Series on identifying minerals

[user 4386]

Series on identifying minerals - part 8 SPECIFIC GRAVITY

We all know that a cube of lead weighs a lot more than a cube of quartz of the same size, and both weigh more than a cube of Styrofoam. Density is simply the mass per unit volume of whatever you are measuring, and the mass of lead is much greater than the mass of quartz of equal volume. Think of mass as weight to keep it simple. So we could express density in terms of kilogram per cubic meter (kg/m 3). But you don't really need to know that, because in practice we measure specific gravity not density.

Specific gravity is a measure of density relative to the density of a reference substance . So specific gravity is the ratio of the mass of a substance (e.g. your mineral) to the mass of a reference substance of the same given volume (e.g. water). Because the density of pure water is so close to 1 (0.9976 grams per cubic centimetre), specific gravity and density are nearly the same value so long as the density is given in g/cc (i.e. density is only very slightly less than specific gravity). But you don't really need to know that either, just follow the instructions below.

Sample preparation

Your specimen must be pure, not a mixture of minerals and not altering to something else. But it is important to make sure that your specimen is dry (dry it in a low oven), and that it is not something that will soak up or dissolve in water. If it does soak up water or dissolve, it is still possible to approximately determine the specific gravity by spraying the outside of the specimen with silicon or epoxy resin (but you must not add significantly to the mass (weight) of your sample, so use a light spray and the largest possible specimen.

The method

The method is simple with the right, fairly inexpensive, balance:

If we use water as the reference substance, we simply measure its mass on a balance separately in both air and water. Divide the first (dry) weight by the second (suspended in water) weight and you get the specific gravity.

http://www.johnbetts-fineminerals.com/jhbnyc/articles/specific_gravity.htm

How useful is it?

How useful is it? Well consider minerals common in gold veins - chalcopyrite has an SG of 4.2, pyrrhotite 4.7, pyrite 5.0, arsenopyrite 6.1 and gold 19.3. This method will distinguish between all of them (and is that a genuine gold nugget or gold-painted lead?: lead metal is 11.3).

 

[Dihusky]

Goldirock... having trouble reading your C: drive from my remote location... 8) 8)

 

[user 4386]

Strange that...... (now edited)

 

[user 4386]

This seems to be the sort of tare-zeroing scale he is using, good to 0.1 g

https://www.direct-sales-online.com...tangent-digital-mini-scale.html#product-tabs1

I think he is using the 300g version:

https://www.idavid.be/mini-scales/tangent-digital-mini-scale-300g.html

but I don't know that one would need to weigh such big specimens in practice (conversely, if the specimen is too small the 0.1g accuracy of these would not give you an accurate enough result - but for most purposes these would be OK).

This local one sounds cheap and good - has tare function and up to 500g with 0.01 g precision:
https://www.ebay.com.au/p/Portable-...m=273058847798&_trksid=p2047675.c100005.m1851

http://vi.raptor.ebaydesc.com/ws/eB...ategory=48724&pm=1&ds=0&t=1522594894000&ver=0

 

[Lefty]

I usually had difficulty getting consistent results when testing specimens for SG. There was nothing wrong with the scales - they would consistently and repeatedly record the same weight for a stone to within a tiny fraction of a carat - but even when carefully following the instructions of a professional gemologist I had some difficulty. I did usually end up averaging the results of repeated tests on stones that I already knew the identity of to something close to what I knew that stone should be but I often remained doubtful of stones I wasn't certain of.

There is no question that SG/relative density is among the most useful of tests for gemstones - you just have to be satisfied with regards to accuracy.

There's a battery of testing means that gemologists have at their disposal - refractometer, thermal conductivity etc. Right through to burning a tiny hole in the stone with a laser and examining the chemical make up of the gases released. A pro gemmo told me that of all the testing methods at his disposal, his favourite is a powerful microscope - though I don't think any pro gemmo would likely conduct only one type of test and write an official conclusion from that.

But yes, specific gravity is a tried and true method, I probably just need more practice. Plus the tools required for it are very cheap, a helluva lot cheaper than a high-quality microscope I imagine

 

[user 4386]

Series on identifying minerals - part 9 COLOUR AND LUSTRE

Colour

Colour is not a very reliable way of identifying a mineral. Some minerals are always the same colour (e.g. native sulphur, gold, galena, malachite, azurite). However a lot of minerals display numerous different colours in different samples of that mineral (e.g. calcite, fluorite, quartz). For example, quartz occurs in different colours in citrine, cairngorm, smoky quartz, amethyst, agate, rose quartz, white vein quartz, transparent quartz crystal - calcite also commonly occurs in up to at least 6 different colours, sphalerite varies from a honey colour to a dark grey, to reddish and greenish. The powder of a mineral is a little more reliable, which is why I discussed streak in part 4. For example, calcite will usually give a white streak regardless of its colour, galena dark grey, sphalerite white. The problem of colour variation and some different mineral colours are shown here:
.
http://www.oum.ox.ac.uk/thezone/minerals/detect/colour.htm

