Crossed-polar illumination rock samples using dedicated Leningrad Optical-Mechanical (LOMO) Stereo Polarizing Microscope (MNC-1)- Transmitted & Reflected Light 16/7/2017

At the RAG meeting in June 2017, I demonstrated plain polar illumination of thin rock sections on a biological microscope modified for polar illumination by myself. The pictures were impressive. Since that time, I have added posts in which I have attempted, with some success, to do crossed polar illumination on that microscope – the Zeiss IM inverted biological microscope.

It is amazing what comes up on ebay – my latest acquisition is an example of the Leningrad Optical-Mechanical (LOMO) Stereo Polarizing Microscope (MNC-1)- Transmitted & Reflected Light. This old and dirty example has seen much use, is highly scratched, but was a good (cheap) price and comes with rotating stage, and a professional setup for both transillumination and epi-illumination in plain and crossed polarised light.

It turns out that the difference having a scope designed for this task masks is quite significant – or so my experience tonight would suggest – as you can see below.

Andy

Leningrad Optical-Mechanical (LOMO) Stereo Polarizing Microscope (MNC-1):

The oculars on this scope are larger in diameter than those on the Zeiss IM/IM35 – but thankfully the Bresser MikrOkular camera comes with an appropriate adapter!

The first two photos I am going to show demonstrate the view through the microscope without any slide. The polarising filter between the stage and illuminator is set to 90 degrees difference between the two pictures. This demonstrates that when this polariser is at 90 degrees to the one between the eyepiece (ocular) and slide then the two polarising filters are said to be crossed and no light gets through (the image is black):

Leaving the microscope polarising filters at the settings for the second slide above (ie crossed polarisation so no light gets through), I inserted a slide of Glauconitic sandstone from Upper Green, Folkestone, Kent, UK. This microscope has oculars (2 choices of magnification), an intermediate lens of varying magnifying power, and a fixed objective (as yet I don’t know the power of this).

The Glauconitic sandstone with x1 intermediate magnifiying lens gave following view:

The picture above demonstrates birefringence in some of the crystals. Birefringence is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. These optically anisotropic materials are said to be birefringent (or birefractive) (https://en.wikipedia.org/wiki/Birefringence). Crystals in the sample refract the light and change its polarisation phase as they do so – so that some of the light is no longer crossed at 90 degrees polarisation and becomes visible. The amount and nature of this change is typical of different crystals and can be used to identify them.

The following comes from https://en.wikipedia.org/wiki/Glauconite:The following comes from https://en.wikipedia.org/wiki/Glauconite:

Glauconite is an iron potassium phyllosilicate (mica group) mineral of characteristic green color with very low weathering resistance and very friable. It crystallizes with a monoclinic geometry. Its name is derived from the Greek glaucos (γλαυκος) meaning ‘blue’, referring to the common blue-green color of the mineral; its sheen (mica glimmer) and blue-green color presumably relating to the sea’s surface. Its color ranges from olive green, black green to bluish green, and yellowish on exposed surfaces due to oxidation. In the Mohs scale it has hardness of 2. The relative specific gravity range is 2.4 – 2.95. It is normally found in dark green rounded brittle pellets, and with the dimension of a sand grain size. It can be confused with chlorite (also of green color) or with a clay mineral. Glauconite has the chemical formula – (K,Na,Ca)1.2-2.0(Fe+3,Al,Fe+2,Mg)4(Si7-7.6Al1-0.4)020(OH)2.nH2O. Glauconite particles are one of the main components of greensand and glauconitic sandstone, and glauconite has been called a marl in an old and broad sense of that word. Thus references to “greensand marl” sometimes refer specifically to glauconite. The Glauconitic Marl formation is named after it, and there is a Glauconitic Sandstone formation in the Mannville Group of Western Canada.

 

Pleochroism is an optical phenomenon in which a substance appears to be different colors when observed at different angles, especially with polarized light (https://en.wikipedia.org/wiki/Pleochroism). In the table above, the peochromatic characteristics of glauconate are that it appears yellow-green, green, deeper yellow, bluish green.

In the photo below, I have increased the magnification of the intermediate lens to 4x and this now shows clearly crystals with all the colours mentioned above:

Rotating the stage is supposed to lead to colour changes in the birefringence so I gave that a try tonight – did not seem to make much difference as you can see below. One of the adjustment screws on the stage is bent limiting how far it can be turned in one direction so I can’t properly centralise the specimen so it rotates around one spot – this means I had to use X and Y controls on stage to keep recentring it each time I rotated the stage – need to get that sorted.

Images showing effects of rotating stage on berefingemence of Glauconitic sandstone, Folkestone (below):

I now changed the sample to Ritland Impactite & some other samples. The following images are all crossed polarised images using the LOMO MNC-1 with 2x intermediate lens.

Ritland Impactite (below) – notice new colours that are not present in the sample above:

Chondrule-rich unclassified NWA meteorite found in Sahara desert 2016 (below) – 2x intermediate lens – thickness of this sample meant it was difficult to get focus at higher magnification:

You probably are wondering the same thing as I did at this point – can I do anything similar with rocks from my own backyard? Thin sections are very expensive to produce – need expensive machinery anyway which I don’t have. The LOMO MNC-1 does have the capacity to use reflected light and I also have a HL150-AY cold light source with swan-light attachments – currenltly no polarised filters on the latter but it can help with reflected light non-polarised images.

So, I popped outside and picked up a relatively flat small stone from the garden path…….it is about 2.5cm longx2cm wide (below):

LOMO-MNC-1-Rock-from-garden-path-0-8x-intermed-lens-160717.bmp (below, this series of images feels a bit like those seen as Rosetta closed on Comet 67P!): The following image is taken using reflected light WITHOUT polarisation:

LOMO-MNC-1-Rock-from-garden-path-2x-intermed-lens-160717I.bmp (below): The following image is taken using reflected light WITHOUT polarisation:

LOMO-MNC-1-Rock-from-garden-path-2x-intermed-lens-160717II.bmp (below): The following image is taken using reflected light WITHOUT polarisation:

LOMO-MNC-1-Rock-from-garden-path-4x-intermed-lens-160717.bmp (below): The following image is taken using reflected light WITHOUT polarisation:

LOMO-MNC-1-Rock-from-garden-path-7x-intermed-lens-160717.bmp (below): The following image is taken using reflected light WITHOUT polarisation:

LOMO-MNC-1-Section-rock-from-garden-path-reflected-light-no-polarisation-160717field-X.bmp – first image below shows this field of view without polarisation, the second image using plain polarisation:

Non-polarised:

Plain polarised (same field as above):

When I attempted to view the above field using crossed polarised filters, nothing showed. In fact, throughout the sample there was very little berefringence seen in crossed polarisation – little but not nothing – here and there some showed. It was difficult to capture what was there due to limited sensitivity of the Bresser MikrOkular camera.

The following is one attempt to photograph birefringement with crossed polarised filters on this rock, on a different field of view from that above and using x0.6 intermediate lens.

The first picture is with plain polarised light and the second with crossed polarised light:

Plain polarised light:

Crossed polarised light, on same field of view as above – birefringence is there if you look carefully but it is very faint:

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