Mystery Crystals Revealed as a Mystery

Results

The results are in on those small crystals I found in tuff near Jean, Nevada. Kerry Day says they are a mystery mineral, possibly orthoclase. That’s a feldspar member, a group of rock building minerals. Although the geologic map for the area lists sanidine as being present, geologists aren’t all skilled mineralogists and sometimes small mistakes occur. Sanidine is another feldspar mineral, closely related to orthoclase.

Feldspar imparts a pink color to many granitic rocks. Feldspar minerals aren’t much collected unless they display good crystal form or come from a rare area. Like the anorthoclase found at the Mt. Erebus Volcano in Antartica.

Mindat.org says orthoclase is “a alkali feldspar intermediate between low sanidine and high albite.” Hmm. I’ll have to read up on that but in the meantime have asked Wendi at Minerals Unlimited to send me specimens of those two. I’ll put them under my microscope to compare them to what I have. Positively identifying any of these minerals by sight,  however, is really impossible.

Minerals.net has an excellent article on anorthoclase which says that distinguishing between albite, sanidine, orthoclase, and microcline requires x- ray analysis, probably X-ray diffraction. The usual visual and physical tools for identification: determining luster, hardness, cleavage or fracture, and so on, don’t matter much when minerals are quite alike. Rather, something like the percentage of potassium in a specimen may make the difference.. And you’re not going to determine that with with a field guide or anything else online. Test.

This is a labeled spectra of the crystal sample I sent Kerry Day. His conclusion:

“Mystery mineral = Not Sanidine, insufficient K. Probably Anorthoclase. (Now orthoclase, ed.) The spectrum does not fit Chabazite.”

With that comes his qualifications:

“That 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. [NA is sodium, ed. note]

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. [the lighter elements, ed. note]

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.”

So, what do we have?

One Method For Further Testing

Calculating a mineral’s identity by its geochemical composition is a good step when the usual ID methods fail. Kerry Day uses a method called qualitative EDS analysis. A qualitative test differs from a quantitative test in that it may be less precise but it is far, far less expensive. Quantitative testing is usually only needed when publishing results in the sciences or when working in an industry with critical concerns. NASA needs quantitative, you probably don’t.

Kerry Day uses his own Scanning Electron Microscope or SEM to perform mineral identification. He writes, “When high energy electrons from a scanning Electron Microscope (SEM) bombard a mineral grain, they generate X-rays of voltages specific to the atoms being bombarded. An Energy Dispersive Spectrometer (EDS) then detects these X-rays and displays them in graph form on a computer. Interpretation of the raw graph is called Qualitative EDS analysis.”

An element-based formula can express a mineral’s chemical makeup. Like this for turquoise:

CuAl₆(PO₄)₄(OH)₈ • 4H₂O

Cu is copper, Al is aluminum, PO4 is phosphate, OH is hydroxide, and H2O is water. The subscript values indicate their proportions.

Day considers the elements reported in a sample. Relying on years of experience, he then tries to match these elements to a mineral formula. His equipment does not test for every element nor do other kinds of analytical tests but the major elements present are often enough to identify a mineral in this manner.

Day charges $8.00 for each spectrum conducted, with the peaks of the spectra indicating which element is present and the height of the peak roughly approximating its proportional abundance. His report generates a labeled-spectra which is e-mailed and which you can see above. Kerry says, “A sand-sized grain or a scraping in a gelatin capsule or taped to paper is sufficient for the this analysis.”

You might pay more for postage than the test itself if you send your material as a package. Fit your sample instead into a No. 10 envelope. The USPS should charge less than $2.50 for a first class envelope to Canada. Although Day accepts cash tucked into a sample’s envelope, you can pay for testing through his Etsy store. That will prevent customs or your shipping company from delaying your sample should they have a problem transporting money.

His website:

http://kaygeedeeminerals.com/sem-eds_service

His Etsy store, under the name KGDOLMC, is here:

https://www.etsy.com/ca/shop/KGDOLMC?

His selection of minerals and rocks, by the way, is wonderful. Inquire if you don’t see something you are searching for. I have bought many things from him. Back to testing.

This testing is primarily for minerals, not elements. For example, an EDS analysis of colored chips from a petrified wood sample will probably reveal the mineral quartz, the host mineral, and not the trace elements coloring the chips. In the case of pet wood, Bob Jones says these colors could come from vanadium, chromium, manganese, iron, cobalt, and many more. Any of the so-called transition metals. Again, these are elements, not minerals and Day’s equipment may not pick them up. An EDS analysis may disappoint when a minerals are expected and instead undetectable trace elements produce an inconclusive report.

If known, you might include in your correspondence with Day the minerals documented to exist in your collecting area. Mindat.org might provide this for a well noted area or perhaps a list might be had from the applicable geologic quad.

What Now With My Mystery Crystals?

I’m wrapping up my efforts with the samples I collected. I’ve noted their location in a Word doc and have included portions of the Jean Quad, the geologic map of the area. Along with the test results Kerry Day provided. It’s not everyday that you find crystals that come in squares, rectangles, and sometime arrowhead shapes. It’s been fun. My mineral friend Rolph in St. David Arizona, says orthoclase is common there and brings about a sparkling character to many areas.

I’ll now store away my samples with this paperwork and go on to finding other things. Perhaps I’ll make this file available online in case the one or two feldspar specialists in the world ever decide they want to look up this occurrence. Maybe some student twenty years from now working on their Masters’ thesis. It could happen! 🙂

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