If I understand the definition of pegmatites correctly, there is a boatload in that area. In other words, there are these quite large fist size to softball sized, basalt or other hard rocks (albeit not spherical, mostly almost triangular but def not equilateral - that’s a crude description, filled with veins of various crystals and other minerals, oftentimes ver bright white, but the host rocks are unlike the surrounding rocks, as are their various crystals.
As I think I mentioned earlier, the whole region is alluvial, where I was, the hills are low lying, rolling and golden, covered with now wild grains that I saw a paper written about, mostly earlier Mediterranean varietals, probably dating to the Spanish conquest. It’s a beutiful area, but there in the specific region where I was searching, there was no large rock formations, just alluvial mixed up compressed soil and sand, with lots of cool rocks.
It’s really hard to find open land there, most is ranch land and fenced in, and the ranchers, at least the couple I met, ar prickly about people on their land, I stick with BLM land, and there’s not much. I found this stone in what I think was a wadi, a cut out a lot in the land, probably from a seasonal creek, but possibly a fault line. I found a much of the rocks that host benitoite, but outside of a tiny crystals that is too small to measure that looks like benitoite, I found none. Some of what I thought was jade may not be jade, but some probably is clearly nephrite, and I pretty sure I found some small pieces of jadite. It’s a beutiful area, and such a cool place geologically.
if there are any pegmatites in the area, they could be nepheline syenite pegmatites. Gabbroic pegmatities also exist… any pegmatitie that is mafic, alkalic or otherwise not a granitic one is another rare geologic occurance…If you found one, then you have found a gold mine of unusual and rare minerals. All a pegmatite is, is the residual hot fluids, gases that are exsolved from very a very slowly cooling intrusive igneous rock mass. These residual fluids carry incompatible elements that are essentially “sweated out” of the cooling rock mass, as it cools and crystallizes. These fluids and gases that are in solution or liquid due to pressure the slowly crystallize in cracks and voids and other channels forced open by the intrusion. This is the last stage of fractional crystallization and creates spaces and a high concentration of incompatible elements (elements that don’t readily fit in to the crystal structure of ordinary igneous minerals such as feldspar)… that lead to the final growth of large crystals of unusual chemical composition…
So far as the crystal faces on your specimen are concerned. cubic system minerals have equal angle crystal faces. Cubic system crystal have different habits… come in cubes, bipyramids, dodecahedrons… the key is that all of the angles between the crystal faces have equal angles… the crystals can be elongated in one direction due to unequal growth but the angles will still be equal… your specimen is large enough to measure the angles using a small cheap plastic goniometer…no need for specialized equipment…you just need to measure three angles between different crystal faces…just FYI, as you are a historian also, .Nicholas Steno, the father of crystallography and one of the early founders of geology discovered this principle back in the mid to late 1600’s.
The last specimen that you showed looks cubic to me. The Mindat photo posted by TroyJ also looks dodecahedral…12 faces of equal angles., the same is true of garnet, which is also cubic… garnet crystals often have unequal lengths along a face, but still have cubic symmetry if angles are measured.
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see my previous post at the end of this thread. Taking it to UC geology would give you an ID, but possibly not 100% certain without testing… visual inspection and a professional opinion would certainly help… In addition, your find is of definite geological importance… The area of New Idria is in the forearc portion of an old subduction zone… this is where oceanic sediments are scraped off the basaltic oceanic plate, compressed in a forearc wedge, thrust upwards, with the rest being subducted into the mantle. The coastal ranges of the West Coast of the US are forearc low mountains…On occassion, pieces of oceanic crust are broken and pushed upwards (obduction). The basalts and seafloor sediments are compressed under low temperature, high pressure conditions… these high pressure/low temperature metamorphic rocks are not that common world wide in subduction zones…serpentinite is the primary rock, with the high P/low T blueschist metamorphic facies hosting jadeitites and jadeite…Jadite is found in Japan, Guatemala, Taiwan, Southern Europe, New Zealand, and the coastal ranges of Northern California and the Klamath Mountains created by the same mechanism… Benitoite is formed by a different mineral paragenesis…hydrothermal fluids from dehydration reactions of chlorite mobilized barium from seafloor carbonates and other sediments, titanium from metamorphosed mafic rocks, and precipitated them in veins with associated rare minerals like neptunite with natrolite forming the matrix. The veins carrying benitoite are not considered pegmatites. Too often we focus on the crystal alone and ignore the matrix… gem minerals often are common, but gem quality crystals aren’t… the matrix is just as important as the crystal for an ID, as gem minerals have specific associated minerals in the matrix… a piece of matrix can be broken off and destructively tested without harming the crystal…it’s an invaluable tool for identifying the crystal itself.
