Posted by: Rob Viens | June 1, 2012

Darwin’s Gneiss Eyes

June 1st and it was rapidly approaching the winter season in the southern hemisphere (though being close to the equator, I suspect it was still pretty nice in Rio). So Darwin went out to look at some rocks.

“Took a long ride, in order to geologize some of the surrounding hills.” (June 1)

Not surprisingly, Darwin probably saw a bunch of granite and gneiss on his trip.  In Brazil’s Gneiss Rocks I discussed a little bit about the general concepts regarding the metamorphic rock gneiss.  Today, I’ll add a few thoughts about the specific gneisses that Darwin was observing. Let’s start with his description:

“This whole district is almost exclusively formed of gneiss, abounding with garnets, and porphyritic with large crystals, even three and four inches in length, of orthoclase feldspar: in these crystals, mica and garnets are often enclosed. … The porphyritic gneiss, or gneiss-granite as it has been called by Humboldt, is only so far foliated that the constituent minerals are arranged with a certain degree of regularity, and may be said to have a ‘grain,’ but they are not separated into distinct folia or laminæ.” (Geologic Observations on South America)

Interestingly Wikipedia Commons had a nice picture of a gneiss from Rio showing more or less what Darwin is describing:

Augen Gneiss from Rio

This type of gneiss is what is often referred to as an augen gneiss. Augen, which means “eye” in German, refers to the large, elongated (often lenticular) mineral crystals in the rock.  In the case of the picture above, they appear to be orthoclase feldspar (exactly what Darwin suggested).

Even more wonderful is the image below from the Sedgwick Museum that shows one of Darwin’s actual samples collected in Rio (possible on this very day):

Augen Gneiss from Rio- collected by Darwin

How cool is that – I can picture him chipping it off the bedrock! In this one you can also see the reddish colored garnet augens along with the orthoclase.

If you recall, minerals in a metamorphic rock “grow” during metamorphism (with the grade of metamorphism determining what type of minerals you get).  Augens tend to form around a hard “core” (in metamorphese – called a “porphyroclast”) and grow outward – perpendicular to the direction of pressure. Since the center of the augen has had more time to grow, it tends to be thicker than the limbs, thereby forming the “eye-like” shape.

Garent augen up close (from hypocentral.com a blog by Ian Stimpson who took on the project of photographing a rock a day for a full year in 2010. Check out his Flickr page for the Rock365 Project – it is pretty awesome and quite beautiful!)

Garnet Augen

Much like the change in his focus later in life from geology to biology, Darwin eventually ends up back in the forest and has some more poetic words on the beauty of his surroundings. In this case, I blame it on the fact that he’s been seeing he same types of rocks for a few months and was getting a little sidetracked by all that greenery.  He concludes today with:

“After passing for some time through lanes shaded by hedges of Mimosas, I turned off into a track into the forest. — The woods even at this short distance from the city are as quiet & unfrequented as if a civilized man had never entered them. — The path wound up the hill: at the height of 5 or 6000 feet I enjoyed one of those splendid views, which may be met with on every side of Rio. — At this elevation the landscape has attained its most brilliant tint. — I do not know what epithet such scenery deserves: beautiful is much too tame every form, every colour is such a complete exaggeration of what one has ever beheld before. — If it may be so compared, it is like one of the gayest scenes in the Opera House or Theatre.” (June 1)

I have to wonder if this is the day that he wrote the following in his Geologic Diary:

“In the immediate environs of the city the geology is uninteresting. — The prevailing rock is gneiss, containing large crystals (even 3 or 4 inches long) of feldspar… & garnets.” (Geologic Diary – Rio)

Just wait, Charles, there are some great geologic surprises yet to come! (RJV)

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