[JimBob (OP)]

Thin sections are very thin slices of rock in which most minerals are translucent. Minerals are said to be anisotropic - having physical properties that are different in physical propeties along different crystal axes.

To make use of the different amount of retardation and identify minerals polarized light is used to produce different optical effects that help identify the mineral being examined.

Using two polarizing filters, one above the microscope stage and one below the stage, allows separation and possible retardation of one of the perpendicular light wave components.


from http://www.uwgb.edu/dutchs/petrolgy/olivine.htm


As the two beams of polarized light pass through a crystal in thin section which IS seen in 3D, not 2D, they travel at different speeds and get out of phase because of anisotropy. The slow ray is said to be retarded and the phase difference is called retardation.

If the retardation is a whole number of wavelengths (upper right), the beams recombine with the same orientation as when they entered the crystal. These wavelengths will be blocked by the upper polarizer.

If the retardation is a whole number of wavelengths plus one-half (upper left), the beams recombine with an orientation perpendicular to the original direction of polarization. These wavelengths will be fully transmitted by the upper polarizer.

It is being out of phase at other any other combination other than 1/2 or 1 wavelength that produces colors of different wave length.

This may be Greek to most of you but the pictures of these properties in different rocks can be stunningly beautiful.

Olivine (a rock made up of all one mineral, olivine see http://en.wikipedia.org/wiki/Olivine) There are two microscopic thin sections below.

from http://www.uwgb.edu/dutchs/petrolgy/olivine.htm

One polarizer.


Same rock under cross-polarized light.




For more beauty from rocks and the minerals that make them up see:

 http://www.microscopy-uk.org.uk/mag/artfeb04/iwouslides2.html

[neilep]

A LAYERED SEDIMENTARY ROCK.


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Jim can tell ewe all about it.................he was there when the layers were laid down !! []

[JimBob (OP)]

This is a layered sedimentary rock under artificial light. IT DID NOT come out of the ground that way. Why I remember when it was just a few piles of a few sand grains.... the rain came and there were suddenly other sand grains dancing around my piles ........ Pardon me, I need the loo

[neilep]

A Beautiful Geode-Amethyst


geodo-amethist_001-600.jpg (42.08 kB . 600x600 - viewed 35497 times)

[Bass]

Let me guess Neil- you found that in your garden?

[neilep]

Let me guess Neil- you found that in your garden?

LOL !!..Not that one...

..but my son has these..which he found in our window box !! [](they're guarded by Darth Vader)


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Iaaaaaamage2.jpg (43.35 kB . 574x251 - viewed 25602 times)

[Alandriel]

....This may be Greek to most of you but the pictures of these properties in different rocks can be stunningly beautiful.

(from first post)

Much of it all is (and probably somewhat will remain forever...) Greek to me but it's great to have this thread with info and pics for dummies like me. Thanks for starting it - fab idea!!
 []


talking of olivine.........one of my favorites 'rocks'

Olivine - some call it also Peridot? or maybe I'm just confuzzeld as ever
I don't mind the bigger green cousin EMERALD either - gotta love green rocks!
 []

[JimBob (OP)]

Hi Alandriel,

Thanks for the purty picture. Yep, also known as peridote when it occurs in gem quality crystals, like the one you posted.

Olivine is a magnesium iron silicate with the formula (Mg,Fe)₂SiO₄ that can range in composition from all Magnesium to all Iron.  A mineral whose composition can vary is a solid solution instead of a compound as the same space in the crystal lattice can accommodate (but not simultaneously) two or more atoms of different but equivalent substances. It includes both forsterite (Magnesium endmember) and fayalite (iron endmember). Peridote is a mineral close in composition to the magnesium endmember forsterite.

Olivine is one of the most common rock-forming minerals in the earths crust.

[Bass]

Uplift(2001) centered around the Norris thermal basin, north of the Yellowstone Caldera, as shown by radar interferometry.  This area has since subsided, suggesting it was due to hot water.  Image from Yellowstone Volcanic Observatory.


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[Alandriel]

I looked up radar interferometry but that did not tell me too much about what the meaning of your picture is Bass.

What do the different colour bands and 'blotches' mean / represent?

[Karen W.]

....This may be Greek to most of you but the pictures of these properties in different rocks can be stunningly beautiful.