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you have a point about focusing on the gemmology side… however, mineralogy has to come first… can’t know what a gem is without knowing what mineral it is… from more of the photos, I think that the crystals are docahedral- cubic…identifying them as cubic will point to hackmanite. Beryl and benitoite are cyclosilicates taking a hexagonal crystal form… the specimens that PaulB has are large enough to measure crystal face angles with a simple plastic goniometer… no need for expensive equipment.
Too often we just focus on the crystal and ignore the matrix. Gem forming minerals are often common minerals but gem quality crystals aren’t. Knowing the mineral paragenesis requires knowing the geological setting…
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Good luck with your DYI equipment…I know the principles on how these machines work, but don’t know how to build one… FFT is essential for Raman spectroscopy… good luck and best wishes…
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I’ll update you and Troy as this takes shape, I couldn’t/wouldnt build this if not for the board/chip-set. It’s kinda crazy that that got designed/built, it’s tailor made for this purpose, lab-instrument grade, and is just brilliantly and richly designed. It is really three separate sensor chipsets/LEDs, ADC’s, and filters, etc. combined, so the entire spectrum from vis to nir, can be analyzed, and there is a whole range of software tunable perimeters (LED intensity, sensor sensitivity, etc), AND the spectrum capture can be continuous or the SW has full featured “shutter” functionality (essentially a camera/sensor/light-source in one), all for about $70 shipped! I am not sure what the market is for such a board, but for a gemstone of mineral spectrometer, it’s a really nice piece of kit!
My coding chops are nowhere near strong enough to write all the sw for this, especially the FFT loops to reconstruct higher res waveforms from the digitally sampled segments, but I don’t need to do much of that, a lot of those functions are already written, and there are a lot of lab-grade visualizers out there I can, in principle, send the output to.
We shall see how this goes, no good product idea survives first contact with the soldering iron and it’s probably a lot harder to make work than I suggest, but I have a very good friend who has encyclopedic knowledge of such things (maybe you know said friend, “ChatGPT” is its name :-), so it should be a fun project. I had a carbon-based friend 3D print a few field cases to mount it in now that I know the hw configuration (beaded the on-board capture box, I’m going to add a port for a future external capture “lens”, that is not fully sealed for field analysis of stones and minerals too big to stick in the enclosed capture chamber), but that’s a future project. Hopefully Troy will clean up my code and add his magic to this and it may actually work.
If you do get your spectroscope built, here’s an open sourced article that you will find useful:
This one is open source.
[Chemistry of Materials](https://pubs.acs.org/journal/cmatex?ref=breadcrumb
Vol 32/Issue 20
Article
ArticleSeptember 25, 2020
Hackmanite—The Natural Glow-in-the-Dark Material
- Cecilia Agamah
- Sami Vuori
- Pauline Colinet
- Isabella Norrbo
- José Miranda de Carvalho
- Liana Key Okada Nakamura
- Joachim Lindblom
- Ludo van Goethem
- Axel Emmermann
- Timo Saarinen
- Tero Laihinen
- Eero Laakkonen
- Johan Lindén
- Jari Konu
- Henk Vrielinck
- David Van der Heggen
- Philippe F. Smet
- Tangui Le Bahers*****
- Mika Lastusaari*****
Another article whose reference I couldn’t copy and paste is “Raman spectra and lattice-dynamical claulation of natrolite” SV Goryainov, MB Smirnoov, Eur.J.Mineral., 2001 13, 507- 519
I mentioned natrolite several times since it’s the matrix material associated with benitoite…its an aluminum silicate in the zeolite family, indicating that the matrix was hydrothermal… same should apply to the matrix on your specimen…although nepheline is more likely…
No argument here! Gemology is rooted in Minerology/Geology. Without these fundamental disciplines, our understanding of the precious gems and specimens of the world would be much less appreciated and prone to biased interpretation. Admittedly, I am not schooled in minerology/geology, so I have to adapt my understanding with instruments and toolsets that can help identify stones both faceted and in rough form. And many hours of research in geology/minerology text books/white papers. 
