Ann: Response to ASX Query Letter, page-27

@eastwest101

Very interesting observation and point you make about the Al.

People need to understand what those percentage breakdowns on the left hand side actually represent and to do this you need to know a little about how a Scanning Electron Microscope (SEM) collects the spectral data from the surface of the sample.

Take the two randomly selected images from the announcement (below) for example.


Firstly you need to understand the scale or magnification. The scale on the bottom right hand side of the first image is in µm (micro meters or microns).

1 micro meter = 0.001 mm or 1/1000th of a mm.

The field of view in the lower image is about 47 microns x 31 microns or 0.074mm x 0.031mm = 0.0023mm2 (about 2/1000th of a square mm)

How does this compare to the best light microscopes?

Light microscopes are limited by diffraction and can only resolve images down to about 200 microns which is 10 x larger than the scale on the lower image.

The image is created by scanning a narrow beam of high energy electrons up and down the field of view and detecting the products that come back from those electrons. The bulk of the image is formed by detecting the energy and angles of secondary electrons which are emitted from the surface from the interaction with the primary beam of electrons. A computer converts this information into a grey scale image which shows the texture/morphology of the surface as you see in the images on the right hand side.

The actual beam of electrons that gets scanned up and down is only about 1 or 2 micron wide (1 or 2/1000th of a mm wide).

Well in very simple terms that's how the image is created.

But how are the elemental spectra determined on the left hand side and how are we meant to interpret the numbers? We can see the % weights add up to 100%, give or take a few % that might have been clipped from the list at the bottom (the top image on the left hand side adds up exactly to 100%).

The first thing that struck me is where is O (oxygen)? Oxygen is the most abundant rock forming element on earth (46% of the earths crust) but it's nowhere to be seen on the % weights in all those scans. You can't have truly representative "weight" if oxygen is missing.

To understand why oxygen is missing you need to understand how the SEM collects the spectral information (spectral information is just the information that tells you what elements exist in the area targeted not the image of the surface like you see on the right).

Well how big is the target area for the spectral information? You can see on the right hand side images the SEM is operating in "spot mode". This means the spectral data is being collected from the area in the green cross-hairs which is only about as wide as the electron beam itself (about 1 or 2 micron wide or 1 or 2/1000th of a mm wide).

The spectral data is not collected by measuring the secondary electrons. It comes from measuring X-rays that are being emitted from the surface caused by interactions with the primary beam of electrons. The device used to detect these X-Rays is called a Energy-Dispersive X-Ray Spectroscope (EDS). I've put a link below if you are interested to know how it works.

The thing is EDSs have limitations on what elements they can detect because some energy peaks overlap. Most EDSs cannot detect very light elements (H, He, and Li), and many instruments cannot detect elements with atomic numbers less than 11 (Na) which includes oxygen (atomic number 8).

The SEM used to report on the samples at Ema and Tres Esdados will be a typical machine that can't see oxygen so the "weight %s" are missing a large fraction of what is in the target area.

Let's also think about the size of the target area. Say it is a circular area with a diameter of 2 microns (2/1000th of a mm) its actual area will be about 0.00003 mm2 or 3 hundred thousandths of a square mm.

The percentage weights are coming from a surface area on the rock chip of 3 hundred thousandths of a square mm. Think about how small that is.

Also if the sample is moved over a few microns (say 5 -20 microns) are the results reproduced over this scale?

This brings me back to my previous question. If the geologists can see with their naked eyes sulphides?, alloys?, metal amalgams? whatever you want to call them on mm scales (ie 1mm or even 0.1 mm), as is claimed, why not put that rock under and conventional optical microscope and produce a petrographic report and give that to the ASX. I hope the ASX is listening. Esh

P.S below are links that will help you better understand the explanation above.

Scanning Electron Microscope SEM
https://serc.carleton.edu/research_education/geochemsheets/techniques/SEM.html

Secondary Emission (electrons)
https://en.wikipedia.org/wiki/Secondary_emission

Dispersive X-Ray Spectroscope (EDS)
http://serc.carleton.edu/research_education/geochemsheets/eds.html