gemstone’s appeal. But in sand grains, inclusions are simply a delight to the eye.
All photos taken by R.E. Lee from Ottawa-area samples. Last updated July 7, 2004. Coding revised July 20, 2013 and June 2016.