A goniometer is definitely a worthwhile tool for the field bag and the lab. However, even the smallest one that I have, is limited in use, especially when the crystal is embedded in its host matrix. Without completely extracting the specimen, we are left with just a few exposed planes of the crystal habit.
A single-plane goniometer is useful when appraising the cut/quality of faceted stones. But it leaves some room for interpretation, which can vary from one appraiser’s assessment to another’s. Being human, we can easily make errors and induce a bias of uncertainty to our analysis and measurements.
This is where the lab/desktop tools can enhance the analysis and identification efforts. Unless one can figure out the intricacies of an apparatus such as this one, let alone afford one:
…the majority of gemologists probably use the hand-held goniometer and/or their experienced eye to make these assessments of a stone’s cut and proportion quality. I know I do.
I don’t want my customers to feel slighted, when they have an independent appraisal done on one of my commissioned pieces, resulting in a negative evaluation, all because I am not consistent with my own skills at evaluating a stone I have set.
Hence my pursuit of advanced gemological instruments that can be constructed and calibrated to be competitive enough to meet or come close to the accuracy of more expensive ones.
Cheers!
Troy
the use of a goniometer could help if the cystal is Big enough, and has enough Exposed faces…Paul B’s largest crystals, look to be in that range… isometric is the easiest…unless you have a big free crystal it’s not good for much else…more complex crystal structures will be far more difficult to measure… again, just a suggestion for a DYI Free home test that doens’t require more than two thick paper strips,cut into thin strips, with short arms that can be placed on a plastic protractor to get three angles…Not very accurate but just another screening test in the field, as you have mentioned.
This the also a problem with photos… if PaulB showed us the same large sized crystal group from slowly rotating photo angles, identifying from shape appearance alone would have sufficed…isometric habits are also easy to recognize visually: cubes, dodecahedrons, octahedrons and trapezohedrons…Sodalite crystals are rare and usually take a dodecahedral habit…In his specimen, there appear to be some faces that are 4 sided- (tetragonal system), with others having five ( dodecahedral)… my initial impression was that the specimen was cublic, but am no longer sure on second look… the central face looks like a square…both the isometric and tetragonal systems have 3 axes at 90 degrees…hence the similarity, the C axis is longer…only isometric has 4 fold rotation symmetry along all axes. tetragonal has one, face is square to rectangular…
leucite is tetragonal and would be found in association with sodalite in the same provenance. But not sodalite/hackmanie…
Thanks for the reply… and Cheers!
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PS: determing crystal class by Raman spectroscopy requires polarizing filters and is a subspecialized application. While you build your machine, I still think getting a professional PhD mineralogist/petrologist’s opinion for your closest UC campus is worth the time… sodalite has not been found in the New Idria mining district so far… also see my last response to TroyJ… consider googling the nearest UC geology department to you and find out if anyone can do a visual inspection first and take it from there.
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using an optical goniometer measures angles of a faceted gem. the facets don’t match the underlying crystal structure. Good for telling how well a gem has been cut but not underlying crystal structure…the only accurate way to determine crystal structure in a cut stone is by X ray diffraction. RI, brifingence, and pleochoism are clues but not definitive…doubly refractive minerals are pleochroic with plane polaized light…birifingence is a related property measuring maximum diffrences in the refractive index at different crystal orientations.
Ha, yeah for sure, the two things (my little science experiment) and the subject of this thread are unrelated, sorry to mix them
up. I agree and have taken your advice, I’ll let you know how it goes.
That said, to further mix topics, I have been messing around with the hardware I referenced and got code written that doesn’t totally suck; it’s actually quite interesting in many ways. For example, an analogue spectrometer and the human eye can capture way more granular degrees of difference between visible light ‘colors’ than the hardware I’m using, but, refraction extends way past the visible light spectrum, and so even though the resolution of my toy will be poorer than even a good eye, I theoretically get more definitive identifying data from analyzing much broader spectrums (all subject to the breath/quality of reflected data, of course). Also, humans see color differently, the woman at the body shop where I had my car repaired complained about that, they would use the computer to mix and match after-market paints, and customers regularly see differences that even she can’t spot. So another kicker is that that variability can be minimized or eliminated. I ran some back of the envelope probability calculations of the theoretical fidelity of measurement I can get versus a humans with an optical spectroscope, and even unenhanced algorythmically (no FFT), the math says it can be easily exceeded. Of course, I don’t have the chops to calibrate and understand all the nuance enough to get to that point, but I’ll take it as far as I can and then opensource the rest. Opensource always innovates faster than closed-source, and for hardware costs of less than $150 (compete with cabling, simply mirrors, etc)., others may want to contribute.
