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Identity Helpspecific gravity
27th Jan 2011 14:29 UTCGord Howe
Thanks
Gord
27th Jan 2011 14:46 UTCReiner Mielke Expert
27th Jan 2011 17:38 UTCStephen Johnson
First measure the weight (mass) of the sample and write the number down.
Second, place the container for water on the scale and zero it out. At this point you're going to dip your sample - I've used dental floss in the past to hold the specimen - but have since worked up a little thin aluminum plate with string (dental floss again). I usually dip the plate in the container as well and re-zero. I then place the the sample on plate and dip it in the container and take my second reading and write the number down.
At this point it is simple math. Take your first value and divide it by the second. This will give you a S.G. value that is close enough for most of what you need to for identification...along with other characteristics like hardness, cleavage, crystal system, etc. This is a simple and quick way to get at a S.G. for most identifications.
If you need to get more than two significant digits, then go ahead and knock yourself out with a more elaborate and expensive system.
27th Jan 2011 18:35 UTCGord Howe
David, that is the article I've been hunting for. Reiner, I will try and find this book, not sure if its still in print. Good info Steve, I know I'll be refering back to it once I get started.
Thanks again
Gord
27th Jan 2011 18:52 UTCKelly Nash 🌟 Expert
The method was posted on his (John Betts Fine Minerals) website homepage on 6/9/09, and is still there in his archive. (I'm taking the liberty of pasting it below):
"I have developed a simple technique for determining specific gravity using a digital kitchen scale:
1. Turn on power and the scale should "zero" itself.
2. Weigh the specimen and record the weight.
3. Find a container large enough for the specimen and fill with water with room enough to submerge the specimen.
4. Place the water-filled container on the scale and "zero" it out.
5. Suspend the specimen from a wire or paperclip in the water, but not touching the bottom or sides.
6. Record the weight reading.
7. Divide the weight in #6 into the weight in #2 and you get specific gravity.
Do not ask me how or why it works. The explanation will only confuse you. All you need to know is that it works, it is fast, it is easy and does not require any lab. equipment. This technique is accurate to the first decimal place. And that will narrow down the possible mineral species of an unknown specimen making it fairly easy to get an accurate identification in your own home without spending money for testing."
27th Jan 2011 20:03 UTCPeter Haas
27th Jan 2011 20:04 UTCDavid Von Bargen Manager
27th Jan 2011 20:17 UTCAlysson Rowan Expert
The Jolly Balance, for example, if the spring is carefully matched to the sample size, is capable of a precision of the order of better than 0.1% - and that is just using scrap materials and common workshop tools. At that level of precision, the difference between tap water and distilled water will be measurable.
Please remember that the article is a work in progress, and is still not actually complete (yet).
27th Jan 2011 21:10 UTCKelly Nash 🌟 Expert
27th Jan 2011 21:27 UTCPeter Haas
27th Jan 2011 21:43 UTCAlysson Rowan Expert
Of course, one must always state the worst-case figures for the uncertainty of a reading. By making your corrections and by using a standard sample (usually a glass bead of known SG) you can reduce the errors introduced by using impure water by a simple correction.
28th Jan 2011 00:02 UTCKelly Nash 🌟 Expert
28th Jan 2011 00:30 UTCEugene & Sharon Cisneros Expert
Gene
28th Jan 2011 00:33 UTCGord Howe
Thanks Again
Gord
28th Jan 2011 03:02 UTCEugene & Sharon Cisneros Expert
Gene
28th Jan 2011 22:23 UTCAlysson Rowan Expert
Other factors that can affect results to a greater or lesser degree include the amount of air entrained in the fluid ans how well you ensure that there is no air trapped on the surface of the sample. Size, shape and surface texture of sample fragments can have an effect - and if air is trapped on the suface of the fragments (poor wetting) it is possible for the experimental error (correctly known as experimental uncertainty) to exceed the results obtained - rendering your effort useless.
I have seen the SG of lead (dust shot) appear to be only marginally above that of water.
Good technique and consistant methodology counts for an awful lot when performing any scientific measurements.
