Qualitative EDS analysis on The Mystery Slab

In General

In my book I discuss different kinds of tests. I won’t explain anything about testing here; this is simply a page for fluorescent mineral collectors and other mineral enthusiasts to drawn their own conclusions.

Kerry Day is well known on Mindat and has been performing qualitative testing for over twenty years. His website is http://kaygeedeeminerals.com/ and he is on Etsy.

He has tested materials for me before and I have bought some fine specimens from him. He is, as we Americans might say, a good egg.


This is a slab of unknown origin bought for five dollars at a rock shop in Kanab, Utah. It is 2.5 in hardness and does not fizz under 10% HCI, although it does produce inactive bubbles. It is about five inches in length. It breaks apart easily and has no sandstone like texture, more resembling hardened clay than anything else. But obviously hard enough and structurally sound to be slabbed.

Visible light

Shortwave UV 18 watt Way too Cool Lamp

What The Following Numbers Refer To

#1 red mineral: The darkest material in visible light, with a faint reddish appearance. This material fluoresces red under SW UV.

#2 green mineral: The light green material in visible light. This material does not fluoresce.

#3 Grey: Colorless or mostly so material forming what I am presuming to be the matrix of the rock. Does not fluoresce.

Aug 22, 2023

Hello Thomas:

Here are your latest:

The following spectrum was created with a Cambridge S100 SEM, a XR-100-CR pin diode detector and DTSA software. X-ray counts are on the vertical axis and X-ray voltage is on the horizontal axis. For various reasons peak heights are not directly comparable.

The accelerating voltage was 25 KeV. This setting exaggerates the higher voltage peaks. Detector efficiency peaks at Ca, thus, all Ca peaks are greatly exaggerated. NA IS VERY POORLY DETECTED BY MY HARDWARE.

Some elements create more than one peak. All elements have been labeled.

My X-ray detector cannot detect Li, Be, B, C, O, N or F.

Uncoated specimens charge up under the beam and generate false peaks such as Al (1.49), Si (1.74), Cl (2.61) and Ni (7.47). These elements are coming from the inside of my SEM chamber. Surrounding minerals also
contribute. Any element I believe to be extraneous I did not label.

#1 red mineral = I suspect Smectite Group - Sauconite. There are localities in both Colorado and Arizona.

#2 green mineral = Malachite in Ca matrix.

#3 Grey = If it didn’t fizz I would guess Fluorite.

Comments by Drew Barkoff

And yes, you were right about the UV light on that slab sample, that looks wild! What is the context of this sample again? There is quite a lot going on but it looks like this was the product of low- to medium-temperature alteration of what was probably originally limestone or some carbonate-bearing rock.

Low-T fluids can contain lots of Cu and sometimes F depending on local geology, explaining the malachite and what is probably fluorite.

While I don’t know what is making it fluoresce, the clay mineral is one of many common clays that form from lower temperature alteration of things like feldspar, or more likely residual clay grains that were trapped in the protolith as Al is not very mobile at all, and therefore very unlikely to have been introduced by the fluids as Ca and Cu have been in this case.


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