Petrology & meteorology through a microscope with thin rock sections

Crossed polar images of Northwest Africa meteorite with and without 1/4 wave compensator plate

I recently acquired a compensator plate for my LOMO Polam microscope – the following images of a chrondrule from a thin section of a Saharian meteorite collected in 2016 and sold online by SDFossils show the difference using the compensator plate – purple images without plate and green with – all are crossed polar images.


Crossed-polarised images of set of five NWA meteorite thin section slides from Sahara (Morocco)

Set of five thin sections of NWA meteorites from the Sahara in Morocco.

These are from un-numbered meteorites and were sold as set of ebay by SDFossils in the UK. I do not know whether they are from different meteorites or five sections from the same meteorite. They are all show chrondules.

A meteorite is debris from space that survives impact with the ground.

NWA Meteorites: Northwest Africa (from Meteorite markets came into existence in the late 1990s, especially in Morocco. This trade was driven by Western commercialization and an increasing number of collectors. The meteorites were supplied by nomads and local people who combed the deserts looking for specimens to sell. Many thousands of meteorites have been distributed in this way, most of which lack any information about how, when, or where they were discovered. These are the so-called “Northwest Africa” meteorites. When they get classified, they are named “Northwest Africa” (abbreviated NWA) followed by a number. It is generally accepted that NWA meteorites originate in Morocco, Algeria, Western Sahara, Mali, and possibly even further afield. Nearly all of these meteorites leave Africa through Morocco. Scores of important meteorites, including Lunar and Martian ones, have been discovered and made available to science via this route. A few of the more notable meteorites recovered include Tissint and Northwest Africa 7034. Tissint was the first witnessed Martian meteorite fall in over fifty years; NWA 7034 is the oldest meteorite known to come from Mars, and is a unique water-bearing regolith breccia.

I don’t think my meteorite thin sections are not from Mars or the Moon! They are probably from a range of different unclassified meteorites – and this explains differences seen in the photos below.

It is quite possible that one or more of these thin sections come from “NWA meteorite 869”. The meteorite bulletin describes NWA 869 as follows at

Northwest Africa 869

Northwest Africa

Find: 2000 or 2001

Ordinary chondrite (L4–6)

History: It is quite clear that meteorite collectors in Northwest Africa have discovered a large L chondrite strewn field at an undisclosed location. At least 2 metric tons of material comprising thousands of individuals has been sold under the name NWA 869 in the market places of Morocco and around the world. Individual masses are known to range from <1 g to >20 kg. It is certain that NWA 869 is paired with other NWA meteorites, although no systematic survey has been done. It is also possible that some stones sold as NWA 869 are not part of the same fall, although dealers are confident that most of the known masses are sufficiently distinctive from other NWA meteorites in terms of surface and internal appearance that the error rate should be fairly low. Scientists are advised to confirm the classification of any specimens they obtain before publishing results under this name.

Petrography and Geochemistry: (A. Rubin, UCLA) A fragmental breccia of type 4–6 material; one thin section dominated by an L5 lithology gave olivine (Fa24.2).

Classification: Ordinary chondrite (L4–6); W 1, S3.

Specimens: A 189.3 g type specimen is on deposit at UCLA.


Below with x25 Leitz objective, crossed polarised filters, Zeiss IM microscope, Optovar x1.0 setting:

Below with x32 Zeiss LWD objective, crossed polarised filters, Zeiss IM microscope, Optovar x1.0 (1st) and x2.0 (2nd photo) setting:

Successful polarised microscopy of NWA Sahara (Morocco) meteorite slide using Zeiss IM microscope & Optovar

It has taken me some time to get there but I am here now – success with polarised microscopy through the Zeiss IM microscope!

The following images are of a Chrondrite-rich NWA meteorite from the Sahara desert, Morocco, purchased from SDFossils on May 2018. There is no date on the slide nor meteorite designation number.

It wasn’t obvious exactly which bits were needed but eventually I found that a Zeiss 47 36 68 polarised filter fits into the filter slot below the objective turret – this gives plane polarisation alone. However the purchase of another polarised filter (this time a modern one) designed to fit in the filter slot above the condenser (if you are used to normal upright microscopes remember this is an inverted microscope so turn everything upside down in your imagination and it then makes sense) allows for crossed polarisation. The latter filter can be turned through 90 degrees.

The following images show the Zeiss 47 36 68 filter and where it fits onto the microscope:

The following images show the second polarised filter and where that fits onto the Zeiss IM microscope – both the above filter and this second one are needed to achieve crossed polarisation:

This is also the first time I have used my new Optovar – this is another ebay purchase with optics in excellent condition.

Optovar on Zeiss IM microscope:

The different positions of the Optovar’s ens wheel showing different magnifications offered – these are in addition to that provided by the eyepiece/camera and objective lens:

The Optovar was not originally designed for the Zeiss IM microscope nor for the dual head block I am using to allow me to place a camera below the binocular head. This extra round accessory with the black lever on it is necessary to act as a spacer between dual head block and Optovar so they can fit together, otherwise the lens on top of the Optovar projects too far out and so does that on the bottom of the dual head block and they won’t fit together. The extra accessory with the level is in fact an aperture diaphragm attachment for use with illuminators and I have another one under my illuminator (see diagram above):

The following information about the Optovar comes from

The Optovar magnification changer was originally introduced by Carl Zeiss Oberkochen / West Germany in 1954 as an accessory to the Stand W. This microscope was the first post-war microscope designed by Dr. Walter Kinder at Oberkochen and incorporated a number of major innovations. As such it deserves a separate essay. What I would like to discuss here is the Optovar magnification changer. Still today it is a much valued accessory to any Zeiss microscope of the Standard series and has, in principle, been incorporated in many newer stands since.

