Dear Mr. Udagedara,

Using SEM-EDS is not suitable for accurate chemical analyzes of garnet.

Exact quantitative chemical analysis of garnet are possible only with an electron beam microprobe.

In almandine garnet may be present, the trace elements:

Sodium, potassium, chromium, vanadium and in very minor quantities, scandium, yttrium, europium, ytterbium, hafnium, thorium, uranium.

In pyrope garnet may be present, the trace elements:

chromium, iron, sodium (in eclogite), potassium, phosphorus, vanadium, nickel, scandium, ytterbium, lutetium.

Sulphur does not fit in the garnet crystal lattice. Measured sulfur contents come from tiny solid inclusions of sulfides such as pyrite, pyrrhotite, chalcopyrite, which are often found in garnet crystals.

Mg and Ca Sodium are interchangeable. Na and K are trace elements.

Best regards,

G. Grundmann

Dear Mr. Udagedara,

Is your S content really homogeneously distributed among the mineral or might you expect also inclusions. Normally, garnet does not accommodate S in its lattice. There are some varieties, such as hibschite,henritermierite,or katoite which contain hydroxyl groups and whose structure is not very well defined shown by the subscript x. In this case S may be incorporated in a non-stoechiometric way (?).

H.G.Dill

Thank you Professor Guenter and Harald,

I suppose Sulphur is distributed nearly homogeneously, it is 0.7% in average. I haven't observed any sign of inclusion even at higher magnification (X5000). How would this be possible?

Dear Tharanga,

The presence of sulphur in garnet may happen if you have contact metamorphism, and sulphur may appear as skarn. There is also possibility of changing oxidation state to reduction state, here also you may get sulphur in garnet.

Thanks Dr Greg and Dr Habib.

There are some signs of contact metamorphism in the study site, at least there is mobilized marble as a faint proof. There is a strong sulphur smell from the samples.

Yes, I would try ICP!

THanks a lot

Presence of S within garnet lattice is unlikely however the sulphur is seen as individual pyrite or skars around the almandine garnet (mainly) as geochemical post mineralisation process. I agree that SEM/EDS is only qualitative and ICP is good option to quantify the content chemistry of your sample. Always try the fresh cut for ICP.

Thanks Dr. Ehsan!

In response to Guenter Grundmann “Using SEM-EDS is not suitable for accurate chemical analyzes of garnet.Exact quantitative chemical analysis of garnet are possible only with an electron beam microprobe.”

This statement needs some clarification. Often EDS is performed standardless while WDS-EPMA uses reference standards. Standards-based EDS quantification can obtain results with a comparable precision and accuracy than WDS (Ritchie et al. 2012). A limit of quantification of ~0.08 mass% for elements with Z>10 can be achieved using EDS. The limitation for EDS are elements with strong overlapping peaks where one element has a concentration near the detection limit (e.g.0.1 mass % Co and high concentration of Fe in Cr-Ni steel).

What kind of EDS system was used [SDD or Si(Li) detector] and what were the measurement conditions? Was the sulphur correctly identified in the spectrum?

It could be possible, that a pile up/ sum peak at 2.265 keV due to combination of the oxygen-K (0.525 keV) and the silicon-Kα (1.74 keV) peak was misinterpreted as a sulphur peak (sulphur-Kα: 2.309 keV, sulphur Kβ: 2.465 keV). This is possible when carrying out analysis a high dead time especially with Si(Li) EDS detectors.

Ritchie et al. (2012) EDS Measurements of X-Ray Intensity at WDS Precision and Accuracy Using a Silicon Drift Detector. Microscopy and Microanalysis, 18, 892-904.

Thanks Tobias and Jorge,

Yes I am planing for analyzing it in another lab.

Grandmann's answer is good!

Dear Tharanga,

the fundemental problem here is the management of the investigation procedures and the worker's expectations for these. In my opinion there's no need (while also making little sense) to identify garnet by diffraction in a pelitic metamorphic rock - what about the good old microscope?, also providing lots of additional information. EDS is utterly unfit for measuring trace elements, all the more that the identification of elements and quantification of the resuls is often performed automatically with all inherent pitfalls. Yes, little chances for garnet to contain the sulfate instead of the silicate anion. But may I have a look at one of your sulfur-richest spectra? - a *.emsa or *.spx or *.txt or whatever ASCII file you might have there as au output would do.

Good luck with your work,

GS

Dear Tharanga,

What doesn't come clear from your initial query is how you prepared for analysis, which in addition to sample and specimen preparation, that by default should include optical petrography as a standard procedure. 

In optical microscopy, you can already observe phenomena like eg. microstructure, fabric/texture, intergrowth, and for individual mineral grains alteration, twinning, zoning, etc, which will help you anticipate the results from EDS or EPMA assessment.

However, far more importantly, optical microscopy in transmitted light will help you select suitable spots for analysis, meaning: without inclusions visible, without underlying wedges of another mineral (grain), of sufficient thickness, and unflawed by specimen preparation artifacts.

Optical petrography on (polished) thin sections is all too often dismissed as 'old fashioned' and/or 'non-quantitative', yet it provides valuable information that no other method can with the same efficacy and without destructing a specimen.

Whether or not SEM-EDS or EPMA is best suited to assess mineral chemistry will mainly depend on instrumental setup and operating conditions, as well as the purpose of the analysis. For simple identification (eg. pyralspite vs. ugrandite), SEM-EDS may be adequate, however, reliable assessment of trace constituents usually requires a well-calibrated, well-operated EPMA instrument.

Back to your original question regarding presence of S in garnet. I would first verify presence of possible inclusions of eg. pyrite or another common sulfide mineral using an optical microscope in transmitted light. With an SEM-EDS and/or EPMA instrument, you are actually analyzing a droplet-shaped volume under the point of incidence of the probe beam, so your analysis may be contaminated by any inclusions present (if). Sulfide inclusions are quite common in garnets, so that wouldn't be something new.

If no inclusions are visible, then the S cannot be attributed to mineral sulfides, but maybe represents a genuine garnet constituent. You will find detailed information in Grew, ES, Locock, AJ, Mills SJ, Galuskina, IO, Galuskin, EV, and Hålenius, U (2013): IMA Report. Nomenclature of the garnet supergroup. American Mineralogist (98): 785-811, and references therein. Given the very broad possible compositional range of garnets and the tremendous flexibility of the garnet crystal structure, I wouldn't be too surprised if some examples were found to contain some S as well (OH, Cl, and F have all been found). Also check the many example analyses in Deer, Howie & Zussman's The Rock-forming Minerals, Vol 1A, Garnet Group.

Furthermore, validating analytical results by judging weight percentages of constituents is arduous. Instead, recast your analyses into mineral composition formulae in Atoms Per Fomula Unit - APFU  following procedures outline by Deer, Howie & Zussman (in the appendices of the 2013 excerpted student edition), or in: Papike, JJ (1987): Chemistry of the rock-forming silicates: Ortho, ring, and single-chain structures. Reviews of Geophysics (25/7): 1483-1526 (also see: Muhling, JR, and Griffin, BJ (1991): On recasting garnet analyses into end-member molecules - revisited. Computers & Geosciences (17): 161-170).

In summary: first check thin sections in an optical microscope for presence of mineral inclusions, next read up on garnet chemistry, third recalculate weight percentages as APFU.

Kind regards,

Maarten

Dr Gavril and Maarten,

Thank you very much for your valuable inputs. I'll follow your suggestions.

and will attach the analysis file.

Additional Info (XRF analysis (FP method) shows S element about 1.6 wt%. but XRD does not show any peak for possible Sulphur bearing minerals such as  pyrite or gypsum. Still the rock samples have strong sulphur smell).

In general: one should always keep in mind that chemical analysis is not suffcient to charaterize mineral phases. Just one classical example: chemical analysis indicates presence of C (carbon); what kind on carbon?

Hello, You ask about garnet and I will get there somewhat circuitously out of necessity. I study fossils that are created in special circumstances. Mudslide victims entombed in TINY molecular clays, deep, cold, platy polar silicates in continuous wet conditions with certain PH values. Anaerobic compaction and certain acidic chemical runoffs can lead to soft tissue preservation. It is a long story but very interesting and yet to be completely accepted.

Silicon, iron, PH and fascia are the preservatives in mud fossils. Some mud fossils are so "perfectly preserved"  that they maintain every articulation and even sometimes color but they are petrified creatures exactly like petrified trees.

Now we come to the minerals that concentrate after VOC sublimation. Different section of the body and different tissue types have different chemical makeup. The metals and carbon that did not sublimate get concentrated, and eventually crystallize (like garnet).

I have some unusual info for you. Look at the chemicals you listed in your specimen and what is in the trace elements of the body, quite curious. I have a finger video that appears to be bleeding garnet from the tip and also sulphur looking deposits where bone should be. I have a finger video about the garnet..  

--------------------------------------------------

The claims are extraordinary so I believe an explanation is in order. My actual area is mud fossilization but this process of crystallization is integral. I am now releasing info about this. DNA tests show DNA and therefore life was the specimens origin. However it is damaged and we are trying to find the mitochondrial sequence for origins of the creatures. I have video links attached that show CAT scans and analysis of Giant Primate specimens.

Here is the wet "mud" fossil process. 

1..Wet soils create natural electric currents.

