the joys of panning for zircon
Oct. 3rd, 2008 06:59 pm![[personal profile]](https://www.dreamwidth.org/img/silk/identity/user.png){kind=small}
kareinaToday I was introduced to the joys of separating zircon crystals from their host rock so that they can be analysed with the Laser-ICPMS to work out how old they are.
I accomplished the first two easiest steps last week
(1) crush rock into small bits
(2) put small bits into the ring-mill for five seconds to make sand
This morning I was taught how to do the next steps. The next few were also reasonably easy:
(3) put sand into a plastic pan with ridges on one side, add water, shake, pour off muddy water. Repeat as many times as needed to separate the mud from the sand and wash the mud away.
(4) add more water, using a back and forth motion, start washing out the lighter weight bits of sand. The zircons, being heavy for their size, sink to the bottom of the pan with a few other heavy minerals. Repeat this step till most of the sand has been washed away, being careful not to disturb the area at the bottom of the pan where the heavy stuff is collecting.
(5) now that there is just a little (~ 2 tablespoons worth) of sand left (of the ~2 cups of starting material) carefully submerge the pan into some clear water and tilt and shake to get most of the rest of the large, light weight grains to separate out from the tiny heavy grains.
(6) carefully pour the tiny bit of remaining sand into the tiny glass plate (~ 5 cm wide) and use the same sort of panning technique as you've been using, only scaled down, to make the heavy grains sink to the bottom, and wash away the last of the light weight grains.
(7) check under a microscope--yup, some of the remaining grains are zircon! You can tell because of the bright colours it has due to birefringence under the crossed polar lenses of the microscope.
(8) set the glass plate into the oven to dry for half an hour
(9) carefully, with a very, very clean brush, put the remaining sand (about 1/4 teaspoon worth) onto a sheet of paper. Set another sheet of paper around the outside of a magnet, and pass it over the sand at a distance of ~1.5 cm to draw off anything which is strongly magnetic
(10) remove the paper from the magnet, set aside the grains thus collected, and put clean paper on the magnet. This time pass it over the sand just above the grains, to draw off the moderately magnetic stuff.
(11) remove the paper from the magnet, set aside the grains thus collected, and put clean paper on the magnet. This time actually touch the sand with it, to draw off everything that is weakly magnetic. This leaves you with a *very* tiny pile of very tiny grains of zircon and anything else in your sample which is small enough, heavy enough, and non-magnetic enough to have survived the cleaning ritual.
(12) prepare the zircon mount by cleaning two glass plates. Put a sheet of special double-sided tape onto one of them, remove the cover-paper, then set the ring-mould on that and trace a circle onto the tape, and add lines bisecting the circle at right angles to one another.
Now the truly difficult part begins
(13) Carefully sprinkle a small amount of the zircon containing sand onto the plain glass plate and look at it under the microscope set up with crossed polars so that they zircons can be distinguished from the non-zircons (I did find an amazinglly pretty picture of a zircon showing the bright colours it gets in cross-polarized light. This contrasts with this picture (page down) of a bunch of zircons in plain polarized light). The non-zircons in this sand are either dark, or white in crossed polarized light.
(14) using a clean, specially prepared paint brush, which has most of its bristles cut off to tiny nubs, and one single hair whcih sticks out 3 or 4 mm, carefully pick up one zircon at a time from the clean glass plate and transfer it to the center of the tape-covered tape.
That *sounds* ever so much easier than it is! First of all, the microscope with which we are looking at the zircons on the clean glass plate is a normal one, which means what you see through the scope is upside down and backwards. It is also needful to use the 20-times magnification lens to be able to see the zircons, they are so tiny. This makes it difficult to even get that single hair into the field of view in the first place, and it is ever so much harder to actually move it the direction it needs to go to touch the zircon grain once you've found it. In addition, the thing which makes the part about "pick up" the crystal possible is either static electricity (rub the brush against your hair for a bit) or saliva (lick the brush)--either of which is a limited-time-only phenomena--if you take too long getting that hair into place, the grain isn't going to stick to it!
I soon found out that since I am not practice at this skill I had a choice. I could *either* touch the zircon at which I was aiming, *or* I could pick up one (or often, many) grains with the hair. I could not manage both at once! So I contented myself with aiming at specific zircons, picking up grains of *something*, and transferring them over to the tape-covered glass plate, and trying again.
When J, who was teaching me this technique does this, she winds up with nice, evenly spaced rows of zircons and nothing but zircons on her tape. When I did it I wound up with clumps of a variety of grain types, all of which look very much the same in the plain light of the microscope under which the tape covered glass sits, so we don't even know if I'm getting enough zircons!
Once we finish the mount and take them to the laser to analyze them, it will be easy to tell the zircons from the non-zircons. However, it is necessary to analyze at least 60 different grains of zircon, therefore it is usual to prepare a mount of at least 80 different grains, just to be certain that you've got enough with good results. Since I don't know how many of these are zircon, I need to do even more.
Eventually, about six hours after I started work this morning, I gave up, set a cover over the zircon crystals I'd managed to collect thus far, carefully packed up the remaining zircon etc. sand, and went home for the day. Next week, I'll return to it, and see if, perhaps, I find the task of touching what I'm aiming for under a microscope any easier.