The Optovar is an intermediate tube with a magnification changer, it also features an Amici-Bertrand lens (=auxiliary microscope) and an analyser.

The multi-step magnification changer introduces factors of 1x, 1.6x, and 2.5x .Later versions have the factors 1x, 1.25x, 1.6x, and 2x. The Optovar integrated in the larger Universal, Photomicroscope, and Ultraphot offers the factors 1.25x, 1.6x. and 2x. This allows the microscopist to bridge over the magnification gaps between objectives in small steps and saves him from having to change the eyepieces frequently. The optical systems to achieve this are arranged on a rotatable disc and can be switched in as desired.

For instance, with a standard set of objectives 2.5x, 10x, 40x, and 100x and an eyepiece 8x the following magnifications can be obtained:

20 – (25) – 32 – (40) – 50 – (63) – 80 – (100) – 125 – (160)

200 – (250) –320- (400) – 500 – (630) – 800 – (1000) -1250 -(1600) and

2000x (= the DIN series in steps of 1.6x, steps of 1.25x in brackets – figures are rounded off).

The Amici-Bertrand lens as it is officially called, together with the eyepiece, forms what is commonly also known as a phase contrast centering telescope or an auxiliary microscope, and is well known to users of polarizing microscopes. It serves to view the rear focal plane (exit pupil) of the objectives. In other words: it shifts the filament plane into the image plane. In the Optovar, the A-B lens is set in a helical mount that can be controlled by a separate wheel so one can focus up and down to reach the rear focal plane of objectives from 16x to 100x. With it the microscopist can see, focus on, and center the phase rings of his phase-contrast system. He can also check the setting of the condenser aperture diaphragm and the correct centering of the filament of the light source.

It is a most useful tool to inspect the optical system for any misalignment, vignetting, dirt in the objective or air bubbles in the oil immersion. With it one can also detect any cracked or damaged lenses, fungus or delamination in the objective. The main convenience is the elimination of having to remove the eyepiece and to insert the auxiliary telescope each time one wants to check the objective’s aperture or the phase rings.

Lastly, the Optovar includes a swing-out analyser or slot to insert one. The biologist needs only to place a polarizer under the condenser to render his instrument into a simple polarizing microscope to examine crystals or other birefringent material.

All images below with Bresser 5MP Microcam through Zeiss IM microscope, bright field. The advantage of polarised light is that it brings out birefringence in the crystal structure of the rock which can be used to identify the type of mineral in the rock and gives pretty pictures! The advantage of doing this with a biological microscope is the ability to use a range of objectives and imaging accessories that might not normally be available on polarised microscopes – however the disadvantage is that the biological microscope is more likely to suffer from strain in the glass of the instrument and accessories and objectives, which itself will affect the images produced.

Medium resolution images of NWA meteorite:

25x Leitz objective:

Unpolarised light:

Plane polarised light:

Crossed polarised light:

Effect of Optovar on above meteorite thin section showing extra magnification this accessory provides:

x1.0 lens:

x1.25 lens:

x1.6 lens:

x2.0 lens:

High magnification images of NWA meteorite chondrite:

x63 objective + Optovar:

x63 objective, x1.0 Optovar lens = x63 (plus magnification from camera = approx. real magnification of 1758 times – see calibration slide below):

x63 objective, x1.6 Optovar lens = x101 (plus magnification from camera – see below for calibration to convert this into actual magnification). Individual crystals can be seen and it is also obvious that they have a preferred direction:

Calibrating above to give actual magnification – for this purpose I used a calibration slide where each division is 0.01mm.

Calibration slide 0-01mm per division x63 obj x1-6 Optovar 090618:

Therefore across the slide photo there are 7 divisions or 7 x 0.01mm = 0.07mm. On my laptop computer screen the photo appears as 197mm across, meaning that magnification of the on-screen image compared to the original structures on the slide’s actual size = 197/0.07 = 2814 times magnification.

Crossed polarisation of salt crystals

The following salt crystal is seen using x20 objective on LOMO Polam P-113 Polarising Microscope.

The crossed polarisation images required a lot longer exposure on my Bresser Mikrocam 5.0 MP microscope camera and this has shown up a lot of hot pixels.

So, for the first time ever, I took a dark frame and subtracted it from each image using PIPP software (Planetary Imaging Pre-Processor,

You can see the effect below – it is quite dramatic!

The following is from :

“A protein crystal, unless it is cubic, will typically be weakly birefringent under cross polarizers. Salt crystals are typically strongly birefringent under cross polarizers. Some plastic plates and materials are also birefringent so this test is more easily performed and interpreted in an all-glass environment or in a plate made from a low birefringent plastic.”

The initial photos show birefringence but I would not describe them as highly birefringent…..that is until you get to the 11x objective photos at the bottom of the post – now that is a highly birefringent salt crystal!


Crossed-polarisation image x3.5 objective (dark frame subtracted in PIPP, cropped and changed to greyscale in GIMP2). The salt crystal shows birefringence:

Bright field image, x20 objective:

Crossed-polarisation images before dark frame subtraction in PIPP, x20 objective:

Same fields of view as above but this time after dark-field subtraction using PIPP, x20 objective:

x11 objective, post subtraction dark frame with PIPP – for some reason the salt crystal on top right is particularly highly birefringent (below):

LOMO Polam Fossiliferous Steinheim Impact Crater Lake Bed Miocene crossed-polarisation using LOMO Polam microscope

LOMO Polam Fossiliferous Steinheim Impact Crater Lake Bed Miocene crossed-polarisation using LOMO Polam P-113 microscope.


x11 objective:

x20 objective (panorama of 15 frames) – two fossils present with birefringence in crystals in the matrix in which they are embedded: