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Do all mineral crystals have an RI?


#1

I recently read a gemology book in which over 100 stones were
described with RI for each. Some are very dark stones. Hematite
(“black diamond”) for example has an RI. Is there ANY mineral crystal
which, when sliced thin enough, does NOT have an RI?


#2
Is there ANY mineral crystal which, when sliced thin enough, does
NOT have an RI? 

Algodonite
Bornite
Breithauptite
Calcopyrite
Cobaltite

That is five from the first three letters of the alphabet and I did
not finish C. I will leave the rest to you to research.

John


#3
Is there ANY mineral crystal which, when sliced thin enough, does
NOT have an RI? 

In theory, everything that can transmit light has an RI. However it
may be too difficult, or too small, to be easily measured.

Jason


#4

Everything has a refractive index (RI).

Look at these two links:

http://tinyurl.com/py7pq2
http://tinyurl.com/cawgrt

Mike DeBurgh, GJG
Henderson, NV


#5

As I emailed to Mr. Rasmussen which the moderators of Orchid did not
accept for posting, in theory everything with a crystal lattice,
just one lattice layer thick must be suspected of transmitting light.

Why? Because the physics experts say that the distance between my
ears is > 99.999999+ % empty space. So what happens to a photon
reaching one lattice layer of the most “opaque” mineral? Two layers?
Three…

If that is correct then mineral crystal opacity would best be defined
both as a property of the mineral and of the number of lattice layers
of thickness.


#6
Why? Because the physics experts say that the distance between my
ears is > 99.999999+ % empty space. 

Wow, Peter. That’s like hanging out bait for what some might say is
the obvious response. But I’ll pass,…in the interest of civility…
(grin)

Besides. those same experts sometimes have said the same about me on
occasion.

Peter Rowe


#7

To expand on Mike’s comment… yes everything does have an RI.
Everything.

Refractive Index is a properity of solids, gasses and liquids. It is
the difference between the speed of light in one medium vs another.
It is a measure of density mostly. The denser the material, the more
it slows down light. That is why if you look at something in water…
the image is offset from what you see by the RI of the water. Spear
fishers know this and aim accordingly. Light bends as it hits a more
dense or less dense media. This is useful for the identification of
gems because gem material have predictiable RIs (more or less)
because of their chemical compositions. Everything has an RI… just
as everything has a specific gravity. It is a physical definition.
Those physical characteristics are what enables gemmologist to
determine what the are looking at is a garnet vs. a ruby - along with
a lot of other metrics. What SG or RI can’t tell you… are the more
important things… is this stone natural or man-made, has it been
altered, where is it from, etc. Every stone has a story to tell.


#8
Refractive Index is a properity of solids, gasses and liquids. It
is the difference between the speed of light in one medium vs
another. It is a measure of density mostly. 

I think I understand the intent of the quote, but language needs
tune up. Diamonds are less dense (in physical sense) then sapphires,
but diamonds have higher refractive index.

This situation is not unique, by any means.

Better way of saying is that light will experience amount of
resistance to passage through the medium, which is directly
proportional to the number of atoms encountered on the way.

I suspect that in the above quote, term density was suppose to mean
number of atoms per unit of space. In some literature density is
associated with weight, and that can create a confusion.

Leonid Surpin
www.studioarete.com


#9

Do you think that each mineral crystal has an RI as unique as a
finger print and that when two crystals are assigned the same RI in a
gemmology text, this is experimental error?

In practical terms this is the era of nanotechnology in which even
single atoms can be “imaged”. Nanogemmology could become an important
assaying specialization if that is so.

This week I broke into some volcanic stone (basalt?) near a major
fault line on a mountain side and it is rife with silvery and yellow
cubic crystals which I assume to be pyrite. One piece was very
special. Half a dozen of these crystals were in a cluster. But the
colour display is awesome. There are flashes of red and orange,
purple and yellow and silver and blue coming from the crystals so I
expect they are more than pyrite. Until nano-scale methods in
minerology advance I suppose assaying crystals like this will be
guess work as they are no bigger than one mm across.

With all due respect to those who operate jewelry shops (which I
visit sometimes too), I prefer “God’s jewelry” in the wilderness. A
finding like the one above is as enjoyable to me as when I first
arrived on the site at Discovery mine (60s) with the rest of the
surveying-geology team and we found that blasting the night before
opened up rock walls shimmering with gold that could not have been
surpassed by King Tut’s tomb. IMO 100 square feet of gold flakes and
chips and nuggets on a quartz matrix is far more beautiful than a
few one ounce bars in the palm of the hand. It is too bad we could
not have just removed the entire wall and sent it to the National
Museum in Ottawa.


#10
Do you think that each mineral crystal has an RI as unique as a
finger print and that when two crystals are assigned the same RI
in a gemmology text, this is experimental error? 

Peter, you’re expecting too much from refractive index. It is
essentially only a measure of the speed of light within a given
material. There’s no particular reason why two different materials
might not have a similar refractive index if the optical environment
for light is similar. Now, many materials (any mineral crystalizing
in other than the cubic system) can have differing refractive index
in different directions, or for different polarization directions of
light even in one direction through the crystal. When you start
keeping track of these aspects of the optics, as well as behavior
under polarized light and other optical measurements one might make,
then the whole batch of data becomes more and more unique and
diagnostic. Yet even so, it’s not impossible to posit two different
minerals that, despite different chemical formulas, might not have
essentially the same optical properties. I can’t off the cuff think
of any examples, but that does not mean they cannot exist,
especially when considering the whole range of minerals instead of
just the small subset used as gems.

