Sunday, October 2, 2011

Piles of Sand

Am I doing any work or just exploring the city? What have I been up to in the lab?  I'm so glad you asked, and I have an answer for you this week.  Prepare yourself- this is a (lengthy) science update post!

I am now officially one month into my time here in Boston.  Besides adjusting to life as a grad. student, life in a new city, and my two classes for the semester (geochemistry and a mathematical modeling course), I have plenty of ambitious research goals for the semester.  In fact, I have at least four projects that I am supposed to be starting or working on at some point in the next couple months.  I have a feeling you'll hear about them over the coming months..or years.

But before I can produce large amounts of relevant data in the lab, I am currently in that joyous stage of being the newbie, the trainee, the person who has no idea how to do most of the routine procedures in my new lab and has to ask someone for help every time I come up against the next step.  Clearly there are some benefits to that situation (like being limited in what labs I have been trained in so therefore feeling no guilt at not spending every waking moment in the lab or the constant learning of new and exciting things) as well as drawbacks (like not being even remotely self-sufficient and making slow research progress).  I have been assured by my adviser that it could take me a good year to two years until I can confidently handle any and all of the research tasks expected of me- so I suppose I am doing well for a month in. It's all about perspective.

That being said, I am happy to share with you what I have accomplished this month in the lab.  Really, it all centers on making piles of sand.

Glen Clova, Scotland
The first of my four projects involves sand collected from a river flowing through Glen Clova, Scotland.  Glen Clova is a famous glacially-carved, U-shaped valley and is part of the type locality for the Barrovian metamorphic sequence.  I visited in June and collected the sample, along with my adviser and field assistant.  (If you missed my post on summer fieldwork and want to see more details on my trip to Scotland or pictures, check it out here.)  The current goal for the sample is to separate out the mineral garnet and attempt to establish an age for individual garnet grains.  This sample will be a part of the method development for my dissertation.  My overall PhD project is focused on developing a system for dating detrital (eroded and then redeposited elsewhere) garnets, which has essentially never been done and would open a whole new tool for geochronology.

The first step is taking my bucket of sand and separating out the grains of garnet from the overwhelming amounts of quartz, feldspar, and other minerals present.  While you could theoretically just sit there with tweezers and hunt through the bucket, that would be terrible and not very efficient.  Therefore, we can use properties of the minerals to separate them into different "piles."  Many geologists (and most prospectors) have gone through a similar series of steps as a part of the basic sample preparation that is my current lab activity.

The first step is to send the sand through sieves with different size mesh screens.  This divides the grains into piles with similar grain sizes.  This is important not only for later mineral separation steps, but allows me to gain an idea of the grain size distribution for garnet in this sample.  Since I am developing a new method that uses a single grain of garnet at a time, I would ideally like to find the largest grains that exist to start with, before working down to smaller grains.  Also, since my samples have much larger grains than what my lab is used to working with (finely crushed "pure" garnets), I got to determine that we need some sieves with larger mesh sizes and order them.  Thus, my first contribution to making my lab more versatile! Hooray!

Once the grains are separated by size, I have been using a Frantz electromagnetic separator to divide the grains based on magnetic susceptibility.  Most mineral grains are magnetic to at least a small degree, especially if you use a big enough electromagnet and set it appropriately.  The differences in magnetic susceptibility between minerals of different types allow sand to be separated into piles based on the degree of magnetism.
The Frantz I've been using the past couple weeks
Here's how it works:

  1. Clean the machine thoroughly.  (You definitely don't want grains from the previous user's sample contaminating your sample!)
  2. Turn on the magnet and set the desired level of magnetism.  Since I'm aiming for garnet, I usually start with a high setting to eliminate the least magnetic minerals and work my way towards gradually eliminating more magnetic minerals.  People who want to separate out other minerals may approach it differently.
  3. Load whatever sample you wish to separate into the glass container that will feed the grains into the machine.  Usually you want to use a hand magnet on this sample before you load it, because that will remove one mineral (magnetite) that is the most magnetic and keep very magnetic grains from sticking to the magnet.
  4. You can control how quickly the glass feeder container and a tray between the two portions of the electromagnet vibrate to control how quickly grains move through the magnet.  Grains fall out of the feeder (ideally basically one grain at a time so they don't influence one another) and move down a tray between the electromagnet.  The tray is tilted so that by gravity, grains prefer to move down the side away from the operator.  However, grains that are more magnetic than the magnet's current setting will be pulled up to the portion of the track closer to the operator.  The grains are then separated into two containers at the other end- the closer one being more magnetic and the one further away less magnetic.
  5. Since I want garnet I continually rerun the pile collected in the container closest to me at a lower magnetic setting so that it is separated into two new piles.  This continues until I have one or two piles that have a larger proportion of garnet than any of the other piles.
Some people use heavy liquids or water tables to separate minerals by density and to reduce the amount of time required and increase the efficiency of the Frantz.  We have been discussing having me implement these strategies in our lab eventually, especially since I may have to separate relatively large amounts of sand.  We shall see what the future holds.

I also got to make another contribution to improving my lab's efficiency by constructing a "backboard" to keep my grains from bouncing out of the Frantz as they dropped out of the feeder.  Evidently the combination of larger grain size plus an old Frantz model leads to quite a few grains trying to jump ship before they head down the tray- regardless of how high or low the vibration of the feeder and tray is set.  I improvised a shield from weighing paper and label stickers- nothing like scientific McGuyvering.

That pretty much sums up month one in the lab.  So far it has been basic sample processing (all of which I actually did once upon a time as an undergrad. for a term project) but at least it's progress!  I'll share the next step for these piles of sand in an upcoming post.

1 comment:

  1. I like this Frantz machine. You should sell the magnetite grains to Magna-Doodle if you aren't using them! ...Make some money on the side that way...