2..Molecular silicates are Tiny, platy and polar, some 2 atoms in size.

3..Creatures buried in mudslides can be sealed anaerobically in cold deep platy polar silicates (mud) in these wet natural electric earth currents.

4...Silicon is 50x higher in skin and connective tissue than other bodily tissues.

5...Leachates from runoff above percolate and interact with the buried creatures exterior surfaces. Chemicals in leachates determine fossil properties. An example is Opals which are fossils.

6...Natural electroplating of the creatures ionic flesh occurs as Silicon in the skin bonds and becomes SiO2 and SiO4 (amourphous quartz and feldspars), This occurs on the surface but the insides remain preserved under extreme pressures due to deep anaerobic mud on top. I have recreated the process with 12vdc and chicken tissues in sandy soils over the winter in the cold, and this appears to create feldspar "IN" the surface skin. I have video of electro-fossilization also  

7...As plating continues the surface becomes more and more restricted until OH- becomes the primary invader. OH- is extremely reactive and destroys calcium and phosphorous in bone. This is why bone is hard to recognize in mud fossils. Keratin, present in the skin, hair, and nails, is particularly high in the amino acid cystine, which is found in sulfur. The sulfur-sulfur bond in keratin gives it greater strength. Sulfur is the third most abundant mineral in the body, about half concentrated in your muscles, skin and bones, and is essential for life. Sulfur is definitely in the mix.

8...Normal oxygenated decomposing bodies literally explode from the gases as they expand. Sometimes skin is "degloved" and Fluids are expelled from weak areas and orifices but very little is expelled in mud fossils due to compaction.

9...Eventually glacial tilling disassembles the creature along fascial planes and body parts appear.

I would welcome others to look at and test some of my specimens and procedures.

Material analysis will be necessary and much more DNA work. My lab has a DNA  database that is fairly complete of "known" extinct and extant creatures.

[email protected]

https://www.youtube.com/watch?v=OutkhbHut-Y

https://www.youtube.com/watch?v=akTn3eVBrkw

https://www.youtube.com/watch?v=ccnOCw7WuMc

Dear Tharanga,

Thanks for your reply.

I am not surprised that you don't find any pyrite or gypsum by XRD: the amount of pyrite needed to give off a detectable S-signal in microanalysis is MUCH smaller than your XRD lower limit of detection LLD, which is on the order of 1-5wt%, much depending on sample (notably the type of comminution) and specimen preparation of bulk rock powder, as well as its mineral content. (In fact, modal analysis of bulk powder or powdered rock including trace minerals by XRD can be challenging; eg. check literature on Reynold's Cup!)

Really, by far the most practical method to verify the presence of pyrite inclusions in garnet is optical microscopy, in transmitted light where they will appear opaque, as well as in incident illumination (as suggested by Greg Raab) where they will appear as bright spots if at the surface. Thin- and/or polished-section petrography will also reveal any other S-containing minerals present, which must show up if you report 1.6wt% S by XRF, or alternatively ~4wt% SO3.

To conclude, I confer with Giovanni Ferraris: chemical analysis alone is INsufficient to determine mineralogy. Imagine you are analyzing carbonate by EPMA and find 56wt% CaO (a very good analysis, even when far short of 100wt%), how would you know whether this would be calcite or aragonite (or vaterite)? The simple answer is: by thin-section petrography, which through optical properties provides information on which polymorph you are looking at.

Kindest regards,

Maarten

Almandine Fe2+3Al2(SiO4)3 though in general it can be extend as (Fe,Mg,Mn)3Al2Si3O12

Aluminum  10.84 %  Al   20.48 % Al2O3

Iron      33.66 %  Fe   43.30 % FeO

Silicon   16.93 %  Si   36.21 % SiO2

 Oxygen    38.57 %  O

Pyrope Mg3Al2(SiO4)3 with common Impurities of  Fe,Mn,Ca

 Magnesium  18.09 %  Mg   29.99 % MgO

 Aluminum   13.39 %  Al   25.29 % Al2O3

 Silicon    20.90 %  Si   44.71 % SiO2

 Oxygen     47.63 %  O

Garnets generally contain a lot of inclusions including fluid inclusions.

In  Eclogite sample from Maksyutov complex, Russia some Garnets  contain numerous inclusions such as clinozoisite, quartz, rutile, titanite, and apatite, as well as minor chlorite, phengite and amphibole, whereas other garnets contain

 inclusions of carbonates, clinozoisite, omphacite, Ca–Na amphibole, quartz, rutile and apatite.

Probe analysis would solve your problem.

The reason for XRD not showing  any peak for possible Sulphur bearing minerals such as pyrite could be its low concentration in total rock sample, if you do heavy mineral separation and take the XRD, probably there is a chance.