29th Jan 2011 18:01 UTCDana Morong
31st Jan 2011 15:51 UTCDonald Peck
Alysonn, I haven't seen your paper yet, I need to do that. Jolly balances are great, but the springs are so dog-gonned delicate; and I am fumble fingered.
Peter, I learned that "accurate" is the real value (not entirely measurable as errors get included) and precise is how close do repeated measurements group around a point. A measurement can be wildy inaccurate but be very precise. Of course accurate and precise is to be desired. Am I mistaken?
1st Feb 2011 12:37 UTCAlysson Rowan Expert
Coil some 5 or 10 pound monofilament fishing line around a small resin or wooden former, allowing a small gap between turns. Holding the coil taut (I use a wooden former with holes drilled to maintain the tension, heat the monofilament carefully in the live steam close to the spout of a boiling kettle (don't use a whistling kettle unless you really enjoy the noise!).
Once the monofilament cools and has been run under cold water for a few seconds, you can remove the former and you should find yourself with a robust spring.
Using different lengths and weights of monofilament will give you a good range of springs. These springs will return to size even after being almost stretched straight.
21/2 pound fishing line is probably the lightest monofilament you will find, heavy duty strimmer (weed whacker) line the heaviest. Outside of this you will need to use your ingenuity!
For the sand-grain sized specimens, a curled horse-hair will give good results (but don't let it get wet!)
For multi-tonne specimens, steel hawser would work. (I have seen one of these used with 4mm steel cable for obtaining the net density of rocks and soils for soil-mechanics studies - a very serious piece of laboratory equipment!)
... ... ... and yes, both accuracy and reproducability are extremely desirable. An accuracy to +/- 0.005 g/g(w) relative to water is an ideal target as most mineral S.Gs are quoted at X.XX, i.e. 0.01 g/g(w).
Precision (as in precision engineered), by the way, relates specifically to the systematic reproducability of results when consistent experimental technique is used. (i.e. The equipment produces consistent results which are only affected by differences in the way the equipment is handled?) Reproducability relates to the consistency of your results when actually repeating the measurements.
3rd Feb 2011 17:26 UTCTREILLARD Michel
For Mindat'rs yourselfers, I propose a simple installation of hydrostatic balance. Good luck.
First the ingredients: - a support arm with weighted (eg, old desk lamp)
- A precision balance (around 30 euros)
- A container door set at the end of the arm
- Support weighing up a ring of PVC, with one side a pedestal to be placed on the balance pan, and, unlike a stem for receiving the sample. (I used a welding rod is brass and an old electric socket, which are welded to tin) because the device must not be magnetic ... logical.
We put the bracket on the weighing scale platform, we tare to zero, then we put the sample in the old electric socket. We note the outcome of the measure.
Identical operation, sleeve immersed in the container - do not touch the sides or bottom-. We rebuild a tare to zero. Sample is deposited gently into the socket carrier is allowed to stand so that the system is stabilized and there has been the result of the measure. We have a friendly thought for good old Archimedes helps us well for a successful conclusion to these measures.
We have the dry weight of the sample and its mass in water, then applying the following formula:
Density = Weight in air
Weight in air - weight in water
we get a number that is enough to compare to those of many tables that are found in the literature or the internet, and we have an idea of the nature of the sample. The ideal is a small specimen consists only of the unknown material. Of course this is only an index, further testing will be needed, hardness, color and mineral line, form, place of origin, etc. ...
Bests regard's. Michel
4th Feb 2011 15:16 UTCDonald Peck
4th Feb 2011 22:31 UTCAlysson Rowan Expert
The other option is to wrap a bare copper wire around a wooden mandrel and then wrap the fishing line over that - it would heat much quicker than a bolt.
5th Feb 2011 00:14 UTCGord Howe
Gord
5th Feb 2011 16:57 UTCAlysson Rowan Expert
Centering the load is relatively unimportant for a longer spring.
The actual length of the monofilament of the spring is important in that the longer the filament, the larger the displacement under load - however, you must use a rigid enough spring to ensure that it doesn't 'bottom-out' under tension.
