A chemical weathering product of granite.

Granite is an agglomeration of crystals of quartz, feldspar, mica, and amphibole formed when a body of magma cools underground. When granite is brought to the surface, it is weakest at the boundaries between these crystals. The infiltration of water, as well as chemical processes resulting from bringing two dissimilar materials into contact with each other, result in the slow dissolution of the granite at these boundaries.

From the dissolved minerals, the mica crystals already present in a mass of granite grow. In addition, crystals of other clay minerals form. These inexorably force the cores of the crystals apart, and the rock will disintegrate into a gravel of quartz and feldspar shards, packed in with mica and clay.

This gravelly substance is "grus", and can be found anywhere there is exposed granite. (Do not confuse grus with shards of granite formed from mechanical processes such as exfoliation).

Grus, also called The Crane (the symbol of astronomy in ancient Egypt), was first recorded in 1603 by the Dutch sailor Johann Bayer, making it one of the first animal constellations described. There was some early debate as to the name of the constellation; Philippus Caesius, a Dutch author, supposedly named it after the stork shortly after its discovery in his book Coelum Stellatum Christianum. He derived this name from a Biblical passage: All of them turn to their own course, like a horse plunging headlong into battle. Even the stork in the heavens knows its times; and the turtledove, swallow, and crane observe the time of their coming; but my people do not know the ordinance of the LORD. (Jeremiah 8:6b-7)

This constellation is visible primarily in the Southern Hemisphere, although it can also be seen from Japan. It is first perceptible in late August, and often fades by the end of November. Grus is fairly easy to identify; it is located in a fairly remote region of the evening sky below Piscis Austrinus and contains two relatively bright stars: Aldanabu and Alnair.

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