Questions About Rocks and Minerals

The following is a series of questions from junior-high students that are pretty wide-ranging and all very interesting in their own rights. I've put my replies in <brackets> so the teacher could readily compare them with her questions more easily:

Hi,
I am a 7th grade science teacher at Sunset Vista. <Where is Sunset Vista? What state?> We are currently studying rocks and minerals and asked my students to write questions to a geologist. The following questions were submitted by my students.


 What is the best way to organize a rock collection? 
<There are lots of ways; I have several shelves of rocks in my office; one is a colorful mineral collection, and one is full of dull-looking rocks that nevertheless each tell a particular story - like structure, unusual origin, etc. I have a third shelf that is devoted to fossils. I use these for student visitors. If I were in charge of a museum, I would probably display rocks a bit differently. I would first classify them according to Igneous, Metamorphic, and then Sedimentary origins. Then I would break down each of these into their usual sub-classification sets, for instance rank the Igneous rocks from Gabbro to Granite (intrusive rocks) and Basalt to Rhyolite (extrusive rocks); rank the metamorphic rocks according to their degree of metamorphism (Schist to Gneiss), and rank the sediments according to source environment: shale, slate, sandstone, mudstone, and the degree of calcium carbonate in each. >

 How can you tell if a rock is from space?
<Meteorites and tectites generally have a fusion crust: they look like the outside surface has been burned and partially melted, typically with the edges of bubbles visible. This is a tricky thing to be certain of, and it's usually better for an expert to look at it even if you are pretty certain. I have tectites and nickel-iron meteorites in my office, but no stony or chondrite meteorites - but that's because I've published papers on the Wabar
impact site in Saudi Arabia and am particularly fascinated by nickel-irons which almost always penetrate the Earth's atmosphere and reach the ground. A problem here is that relatively few of the stony and chondrite meteorites survive our atmosphere; they usually ablate or burn up in the upper to middle atmosphere.>

 What is the most expensive type or rock? (I know financial questions cannot be asked but
I didn't know if there was a general response for this)
<Value is always a function of how rare something is, and to a lesser degree does it have an interesting story associated with it? The Hope Diamond is worth more because of all the intrigue and deaths associated with it, for instance. I'd probably rank gems as GENERALLY the most expensive, followed by semi-precious stones such as Jasper, Opal, Citrine, and Amethyst. I have seen unusually high prices for certified fragments of a meteorite, however. My wife has been involved in some of the Mars Society's arctic habitat simulations, and I once bought her a tiny fragment of a Nakhlite. These are named for where they were found: it is a certified object blasted from the Martian landscape by a skipping or glancing-blow asteroid impact, that fell subsequently into the Earth's gravity well millions of years later. It's identified as Martian in origin because of its unusual isotopic content. It cost over $100 for a fragment the size and thickness of your smallest fingernail. >

 What inspired you to become a geologist?
<I had earned a Masters Degree in solid-state physics when I realized I was trapped in a laboratory with radioactive samples for the rest of my life. I always liked getting out into the woods and deserts, and one day I was reading my brother-in-law's introductory geology book. I was amazed at the elegance of the idea of being able to pick up a rock and "read" in it a story of its formation. I switched fields almost overnight - it took 27 credits one semester and 30 credits the second semester to play "catch-up football" and get up to speed in the field. That sounds tough, but I already had a MS degree, my fellow students were 18-20-yr-olds, and some of those classes were pass-fail -- but I could do this also because I was so excited and turned on about geology. I'm now technically a geophysicist, someone who uses physics (measuring the strength of the local gravity pull or magnetic field, pulsing the ground with electromagnetic waves and measuring what comes back, etc.) to derive information from the subsurface. I find it really cool to be acting like a detective - from clues in my measurements, computer processing of data into derivative maps, to learn what lies underneath the earth. I currently work on using geophysical methods to learn what lies inside volcanoes, but tI have also worked to find where oil and gas and mineral resources like potash and iron and gold and copper are hidden. I've personally spent time doing all of these. Geology and geophysics are like being detectives... but with a pay-out at the end of your efforts. >

 What mineral is used the most in everyday life? 
<Probably quartz. It's the most abundant mineral in the Earth's crust (as opposed to the Earth as a whole, which would probably be iron because of its core), and quartz makes up the glass in your house, school, car, and hanging on your nose in front of your eyes. Next would probably be iron and aluminum, which are found in the earth in their oxide forms (rocks that look rusty or grungy) and need to be refined by heat into something useful. >

 How hot does a rock need to get to eventually become a metamorphic rock?
<It varies tremendously according to what the constituent minerals are, but it also depends on the overlying pressure. Some rocks could go to 1000 degrees centigrade and show no change at the Earth's surface, but deform plastically at lower temperatures when at great depths. There are so-called "refractory" minerals like Zircon that are used to line blast furnaces routinely maintained at 3,500 degrees. >

Thank you for your time. My students are excited to receive your response! 
<I hope your students sense how exciting the field of geology can be. I've walked the jungles of Venezuela and the deserts of Saudi Arabia, surveyed the paddy-fields of Thailand, mapped sulfide deposits in metamorphic rocks in Greece, and just about anywhere I am in the world I can reach down and pick up a rock and "read" a story from it. That's just incredibly cool. I have also gathered data from Russian scientific literature, did some volume and Monte Carlo calculations, and figured out how much unseen potash exists and can be mined beneath Uzbekistan and Turkmenistan. Other people I know can do similar kinds of studies and decide if it's safe to build a building or a bridge in a certain location, or figure out where pollution in a city's groundwater is coming from. It's a very practical field where you can feel like you are doing something good for your community and the world.

~~~~~