Lustre

Lustre is an additional visual property of a mineral that can help narrow possibilities a bit (it varies for some minerals, but commonly varies less than colour). For example amber is always resinous, diamond is always adamantine, quartz is always vitreous. It is the way that light reflects off a mineral. Because of this it is difficult to represent in a photo. This was the clearest series of images that I could find:

https://en.wikipedia.org/wiki/Lustre_(mineralogy)

Most sulphide minerals and native metals have a metallic lustre (pyrite, gold, native copper, chalcopyrite, galena). Sometimes we might call some sub-metallic (eg common for sphalerite). However the greatest variety of lustres are among the non-metallic minerals.

Because no one site gives clear examples of all types of lustre, you can choose examples from here:

https://www.bing.com/images/search?q=lustre+minerals+images&qpvt=lustre+minerals+images&FORM=IGRE

https://www.bing.com/images/search?q=lustre+minerals+images&qpvt=lustre+minerals+images&FORM=IGRE

Here are a few that are commonly poorly illustrated:

waxy:

silky:

greasy:

resinous:

pearly:

Silky and pearly result from the fibrous nature of the mineral and the differing length of fibres.

 

[20x]

Variable s and impurities makes it confusing, , I have thought about that allready. Q..is there and end point of knowledge of the geology of this planet or do you have to keep up to date with (if there are) new discovery's of minerals or new ways to advance the understanding of how everything was formed and forming and so forth?

 

[user 4386]

Variable s and impurities makes it confusing, , I have thought about that allready. Q..is there and end point of knowledge of the geology of this planet or do you have to keep up to date with (if there are) new discovery's of minerals or new ways to advance the understanding of how everything was formed and forming and so forth?

Yes, although with practice one can get it right 85% of the time using nothing more than SOME of the above techniques (probably 95% if you have three geos arguing together). Also people get good at their own geographic area, so many prospectors and amateurs can do better than me (a mineralogist) in such cases.

I guess it is why we have medical specialists, IT specialists (my kids beat me hands down), electricians, motor mechanics ...and geologists (who specialise as palaeontologists, regional mappers, hydrogeologists, mineralogists, geochemists, isotope geochemists, GIS specialists, geophysicists, marine geologists, volcanologists, exploration geologists, mining geologists, economic geologists, mineral economists and mining property valuers, mining law experts, sedimentologists, regolith geologists (sort of like soils), stratigraphers, palaeoclimatologists, tectonophysicists, environmental geochemists, historical geologists, engineering geologists). When I graduated there were geologists, palaeontologists, and geochemistry was just emerging as a specialty - the first integrated paper on plate tectonics (Vine and Mathews 1970?) was only published after I graduated. I am a dinosaur and started work nearly 50 years ago in engineering geology, geophysics and economic geology, exploration and mining, later worked in hydrogeology, became a geochemist, isotope geochemist and then a mineralogist, and structural geologist and lecturer in tectonics. I would not be permitted to consult in many of these fields now, as they have become too specialized (eg bridge foundations, palaeontology, and only parts of many other branches - eg a hydrogeologist might get me to consult in ground water chemistry).

So the world is now very specialised and no one can be everything - I graduated in geology with subjects in surveying, chemistry, metallurgy and mining. Nowadays you would graduate as a metallurgist, mining engineer, surveyor, and rather than geologist many would probably call themselves a geochemist or geophysicist or mineralogist having that specialty from day 1. Today I consult in mineral exploration (mostly gold and base metals and uranium), geochemistry, mineralogy and minor areas in mining geology, mineralogy, regolith geology and isotopes. I also write histories of goldfields.

As for mineralogy, many new minerals are found every single year (although most of those are uncommon, which is why they were not discovered in the past). A mate of mine discovered 5 new minerals over about 4 years working on a single mine - you cannot name a mineral after yourself, only after someone else, or a place etc., so when someone discovered a new mineral elsewhere they named it after him (he was a good mate and died young from one of the occupationally-related diseases that used to be common in our field, poor bugger).

We tend to call ourselves Earth Scientists nowadays (although that also includes less closely-related meteorologists and oceanologists). When one of us is consulting and feels we are straying outside our field of expertise, we call in another expert to work with us, perhaps up to half a dozen different areas of expertise over the life of a project.

