Monday, June 26, 2017

Tourmaline-bearing granite

A granite is very distinctive looking if it contains "clusters, spots, clots, or patches" of jet-black tourmaline crystals surrounded by white feldspar crystals.  Such a white-colored granite is   leucocratic (i.e., dark-colored minerals absent or, in this case, concentrated).  Black tourmaline is called schorl, and it is black because of its iron content.  Tourmaline, which is a boron-silicate mineral, is commonly found in pegmatites.  In my previous post, I discussed that pegmatites are associated with the late stages of the cooling history of granite-producing magmas.

A single large crystal (4.75 cm tall = 1.87 in.) of schorl is shown in the following image.  The overall shape of the crystal is triangular  and has striations on all of its sides. The provenance of this crystal is unknown.

Three small boulders (all about 13 inches maximum length) of tourmaline granite are shown below.  A 3/4 of an inch in diameter penny (United States) is used for scale. The "clusters" of tourmaline can be as large as 8 cm across.  The provenance (original location) of this granite is not known to me, but the boulders occurred as rock debris emanating from a man-made dam built in a stream bed in northern Los Angeles County.  I was not sure about the identification of the black mineral in these rocks, so I asked my friend and colleague, Dr. Larry Collins, who is a professor emeritus of geology to take a look at the mineral. He is an expert in mineralogy and petrology, and he recognized the mineral as tourmaline.

In this image, the tourmaline crystals are more spread out, with feldspar and quartz in between. 

This image is a closeup showing a divergent fibrous aggregate of acicular (needle-like) tourmaline crystals, which are concentrated in the upper half of the image. The tourmaline in the lower half of the image is blocky. The entire field of view is about 1 cm in height.

The image below shows a small of piece of a tourmaline-bearing granite (5.5 cm width) from a pegmatite at the Stewart Mine near Pala, San Diego County, Southern California. These crystals of schorl are somewhat massive (structureless).

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