What is this Rock?

Perhaps the most common question we receive at Ask-a-Geologist is: what is this? People find an unusual rock in their backyard and naturally want to know what it is. Often we get the query with a clear or blurry photo attached - sometimes the person will ship an entire rock to us! I even once received a weathered chunk of basalt that looked remarkably like part of a ferret's head. The following exchange is one of the more complete ones I have, and is useful in showing how a geologist goes about identifying a rock.

NOTE: Like all these short chapters, there will be words you may or may not recognize ("anchorite", "anhydrite", "limestone", etc.). These are part of what I call "Geologese". Like all professions, geologists have a 'local vocabulary' - like habeus corpus for lawyers - that allows them to unambiguously describe something with one or two words without having to repeat an entire paragraph every second sentence. To that end, this book will conclude with a small Glossary.

Q:

Hello.
I have a project that requires me to classify this specific kind of rock.
I have searched many websites for a couple hours but i am not able to 
find any information on it. I have enclosed a picture of it. If you can help
at all, It would really help me! Thank you so much!!!
Rachel B.

A:

There is really no way to identify a rock from a photo - a geologist needs to assess the physical properties and constituent minerals, and also know something about its context (where it came from, the neighboring rock units, etc.). I'd have to get my hands on it and look at individual grains with a hand-lens, scratch it with a knife blade to check its hardness, perhaps put a drop of acid on it, check it with a magnet, etc.

From what I can see I'd hazard a guess that your sample is a piece of a quartz vein -- quartz deposited in a fracture by a water-based solution over a long period of time. This is pretty common, especially in mineralized areas.


R:
It was silly of me to just send a picture. I have some more information
that might help give a better educational guess. From what I can tell,
it seems to have layers. The top is course grained, rough, and can easily
be scratched with a finger nail. The middle seems to have a crystal 
structure and is hard to scratch. This might be the quartz. The bottom is 
the same as the top: course grained, rough, and can easily be scratched. 
This rock doesn't seem to have much of any magnetism. It has a light 
color, so it is probably high in silica. Because it has layers, I'm guessing 
it might be a type of sedimentary rock (but I am not sure). Sadly, I don't 
know where it came from or what it is made of. Again, if you have any idea 
of what this mysterious rock is, it would help me. By the way, what is the 
acid used for? Thank you!!!

A:

The drop of acid would tell you if the symmetric sides around the lighter center are a carbonate rock or not (e.g., limestone). The scratch test tells you if the light-colored stuff is anhydrate or anchorite (if it scratches easily), or quartz (if it doesn't). The apparent symmetry I can sort of make out in your photo suggests to me that it is even more likely to be a vein - part of a filled-in fracture. Usually these are filled with quartz, but sometimes we see a mineral called anchorite that derives from high-CO2-saturated water. In Venezuela, California, and other places where gold is found, this is the mineral where most of the gold is hosted.

It's common to see one mineral bounding the other in these veins... e.g., the first fluids moving through the fracture deposit the anchorite on the fracture walls, and subsequent fluids from another source deposit the quartz in the center until it fills (and thus plugs shut) the fracture, effectively shutting down the hydrothermal system. The resultant filled fracture is called a "vein". What you likely have there is a fragment of a vein with most of the original host rock either broken off (or more likely) weathered away. That rugged surface on the top would then be sort of a "negative" imprint of the original fracture surface. 

These are pretty common in areas where there have been intrusions of granite or monzonite into an older rock. The intrusion process (a mass of magma coming up from below) causes the fracturing in the overlying rock, and the groundwater in this and the surrounding country rock is heated up and driven into hydrothermal cells - circulation powered by the heat of the intrusive magma. These hydrothermal cells leach out quartz and other minerals (including gold and copper) from surrounding rock, perhaps many kilometers distant. When the hot, saturated fluids reach the fracture, the temperature and pressure drop and the minerals precipitate out onto the fracture faces (from outside to in). This is a typical pattern we see in mining districts (defined by where there is enough gold or copper to be economic).

I'm only guessing here, as I haven't handled your rock personally, but this is a reasonable possibility if not a probability.

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