Anyhow, I’m no pro with such stuff, but I enjoy it, so no downside, mistakes and all!
The spectra that unenhanced optical spectroscopes give are broad peaks… overlapping peaks make it harder to interpret…FFT requires digitizing information, an AD converter, an FFT program that breaks down sinusoids (frequency) into component wave forms and programming to select the desired frequencies. I used it daily in my own work until I retired… I was looking at frequency related power over time, frequency power shifts…time was on the X axis, frequency on the Y axis, power was coded by color, giving a two D representation of a 3 D space… human eyes see colors slighlty diffrently from another pair of eyes… the most extreme being color blindness. Color discrimitation is entirely different…paint mixers, artists who work in color have good color discrimination, meaning that subtle differences in hue are detected when others’ are not able to do so… since we want to break down the color into spectra, FFT is a useful tool to sharpen peaks by reducing noise…That’s not to say that raw spectral data is no good… it’s quite good if there are distinct absorbtion/emission lines. With Raman, you’re measuring quantized translations between vibrational energy levels…lot of pitfalls due to planar orientation, different crystal axes, etc. Good luck… still think that the easiest way is to track down a professional academic to look at your specimen… the problem with many gemologists is that they don’t have the PhD level training in the earth sciences required for a definitive ID…nor the expensive equipment that is sometimes needed… gemology is mineralogy to begin with… mineralogy is determined by provenance…unidentified provenance can be back tracked using trace element and isotopic analysis… provenance means site geology, in turn created by plate tectonics…
I have to thank you for your post, I really learned a lot about the New Idrian mining district from searching the internet…all that I knew when I started was the occurance of jadeitite rock there… and that it had to be blueschist metamorphic facies… I knew nothing about the alkalic intrusion there that created silica unsaturated minerals… my primary interest is in alkalic igneous rocks…they are not that common but host base and precious metal ore deposits, rare earth deposits, niobium… all essential for modern technology… and all in short supply… How these rocks form within the mantle, fractionate isotopes, and concentrate certain elements 10K fold is impressive… But nature works on geologic time scales and many cubic kilometers of rocks, reproducible in the lab at accelerated time scales and detected by LA ICP MS…but only in trace amounts.
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It’s too soon to know, since I am still figuring out the sw and some physical elements of orientation of the board versus the specimen sampled, but in theory, I won’t need to use FFT or any algorithmic enhancements or grooming of the sensor readings. The suggestion I made earlier about possibly using FFT was premature, a product of lack of familiarity with the detailed specs/designs of the board. FFT is great for time-varying digital samples of an analog signal, but the board I’m using (attempting to use) is not that. Instead, it measures up to 18 channels of discrete light wavelengths simultaneously. This is not really analogous to the bins of digital sample data in an audio signal, which was how I was thinking about it, but that was in error. That said, the usable (meaningful for this use-case) sample fidelity of the digitized data within each channel, times the number of channels/spectral range, should be, in theory, sufficient to aid in identification, or straight-up identify, many types of stones and various (but by no means all) mineral and chemical components of such stones, to a degree that exceeds the human eye and an analog (passive) optical spectrometer.
So I think the way to think about this is not in terms of how it might stack up versus various lab grade instruments (which I confess was how I was thinking about it earlier). Of course it won’t touch the fidelity and accuracy of many such devices, nor will it be absolutely conclusive in its analysis of stuff within the scope of the hardware/software. But 1. it can, in principle, provide real, functional, insights that can aid in identification of gemstones and various element of their composition in ways that are less colored (pun intended) by human perception, 2. at a crazy low cost, and 3. do all that in a way that is, to me, anyhow, both educational and fun.
And right back at you re. my thanks for the insights you have provided, they have been super informative and I always appreciate your comments. Sorry if I got way off track with this thread, but I hope there is some value to the community in all of this.
All I can say is good luck on your project and hope it works. If you can tweak it, in principle, it should work as well as any lab grade equipment. Optical analysis with multichannel bands covering the visual spectrum should in principle work without digitization. I worked with digitized data that measured spectral power varying with time, which as you say is great for that purpose. You won’t be able to work backwards from a color spectrum to distinquish different colored mineral cystals all of the time… that would be one limitation…good luck and best wishes… post again when you’ve got it working!
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