One of the Most Important Things I’ve Learned

Bowen’s Reaction Series explains how minerals crystallize in cooling magma.  It is a diagram which explains why certain minerals are found together very frequently while other minerals are hardly ever found together (which is very helpful!).  There are two branches (the continuous branch and discontinuous branch) which meet in the middle and form a Y-shape.

On the continuous branch: there exists mafic calcium-rich plagioclase feldspar as well as intermediate sodium-rich plagioclase feldspar.  The calcium-rich plagioclase feldspar breaks down uninterrupted by chemical reactions.

On the discontinuous branch: there exists mafic olivine and pyroxene as well as intermediate amphibole (hornblende) and biotite.  The magma composition changes as chemical reactions occur between each location of this branch, thereby forming new minerals at each point.  Essentially, some magma precipitates out and forms the different minerals.

These two branches meet at the bottom where the felsic minerals potassium feldspar, muscovite mica, and quartz are formed.

What’s the big deal about being mafic, intermediate, and felsic?
Mafic: high in magnesium content, less viscous, darker in color
Intermediate: a mix or in-between point with both mafic and felsic rocks present
Felsic: high in silica content, more viscous, lighter in color

When someone is looking for different types of rock (or different minerals) the Bowen’s Reaction Series is a great way to lead the way with trying to find out the best places to look for specific rocks and minerals.  Granite, for example, is a very common felsic igneous rock which is prevalent throughout much of the mountains in North Carolina.  Quartz, mica, and feldspars are all very common compositional minerals for granite rocks.  Therefore, it makes sense that all of these minerals would be felsic because they are found in felsic granite.  This is more significant and practical when someone first decides “I am going to try to find some _________ in North Carolina” (or elsewhere) on a rock-hunting trip.  One of the best ways to find specific minerals (and the gemstones which can come from so many minerals) is to know where to be searching for these based on the Bowen’s Reaction Series.  It would not be a good idea to search for olivine (where peridot can come from) in granite, for example, because it is very unlikely to find this mafic mineral in felsic rock.  It is easy to do some research about the best types of rocks to find specific minerals in by reading online or in the plethora of mineralogy books available.

Higher temperature minerals and rocks (which are more mafic) are often the ones which weather and erode away the quickest.  This is because the surface of the earth is so much cooler now than when these minerals and rocks were forming.  Lower temperature minerals and rocks (which are more felsic) are the ones which we see more of today because they take longer to erode because of how the conditions at earth’s surface are similar to the conditions in which these rocks and minerals formed.  The earth’s crust is mostly felsic.  The earth’s mantle is mostly mafic.  There is so much more which could be said about this, but after I learned more about Bowen’s Reaction Series I definitely had a greater appreciation for igneous rocks.  Minerals are still my favorite (compared to rocks) because so many of them can be breathtakingly beautiful to me.  The majority of igneous rocks are varying shades of black and gray (which are less than awe-inspiring for me), but now I see the bigger picture of how these igneous rocks can point me toward my favorite minerals that I seem to constantly be looking for whenever I am outside exploring and hiking.

Oh Schist!

Schist is a metamorphic rock which consists of many layers of elongated minerals which exhibit foliation.

 

Yesterday a classmate Lisa (http://lisajaneboyer.wordpress.com) and I went on a hike with our geology professor through Crowders Mountain as a field trip lab.  It was unfortunate that not many of the students from our relatively small class could come yesterday, but the three of us had a complete blast!  This is one of my favorite hiking spots in North Carolina.  I was thrilled to go here with someone who really knows about the rocks and minerals found in this area because I knew Iwould be remembering this information to tell everyone else I would take hiking at Crowders Mountain (because that’s what I do–I tell people little bits of information so that I can try to spark an interest in them).  We had an absolutely wonderful day!

The majority of the rocks we examined were granite (igneous), mica schist (metamorphic), and quartzite enriched with kyanite (metamorphic). I spoke about the kyanite in Crowders Mountain before on at least one blog post, but I did not ever know where exactly to look for it whenever I hiked along the network of trails.  Thankfully, I know where to look now because of my professor’s explanation of how kyanite remains in metamorphic quartzite.  Quartz (SiO₂) is the most abundant mineral in the world.  It is definitely abundant in the soils of the piedmont as well.

