Unread postby Kerensky » Thu Jan 17, 2013 10:57 pm
Most of us know that if a transparent/translucent gem is placed in a bath of a liquid of the same RI, the gem will 'disappear', in the sense that its surface can no longer be seen and one has a clearer view of inclusions etc. contained within the stone. Most of those of us with microscopes will have used this technique at some time or other, some a little and some a lot.
What we find in practice is that, because the RI of our immersion fluid is closer to the RI of the gem under test than is the RI of air but is not the same as thre RI of the gem, whilst we see the surface of the gem less clearly we do, in fact, still see it (i.e. we see past the surface more effectively but can still see the surface). Like most people I had always accepted this without thinking too deeply about it. A recent couple of threads in another forum have called into question about how closely the RIs of fluid and gem should be matched and what use for crystal ID can be made of this phenomenon. This caused me to think through what goes on and to run some calculations to put fact in the place of opinion and guesstimation. The outcome of this was sufficiently illuminating (for me at least) to make it seem a good idea to port the info over here for the benefit of those who might be interested.
The last part of the discussion revolved around whether a of pure rock crystal will disappear in a bath of benzene (there is some stuff on the net that says that it will) and whether this property can be used to separate quartz from any other colourless species and also differentiate between a quartz crystal and silica class. The choice of substances is interesting since the birefringence of quartz is relatively small (0.009) and with an RI of 1.505, benzene's RI is smack-dab between the 1.553 - 1.546 RIs of Quartz. Here's what I found and wrote up:
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Quartz possesses not one but two refractive indices (in common with all anisotropic crystals that are uniaxial. These indices are separated by 0.009. Thus, no liquid (having only a single refractive index) can match perfectly to any anisotropic crystal.
A single crystal of quartz and some glass have identical RIs - except that a glass can only have a single refractive index. It follows that a specific glass may have an exact match to the RI of benzene, whereas quartz cannot quite do so, ever. I.e. a pure glass ot the right refractive index will *completely* disappear in benzene whereas a crystal of quartz cannot do so.
The critical issue is not actually one of refraction but of reflection, i.e. that part of a beam of light that is reflected back off a surface to the eye of an observer (or optical instrument) and not either absorbed in or transmitted through the mineral. Where there is a boundary between two substances of different RIs, a ray of light travelling through the medium of the lower RI will, arriving at that the boundary, always be partially reflected and partially refracted. The relative percentages of the light reflected and refracted are determined by the differences between the RIs of the two media and also by the angle of incidence of the light to the boundary. The percentage of light reflected added to the percentage refracted always equals 100% of the light in every case.
Out of interest, I ran a few calculations for benzene and quartz and came up with the following percentages of the light reflected according their RIs and the angle of incidence of the light:
89 deg - 76% reflected
45 deg - 0.06% reflected
1 deg - 0.03% reflected
Even in the 1 degree case, given good lighting and a high contrast background, the percentage of reflected should be sufficient to be detectable by the human eye. At 89deg (a grazing angle) it most certainly obvious,
Only if the match of the (single) RI's for the two mediums is perfect can there be no light reflected off the boundary at any angle of incidence - because, in an optical sense, there is no boundary condition to create any reflection.
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So, even if you go to extraordinary trouble to balance the RI of your fluid with that or your specimen (and keep them in balance through strict temperature control), only some transparent and colourless glasses and similar stones that have a cubic (isometric) crystal system can be made completely to disappear. So 'cloaking' technology is a bit harder than many of us may have thought
Re: Immersion microscopy
Unread postby mastersculptor » Fri Jan 18, 2013 2:05 am
Thank you for posting this memory jog Living in Guyana near to Brazil there are several types of gemstones that are considered worthless by the local gold and diamond dredge operators. Locals call one such stone "slim man" which is rummorred to be some a type of berryl. Unfortunately proper RI immersion fluids are not available here to inspect beryl rough with.
Beryl is supposed to have an RI of between 1.560 and 1.602. the closest supstitute I have found so far is Oil of Clove with an RI of 1.535. Olive oil helps for the Quartz's... but leaves a lot to be desired.......Does anyone know of a common substances obtainable in developing nations that could get me into the higher RI's including beryl all the way up to RI 2.12 for clarity inspections of "Heavies" to see if I want to get certain culls inspected.Re: Immersion microscopy
Unread postby Kerensky » Fri Jan 18, 2013 3:19 am
In practical terms there is no absolute fix, only some fixes that are more effective that others. Also liquids with high RI have health and safety issues associated with them, particularly when used about 16 cc at a time in open topped containers, warmed by a lighting unit and without a ventilation system.