You need to experiment, but ideally, you will have a good few milimetres change in extension between dry-weight and wet-weight of your specimen.
The physics of spring extension states that while the change in displacement under load doesn't significantly alter the geometry of the spring, then the displacement is proportional to the load.
What this means in practical terms, is that the spring should open only slightly (less than, say, 45 degrees) between neutral load (just the balance pan) and under maximum load (balance pan and specimen). Under no circumstances should readings be taken when the spring is over-extended, as the larger the extension, the lower the accuracy of measurement.
This limitation is reduced in impact in a longer spring by making the changes in extension larger - but at the expense of using a more rigid spring for larger specimens - a 5lbs monofilament spring will only be useful for a few grammes of specimen at most.
As an adjunct, it is possible to make a simple torsion balance using monofilament that will operate in exactly the same way as a Jolly balance. Whilst it is has a less linear callibration, it is a more robust piece of equipment that requires less head room.
Tonight's homework is to work out why the callibration is not linear.
6th Feb 2011 00:36 UTCRudy Bolona Expert
6th Feb 2011 19:44 UTCGord Howe
Gord
6th Feb 2011 19:55 UTCAlysson Rowan Expert
I built the one in Sinkankas's book. It has been an invaluable tool. It's extremely accurate, relatively easy to build and inexpensive. I used a pica points rule instead of the graph paper suggested by Sinkankas, this makes it incredibly precise! Paper clips make good carriers and weights which can be selected and cut to different sizes according to specimen weight.
That is a really good looking piece of equipment, Rudy. Sadly, I have neither the skill nor the patience to actually build a beam balance of that quality (which is why I use a chemical balance or a Jolly balance).
The beam balance, when well made, is incredibly accurate and consistent in use, and, by use of suitable adjustments to the size of the riders, is capable of a broad range of measurement. The limitations to the design are the friction of the pivot and the rigidity of the beam.
There is no limit to the choice of scale used on this type of balance provided that it is linear and is aligned properly with the pivot (the pivot should be at zero units). Indeed, a scrap metal merchant I once visited had a beam balance weighbridge scaled in hundredweights - that is 4" per Ton, and was capable of handling 20 Tons of scrap. (a beam of 80 inches length that is 6ft 8" or 2m).
6th Feb 2011 20:59 UTCRudy Bolona Expert
7th Feb 2011 01:27 UTCDon Saathoff Expert
Don S.
7th Feb 2011 17:45 UTCAlexandr E. Zadov
I am sorry for machine translation.
I am taken by definition of density of minerals many years.
My councils:
1. For the big crystal almost any physical method on the basis of Archimedes law allows to define easily density +/-0.1 gram/sm3. All methods simple. Choose a method under scales available for you. The scale of 0,01 gram is sufficient.
2. Small and smallest crystals demand a special technique. Here careful, laborious work and other scale of scales is necessary.
3. For primary diagnostics accuracy +/-0.1 gram/sm3 is quite sufficient.
4. Accuracy above +/-0.01 gram/sm3 for mineral definition is superfluous. (You will see, than one crystal of the same mineral differs from another. Even if crystals seem faultless. This influence of micropores and inclusion other minerals.)
A.E.
7th Feb 2011 19:48 UTCDon Saathoff Expert
Don S.
9th Feb 2011 19:55 UTCAlysson Rowan Expert
My article does explain the method and, more particularly, how to make a servicable pycnometer bottle.
I have seen this method used to find the specific gravity of a single, small sand grain using a tiny (0.5 ml) pycnometer bottle.
You will require a precision dry-pan balance (Jolly or beam balances are perfectly acceptable) for this method, and the mass of the bottle is irrelevant as it is weighed dry to start with.
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Copyright © mindat.org and the Hudson Institute of Mineralogy 1993-2024, except where stated. Most political location boundaries are © OpenStreetMap contributors. Mindat.org relies on the contributions of thousands of members and supporters. Founded in 2000 by Jolyon Ralph.
Privacy Policy - Terms & Conditions - Contact Us / DMCA issues - Report a bug/vulnerability Current server date and time: April 19, 2024 11:54:49