Crowders Mountain rises about 800 feet from the surrounding piedmont and exists as a monadnock.  Essentially, monadnocks are features which remain because everything else around them eroded away over many, many years.  The sandstone, shale, and mudstone (sedimentary rocks) as well as granite (igneous rocks) are the protoliths of the metamorphic rocks which exist in the Kings Mountain Belt in this region of North Carolina and South Carolina.  This area is abundant in quartz, mica, and kyanite. 

Kyanite is actually quite easy to spot kyanite within the metamorphic quartzite rock.  It remains and kind of “sticks up” to the surface of these rocks because the kyanite is very resistant to weathering.  I was thrilled when Lisa helped me to find some kyanite crystals on the surface of a rock similar to this which actually exhibited their characteristic blue-gray color.  This photo was taken at the top of Crowders Mountain.  Smaller rocks which are still full of kyanite are very easy to spot on the trails leading up to the top of Crowders Mountain as well.

There is abundant evidence of weathering and erosion at the top of Crowders Mountain.  It is obvious why this area is so popular with climbers!

The views from the top of Crowders Mountain are of course beautiful, but the weathered rocks which remain at the surface are my real interest.  We were unfortunately unable to see the Charlotte skyline from the top yesterday because it was too hazy along the horizon.

This is a pretty large area of schist which was right along one of the trails on the way to the top of Crowders Mountain.  Not all of the trails are covered in gravel (I think gravel trails are annoying) within the network of trails.  This trail doubles as a service road for the cell phone tower toward the top of Crowders Mountain.  Don’t worry, the cell phone tower is not at the top of Crowders Mountain where one has the best scenic views of the piedmont and Charlotte.  The joke of the day was talking about any schist we saw (for obvious reasons).

So many gems, so little time

My spring break is coming up soon! I plan on spending at least a few days out getting some new minerals. You could have probably guessed that would be my plan. My friends and family know how excited I am about these trips and have asked me to give them smaller, less awesome versions of the minerals I find. Please check back during the first full week of March because I will have several posts about where I go, what I find, overall impressions, and helpful directions. Seriously, one of the hardest aspects of planning these mineral-hunting trips is finding out a proper address or at least a good parking location. My plans are to travel in the mountains with maybe one stop in the foothills or piedmont.

Garnets (A²⁺₃B³⁺₂Si₃O₁₂)
*A represents Ca, Fe²⁺, Mn, or Mg
*B represents Al, Cr, or Fe³⁺
I got a handful of nice garnets last year with my cousin Connor. I have a greater appreciation for garnets ever since I did some more research on a whim a month or two ago on my own time. I did not have an assignment to learn more about geology and mineralogy–I just wanted to learn more. Garnets come in almost any color. I knew about some of the green garnets such as demantoid garnet and became fascinated by them. Grossular, tsavorite, pyrope, almandine, and spessartine are several other types of garnet which are also common. Garnets are colored by calcium, iron, and manganese. Garnets have a hardness of 6.5-7.5, so they are very desirable for jewelry or as an abrasive in powdered form. Almandine garnets are very common in North Carolina’s garnet-mica schists. A schist is a metamorphic rock which can have many different types of minerals within it. The rock which undergoes metamorphism in order to produce a schist is normally a felsic igneous rock or shale. Felsic igneous rocks contain more than 75% of a felsic mineral such as plagioclase, orthoclase, or quartz. These felsic rocks are characterized by being abundant in silica.

Kyanite (Al₂SiO₅)
I became fascinated by kyanite when I went on a hike with one of my professors to Crowders Mountain in Gaston County, North Carolina. This mountain is a monadnock which remains today because the surrounding landscape eroded away from it. It is a beautiful area which I have visited in every season. Crowders Mountain exists in Crowders Mountain State Park which will protect it in the future. Originally, people wanted to mine the kyanite which Crowders Mountain is full of before it became protected. Kyanite is a blue-grey mineral which has a hardness of 5 or 7 depending on the way it is cut. It is recognizable because of its color as well as because of its elongated crystals which look similar to columns. It occurs in the metamorphic rocks schist and gneiss as well as in igneous pegmatites. I think it is interesting that kyanite is used very frequently in spark plugs. It seems like a waste of a very beautiful mineral. I would LOVE to find some new kyanite. I definitely will not be trying to get any of the kyanite from Crowders Mountain. My sights are set on somewhere much more remote near the border of Tennessee.