There is a generally linear relationship between heavy liquid SG and RI. The benefits of using such liquids to establish the RI of a specimen are well outweighed by a combination of complexity/time in balancing the fluid to a specific specimen.cost and H&S risks. However, if you really want to go all the way, lay in a supply of Clerici's fluid (Thallium Malonate in aqueous solution). This is now unobtainable in the EU, US and some other places except by registered labs, because of its toxicity. As it dissolved in water, it is easily absorbed through the skin with minute doses causing irreversible nerve damage and with small doses being lethal (Trawl the net for its Material Safety Data Sheet)
For those wanting to check RI's higher than than the standard TIR refractometer can cope with, consider either the purchase of a good reflectometer (with all the limitations and vagaries of that technique) or modify a high quality microscope adapted to use de Chaulnes method to determine any RI of a transparent specimen. http://www.opticsinfobase.org/josa/abst ... -70-9-1160
Re: Immersion microscopy
Unread postby PinkDiamond » Fri Jan 18, 2013 4:45 am
The safest and easiest way to get the information you want requires no equipment or nasty smelling toxic chemicals. It's Visual Optics, The Hodgkinson Method, by Alan Hodgkinson, FGA. You should be able to find copies readily available, and if not, I suggest you contact Dr Bill Hanneman, since I got my copy from him, and if he has no more left, I'm sure he can tell you where to get one. You will find it very useful when you find stones and have no equipment with you.
Re: Immersion microscopy
Unread postby mastersculptor » Fri Jan 18, 2013 9:26 am
Thank U both for such useful responses...Pink... Thats, must have reading!!! found a tread about understanding it on GO....But I don't think I made myself understood on this one. I am wanting to inspect the interior of river worn rough that has an exterior rind like frosted glass you cant see through. Even the Diamonds get that way. it's not necesarily immersion microscopy but it is immersion magnification.
"REFRACTOL" has an R.I. of 1.567. (Good news is that it has no odor!) Since quartz (citrine) has an R.I. of 1.54 it works OK. Bad news tourmalines and diamond are higher plus IT'S HAZMAT so it is cost prohibitive to make the hazmat declaration to have it shipped here to Guyana ruling out all the others as well.
Oil of clove has an RI of 1.535 and oil of Wintergreen is 1.536 and both can be bought here but they do have an odor and I don't care I just need to be able to see past the outer Frosted Rind so I can see inside for inclusions, cracks and zoning.
I am looking for non hazmat common items or mixtures of common items to get up RI's of 1.6, 1.7, 1.8, 1.9, 2.12 (diamond)
Re: Immersion microscopy
Unread postby Kerensky » Fri Jan 18, 2013 10:52 am
As already set out, the *practicalities* are that you won't get perfect surface invisibility with any anisotropic stone and I doubt the perfect matching for isotropic crystals could be worth the trouble. The larger the difference in RIs of your two mediums, the wider the range of angle of incidence over which you will suffer some interference with interior examination because of surface reflections. Where the match of RIs is well off, you can to much to minimise the surface reflectivity by playing about with the position of your light source (try transmitted light rather then incident light?)
The reality is that even a water bath is markedly better than having air in contact with the stone (RI of 1.3 as opposed to 1.0. Use isopropanol (from your local chemist and cheap) and you can have 1.38. Monobromonapthalene will take you to 1.66, above which you run into the triple fanged monster of toxicity/other hazard, cost and colouration/viscosity. This might help..... http://www.chemnet.com/Global/Products/ ... s-0-0.html
If you want to examine inclusions critically, you really do need to use a microscope and various forms of lighting rather than a loupe. Remember that as the magnification goes up so your field of view and depth of field reduce. With my rig, the FOV is 23mm at x10. This reduces to 3.6 mm at x65. Using auxillary optics, I can reduce the FOV much further (and have higher magnification), reducing FOV to as little as 0.6mm at the system's limit. What this means, that from x65 upwards, I'm effectively looking through a 'keyhole' so small (and with so little depth of field) that avoiding trouble caused by surface reflections is not too hard anyway. There are more ways of skinning a cat.....
Re: Immersion microscopy
Unread postby mastersculptor » Sat Jan 19, 2013 7:40 am
Thank you very much,I find this not only useful but educational as well...... It's more for the Dredge operators than for me directly. They do an improv SG test to ascertain what comes out of the rain forests of the interior. All gold sales and lapidary operations are government owned so it's technically illegal for me to even teach myself to facet on gemstones here.
I had started writing to tell you why I am trying to do things this way but to do so could easily turn out to be something that I may or may not regret in putting certain information together.Like I said since the Government is in controll of the Gold and Diamond market here, owns and operates the lapidary, requires licensing to buy diamonds, and gov approval for export, just like they are trying to do in Ethiopia. But they even took it a step further and put a 45% customs duty on colored gemstone imports
What would appear to be a Lack of identification knowledge, cost of testing equipment, rugged conditions, and cost of extraction for local Guyanese dredging operations, is part of why the colored gemstone market is pretty much ignored here except by the smugglers market. Most Dredges are leased to Brazilian miners that know Diamond, tourmaline, baryls, aqumarine, morganite, quartz crystal, agate, and amethyst all end up in the rivers here..........yes, I would love to be a part of sourcing it but even a guyanese with piloted extraction is risky not to mention an American white man with or without an armed escort in the rain forest away from normal tourist destinations is asking for it! One would think its a modernized rain forest movie version of the old Yukon gold rush with a government stranglehold twist to it! 