 

[20x]

What a beautiful answer to a big question(I'm sure you enjoyed). I love opening cans of worms but had no idea the can was so big mate wow!!!
Q to find Uranium you study magnetic satellite imagery for the bleeds?

 

[user 4386]

We geos like to be appreciated.... Seriously though, it helps everyone to know how the world works (eg it is to the country's advantage, and in my experience Australians are already a bit better informed than say Americans - America is more like 4 countries not one, compared to us). For example, 50% more Americans are completely illiterate and 100% more are numerically illiterate than Australians (one in every five people in say South Carolina). And we help those who are poorly educated or poor much more here and don't try to make them feel that what they don't know is some sort of personal character defect. Of course that is really a generalisation about American government in some areas and at various levels, not Americans as people - we live in a great country overall.

Perhaps move your questions like this elsewhere - it may be a bit of a private conversation here.

No, we use a number of techniques and satellite imagery is only a tiny part (used for geology that hosts uranium, not to directly find uranium). Theory decides where you look (because in Australia and much of America and South America most cannot be directly detected at surface). The main applied technique is detecting natural gamma radiation from radioactive decay of the three common radioactive elements (either by aircraft or on the ground) - radioactive potassium, thorium and uranium (we actually detect their decay isotopes, things like isotopes of bismuth, argon, radon and particles given of when they emit helium nuclei). We refer to alpha particles, beta particles and gamma radiation that they emit. India is now moving to thorium reactors so the same comments apply for thorium exploration. However 20 cm of barren soil or sand or a few metres of weathered rock would completely hide Ranger uranium mine from detection this way, hence the importance of theory (it is less of a problem in Canada because it was recently glaciated and exposed fresh rock at surface, less of a problem in the USA because it was uplifted 2 km recently and rapidly eroded exposing fairly fresh rock in mountains and unweathered plateaus). Uranium readily leaches out of rocks by rain during weathering, moving downwards and leaving none at surface to detect. Plant roots take up uranium from deeper so sometimes we analyse plants (eg in Canadian swamps), sometimes surface water (Canada has a lot) or groundwater from boreholes. So existing water bores and drilling new bores are a common exploration technique (drilling is not just used to evaluate uranium once a mineralised site is found, but to explore for new sites). Uranium travels further than gold by far in old buried river channels, many tens of km or even more, so you delineate a channel using geophysics (e.g. aeromagnetics works in many and can see to 70 m depth, otherwise gravity and seismic surveys - channel sediments are less dense so have a different gravitational field and the floors and margins of channels have faster seismic velocities). Channel uranium deposits are just one type though, so after doing theory and radiometrics we use different techniques - some are loaded with pyrite and carbon so we use electrical methods.

If you have any other questions relating to this, make it a new topic and move all this uranium discussion there.

 

[Wishfull]

Gday all. My step daughter found this stone in our front yard. It has obviously been tumbled either naturally or mechanically at some time. It's not one of my stones. It may have been tossed into our yard. Anyway has anyone got an idea what it may be ? It looks to have quartz in it from looking at the base and red and black brown and yeah quartz.

 

[user 4485]

Possibly Tigers Eye?

 

[Wishfull]

Yeah could be. I googled that and yes it does look similar. Just can't understand why it was in my front yard.

 

[user 4386]

Yeah could be. I googled that and yes it does look similar. Just can't understand why it was in my front yard.

The next person who buys my place will inherit stones from Africa, Russia, USA, Fiji, and every Australian State in their backyard.

Yes, some sort of silica rock I guess but a bit out of focus. For about $12 you can buy a tiny tripod for a camera and $5 a cable release (and with a phone you can often set it up with it timed to fire once you are not holding it). Often blurr is not focal length but camera shake....

"Your mineral identification questions" is a better place for identification of specimens - this one is more how do you go about the process of identifying minerals. I don't check here as often.

 

[Wishfull]

Might have been operator error. I might have had a beer or 3 ...ish ?

 

[user 4386]

Yep, that might give you the shakes.....

 

[Phillip78]

properties, one by one, used in identification. The idea with "Your mineral identification questions" is that most questions be asked here where people

 

[user 4386]

properties, one by one, used in identification. The idea with "Your mineral identification questions" is that most questions be asked here where people

Something went very wrong????? A question?

 

[Deepseeker]

properties, one by one, used in identification. The idea with "Your mineral identification questions" is that most questions be asked here where people

Something went very wrong????? A question?

A General John Sedgwick moment perhaps.....................?

 

[tegmclub]

Really liked your articles on identification of minerals Goldierocks. Great Job!.
As it happens I was going to post an article on exactly this subject in my clubs next newsletter.
Can I please ask your permission to quote it, with acknowledgments of course to yourself and Prospecting Australia.
Russ Swan
President