Corundum (Al₂O₃)
North Carolina’s rubies and sapphires are very famous. That is a good enough reason for me to want to get some. Corundum includes the aluminum oxides called sapphires and rubies. Sapphires are colored based on the trace amounts of elements which are in them including iron, copper, and magnesium. Chromium is the element which makes a ruby red. So, a ruby and sapphire are the basically same thing. Both make excellent gemstones for jewelry because they are very hard (basically only diamond and “fake-diamond” moissanite are harder). You may have seen clear corundum (lab created, of course) used for a watch face so that it would not get scratched. Corundum crystals are easy to spot because of their hexagonal structures.

Wish me luck! I am very excited to find some beautiful specimens. Maybe I will even find something I can have cut!

Granite and Quartz

Granite is one of the most common types of rock in this area, especially in the North Carolina mountains. Not many crystal specimens form within granite because granite’s structure often prevents outward crystals from forming. This is not always the case, however. Granite is an intrusive rock which forms from slowly cooling molten material within the earth’s crust. The most common minerals found in granite are quartz, micas, hornblende, and feldspar (both potassium feldspars and plagioclase feldspars). These minerals, in addition to smaller amounts of other minerals, are what give different colors to various types of granite. Micas are often responsible for the shiny bits which are so commonly seen in granite countertops, for example. All igneous rocks, including granite, form from melted or molten rock called magma. Granite is also an intrusive igneous rock which forms underneath the surface of the earth and also breaks through existing layers of rock. Many mountain chains such as the Appalachian Mountains are formed with a lot of granite in them.

I took this photo of this vein of quartz which is clearly visible from the trail of Table Rock within the Pisgah National Forest.

It is particuarly interesting to me that a lot of granite is radioactive because of how much uranium is found in it. It is more radioactive than other common rocks, but the structure of granite also contributes to how much radioactive material can actually escape and pose a threat to the environment and humans. According to the Environmental Protection Agency, the likelhiood that radioactive material which is found in granite would actually pose a threat to humans is actually not as high as we would think after hearing such a statistic. This uranium does not often escape because granite is not very porous. On the other hand, it is interesting that the majority of uranium ore deposits worldwide are attributed to granite. I never knew that before doing more research for writing this blog post.

These small pockets of quartz are also visible from the Table Rock trail located within the Pisgah National Forest in North Carolina. Linville Gorge, where Table Rock is located, is an absolutely breathtaking area which is fascinating to study from a geologic perspective. So many millions of years of erosion are visible in the gorge. I love hiking here and traveling here.

I discussed quartz (SiO₂) in my previous blog post North Carolina Beryl in some detail, but it is important to go over some more aspects which make it form so commonly in granite and other igneous rocks. The hardness of quartz is 7, and it often forms six-sided prisms which end in six-sided pyramids. There is indistinct cleavage with conchoidal fracture. Indistinct cleavage means that the mineral has cleavage which is hard to even notice because it barely even occurs. Conchoidal fracture means that the mineral is very brittle and breaks without defined planes of separation. Additionally, the melting point of quartz is near 1670-1700°C. All of these factors make it easier for quartz to form in granite because it is hard and very stable. Its low density of 2.66 g/cm³ also helps it to form in granite because granite needs to slowly form underneath the surface in a deep area. This may not make sense because a low density of quartz would make it seem as if it would not form in a very deep area within the earth, but actually the granite mountaintops people see while hiking (such as at Table Rock where I took these photos) were heavily eroded. The cleavage and fracture of quartz prevent it from breaking apart or dissolving when it forms in granite as well. The other constituting minerals (micas, hornblende, feldspars, etc.) of granite form before quartz does, thereby allowing quartz to fill the remaining spaces in the granite.