Peter Rowe


#11

I suspect also that twinning, tripling, etc. might affect the RI of
an individual stone.

Rose Alene


#12

I don’t know if I am expecting too much or not. Rice U. just
announced that they can produce one atom thick sheets of graphite
(graphene). Given this era of nanotechnology, why not one lattice
thick sheets of any mineral? Once that is done, does each have a
unique RI?

My guess at this stage would be yes.


#13
I don't know if I am expecting too much or not. Rice U. just
announced that they can produce one atom thick sheets of graphite
(graphene). Given this era of nanotechnology, why not one lattice
thick sheets of any mineral? Once that is done, does each have a
unique RI? My guess at this stage would be yes. 

Interesting questions. Do we know what factors go into RI? Can we
take other known properties, or a X-ray diffraction pattern, or
something, and calculate what the RI will be?

Does the concept of RI even make sense for a sheet 1 lattice unit
thick? Can you really say there is refraction? Or only diffraction?

Al Balmer
Sun City, AZ


#14
Given this era of nanotechnology, why not one lattice thick sheets
of any mineral? Once that is done, does each have a unique RI? 

because it would not be a mineral. There is a thing which is missing
from this discussion - called unit cell, which is mineral building
block. If you make material as you describe it, it would not have
the property of a mineral, and would not be a mineral at all.

Leonid Surpin
www.studioarete.com


#15

I do not know enough about the physical chemistry procedures
applicable to very small samples to say with any certainty how well
RI testing might find practical use to assay microminerals but last
night a one hour special on Mount St. Helens made me think that era
is close at hand.

They showed a geologist examining and identifying microcrystals in
MSH rock samples sliced thinner than a human hair.

My “field gemology” experience would be that this is approaching the
size of the microcrystals which catch my attention in the field on a
bright sunny day. Point being that if a microcrystal can be sliced
that thin, why might we not expect to pass a narrow beam of photons
through it for RI determination? I also note a report from Rice U
concerning their breakthrough in making one lattice-layer (one atom
thick) sheets of graphite.

In field work we use “vectors” in which small sized minerals point
us toward bigger samples and deposits. Flour gold in Slesse Creek
points toward the Mount Baker gold mines from the 1800s for example.

If my tiny green crystals found near a major fault line and volcanic
activity are emerald, maybe diamond drilling would be the next step
to probe further into the fault. So the vector points down rather
than horizontally as for Baker gold.


#16

I don’t know if I am expecting too much or not. Rice U. just
announced that they can produce one atom thick sheets of graphite
(graphene). Given this era of nanotechnology, why not one lattice
thick sheets of any mineral?

Graphite/graphene is very interesting in that it makes incredibly
strong sheets that don’t have significant attraction to each other
or anything else. This is in part why it is such a good lubricant.

Recently researchers found that a good, practical, consistent way to
separate graphite into graphene was to use regular tape.

http://pubs.acs.org/cen/coverstory/87/8709cover.html

By taking a small amount of graphite, putting it on some tape,
folding the tape over onto itself, and the repeatedly sticking and
unsticking the tape. The sheets of graphene are easily pulled away
from each other leaving individual layers.

This type of easy separation is very unusual, and the consistency of
a unilayered substance will be very difficult to match in any
substance that prefers to make 3-dimensional bonds, like minerals.

Jason


#17

Peter, you are still barking up the wrong tree as far as your
characterisation of minerals and the usefulness thereof goes. A
standard petrological microscope slide or rock thin section is 30
microns. This is, as quoted, a thickness and uses a difference in
light travelling through the different optical axes of a mineral to
determine its characteristic birefringence, which gives a different
light colour using plane polarised light. This does not determine RI.
RI can be determined optically by reference to glass, known minerals
and comparison tables. None of this will work on the bits of debris
you have gathered. You can use RI liquids but you still need to do
careful picking to get a meaningful sample to test.

I have told you what you need to do to determine your mineralisation
until I am blue in the face but you will not take any notice. My
first degreee was in geology and I have worked for 32 years in
materials characterisation, chiefly on crystalline materials using a
wide range of state of the art instrumentation. You either need to
spend more time doing proper geological mapping of your area or
listen properly to what others have said. Until then your postings
are wasting valuable electrons that the superhighway
could usefully use elsewhere. Ah, now that brings me on to quantum
mechanics, lattice defects and imperfect monolayers should you wish
to be bored further.

Nick Royall


#18

Let’s start with graphene which is one-lattice layer of graphite.

How would you do photo-chemical testing of graphene to prove its
unique chemical makeup?


#19
How would you do photo-chemical testing of graphene to prove its
unique chemical makeup? 

Have you got any to hand?

I wouldnt do chemical testing for it, nor would I try hitting it
with a hammer to see if it rings right. Orientated lattice diffraction
patterns are the way to do this, you need an ultra high resolution
TEM and the correct sample preparation technique. There is 1 in the
USA and I used to look after the one in the UK until I took early
retirement.

Nick Royall