Despite being worn out from that, I managed to make the effort to do some grocery shopping on the way home, then spent a pleasant hour reading fiction, and another couple on line (including writing this). I think I've recovered enough from my ordeal that I can get to work on incorporating more of my advisor's edits/comments into the thesis...
I accomplished the first two easiest steps last week
(1) crush rock into small bits
(2) put small bits into the ring-mill for five seconds to make sand
This morning I was taught how to do the next steps. The next few were also reasonably easy:
(3) put sand into a plastic pan with ridges on one side, add water, shake, pour off muddy water. Repeat as many times as needed to separate the mud from the sand and wash the mud away.
(4) add more water, using a back and forth motion, start washing out the lighter weight bits of sand. The zircons, being heavy for their size, sink to the bottom of the pan with a few other heavy minerals. Repeat this step till most of the sand has been washed away, being careful not to disturb the area at the bottom of the pan where the heavy stuff is collecting.
(5) now that there is just a little (~ 2 tablespoons worth) of sand left (of the ~2 cups of starting material) carefully submerge the pan into some clear water and tilt and shake to get most of the rest of the large, light weight grains to separate out from the tiny heavy grains.
(6) carefully pour the tiny bit of remaining sand into the tiny glass plate (~ 5 cm wide) and use the same sort of panning technique as you've been using, only scaled down, to make the heavy grains sink to the bottom, and wash away the last of the light weight grains.
(7) check under a microscope--yup, some of the remaining grains are zircon! You can tell because of the bright colours it has due to birefringence under the crossed polar lenses of the microscope.
(8) set the glass plate into the oven to dry for half an hour
(9) carefully, with a very, very clean brush, put the remaining sand (about 1/4 teaspoon worth) onto a sheet of paper. Set another sheet of paper around the outside of a magnet, and pass it over the sand at a distance of ~1.5 cm to draw off anything which is strongly magnetic
(10) remove the paper from the magnet, set aside the grains thus collected, and put clean paper on the magnet. This time pass it over the sand just above the grains, to draw off the moderately magnetic stuff.
(11) remove the paper from the magnet, set aside the grains thus collected, and put clean paper on the magnet. This time actually touch the sand with it, to draw off everything that is weakly magnetic. This leaves you with a *very* tiny pile of very tiny grains of zircon and anything else in your sample which is small enough, heavy enough, and non-magnetic enough to have survived the cleaning ritual.
(12) prepare the zircon mount by cleaning two glass plates. Put a sheet of special double-sided tape onto one of them, remove the cover-paper, then set the ring-mould on that and trace a circle onto the tape, and add lines bisecting the circle at right angles to one another.
Now the truly difficult part begins
(13) Carefully sprinkle a small amount of the zircon containing sand onto the plain glass plate and look at it under the microscope set up with crossed polars so that they zircons can be distinguished from the non-zircons (I did find an amazinglly pretty picture of a zircon showing the bright colours it gets in cross-polarized light. This contrasts with this picture (page down) of a bunch of zircons in plain polarized light). The non-zircons in this sand are either dark, or white in crossed polarized light.
(14) using a clean, specially prepared paint brush, which has most of its bristles cut off to tiny nubs, and one single hair whcih sticks out 3 or 4 mm, carefully pick up one zircon at a time from the clean glass plate and transfer it to the center of the tape-covered tape.
That *sounds* ever so much easier than it is! First of all, the microscope with which we are looking at the zircons on the clean glass plate is a normal one, which means what you see through the scope is upside down and backwards. It is also needful to use the 20-times magnification lens to be able to see the zircons, they are so tiny. This makes it difficult to even get that single hair into the field of view in the first place, and it is ever so much harder to actually move it the direction it needs to go to touch the zircon grain once you've found it. In addition, the thing which makes the part about "pick up" the crystal possible is either static electricity (rub the brush against your hair for a bit) or saliva (lick the brush)--either of which is a limited-time-only phenomena--if you take too long getting that hair into place, the grain isn't going to stick to it!
I soon found out that since I am not practice at this skill I had a choice. I could *either* touch the zircon at which I was aiming, *or* I could pick up one (or often, many) grains with the hair. I could not manage both at once! So I contented myself with aiming at specific zircons, picking up grains of *something*, and transferring them over to the tape-covered glass plate, and trying again.
When J, who was teaching me this technique does this, she winds up with nice, evenly spaced rows of zircons and nothing but zircons on her tape. When I did it I wound up with clumps of a variety of grain types, all of which look very much the same in the plain light of the microscope under which the tape covered glass sits, so we don't even know if I'm getting enough zircons!
Once we finish the mount and take them to the laser to analyze them, it will be easy to tell the zircons from the non-zircons. However, it is necessary to analyze at least 60 different grains of zircon, therefore it is usual to prepare a mount of at least 80 different grains, just to be certain that you've got enough with good results. Since I don't know how many of these are zircon, I need to do even more.
Eventually, about six hours after I started work this morning, I gave up, set a cover over the zircon crystals I'd managed to collect thus far, carefully packed up the remaining zircon etc. sand, and went home for the day. Next week, I'll return to it, and see if, perhaps, I find the task of touching what I'm aiming for under a microscope any easier.
Despite being worn out from that, I managed to make the effort to do some grocery shopping on the way home, then spent a pleasant hour reading fiction, and another couple on line (including writing this). I think I've recovered enough from my ordeal that I can get to work on incorporating more of my advisor's edits/comments into the thesis...