(from first post)

Much of it all is (and probably somewhat will remain forever...) Greek to me but it's great to have this thread with info and pics for dummies like me. Thanks for starting it - fab idea!!
 []


talking of olivine.........one of my favorites 'rocks'

Olivine - some call it also Peridot? or maybe I'm just confuzzeld as ever
I don't mind the bigger green cousin EMERALD either - gotta love green rocks!
 []

MY birthstone and my favorite color... I have never seen one in the raw like that! Very nice.....Beautiful!

[Bass]

I looked up radar interferometry but that did not tell me too much about what the meaning of your picture is Bass.

What do the different colour bands and 'blotches' mean / represent?

By taking two different passes using satellite radar (INSAR), usually sepatated by a year or two, you can create an intereference pattern if the height of the ground has changed. 
"InSAR provides infrequent (annual to semi-annual) but highly detailed images of the amount of movement of the ground relative to a satellite. The technique is very useful for monitoring ground movements or ground deformation at volcanoes because detailed data can be collected for a relatively large area from space. By aiming a pulse of radar energy at the ground and then recording information about the return signal, such as the time traveled by the radar waves, InSAR quickly provides data for an entire area. By returning to the same area at different times, we can gain information about relative ground movements over time. The InSAR data are processed into images, called interferograms."http://www.thenakedscientists.com/forum/index.php?topic=11364.msg139151;boardseen#quickreply

In other words, an interference pattern is created, and the color bands represent discrete changes in elevation.  For this case, the center of the pattern was uplifted about 125mm (or 5in for the metrically impaired on this side of the pond).  This has proven useful for volcanoes and earthquake prone locales to measure surface deformation over a large area.  Think drops of oil on water- the thickness of the oil creates interference patterns.

[neilep]

The Giant's Causeway (or Irish: Clochán na bhFómharach[2]) is an area of about 40,000 interlocking basalt columns resulting from a volcanic eruption. It is located on the North East coast of Northern Ireland, about 3 kilometres (2 miles) north of the town of Bushmills. It was declared a World Heritage Site by UNESCO in 1986, and a National Nature Reserve in 1987 (by the Department of the Environment for Northern Ireland). In a 2005 poll of Radio Times readers, the Giant's Causeway was named as the fourth greatest natural wonder in the United Kingdom. The Giant's Causeway is owned and managed by the National Trust. The tops of the columns form stepping stones that lead from the cliff foot and disappear under the sea. Most of the columns are hexagonal, however there are some with four, five, seven and eight sides. The tallest are about 12 metres (36 ft) high, and the solidified lava in the cliffs is 28 metres thick in places.


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[Alandriel]

In other words, an interference pattern is created, and the color bands represent discrete changes in elevation.  For this case, the center of the pattern was uplifted about 125mm (or 5in for the metrically impaired on this side of the pond).

So, in other words, the yellowish-orange centre of the photo marks an area that's been uplifted by 12.5cm (just to confuse the metrically impaired  []) What about the other circles, e.g. blue etc.
Do the colours represent 'staggered raises' at regular invervals (more or less of course) from the outside to the inside with the innermost circle 'peaking' at 12.5cm ?



The Giant's causeway ***sigh***** been wanting to go see that for ages. Maybe next summer!

[Bass]

Alandriel- exactly right.  who says geophysics isn't your cup of tea?

[JimBob (OP)]

err Bass - that is remote sensing, not geophysics, Isn't it? Thought I'd just keep you honest, not a jackenape like me.

[Alandriel]

You guys crack me up 

Geophysics sounds soooo much more posh(*) and glamorous than remote sensing.

Bass was simply being charming  []


Besides - the guys on TV (timeteam I get coerced into watching) use geophysics but it looks like remote sensing LOL.
What's the difference?



(*)portside out starboard home  []

[Sarah Elizabeth]

you might like to know that olivine, or peridot comes from the word peridotite as it coes from mantle peridotite and is formed under very high pressure conditions.

[Bass]

It's a well known fact that cattle align themselves towards large ore deposits.  These "bovinity rays" are more noticeable during the morning and evening hours.

Well developed bovinity rays near a Nevada gold mine:

 
cattle2.jpg (40.18 kB . 288x216 - viewed 24974 times)

Moderate bovinity rays:


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Obviously no ore nearby:


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[JimBob (OP)]

Now this seems like a load of bull to me - get it? BULL?

Oh well, forget I said anything.