FMF - Friends of Minerals Forum, discussion and message board

[Joseph DOliveira]

I recently purchased these 3 specimens that were unidentified with the exception of the location, Mt. Malosa, Malawi. Based on the heavily striated crystal faces on the two specimens to the left, I am assuming that they are Arfvedsonite, the other crystal on the right shows no striations and appears to be Aegerine. Is the presence of striations a simple way to discern between Arfvedsonite and Aegerine, or am I mistaken. Any other suggestions of a simple way to differentiate between Arfvedsonite and aegerine would be appreciated.

[Jordi Fabre]

[Joseph DOliveira]

Thank you Jordi.

[Ivan Blanco (PDM)]

Hi Joseph

try to look at the basal planes. The section of a pyroxene (like aegirine) is usually a square with modified corners (like an octagone) and the cross-section of an anphibol (like arfvedsonite) is usually an hexagon. However the best way is to look at the cleavage planes at that basal plane. Pyroxenes have two main cleavage directions that cross-cut each other at about 90°. Anphiboles however have two main cleavage directions that cross-cut each other at 60° and 120°.

[kakov]

I would also try with Ivan´s answer with the angles.
There is a further possible observation that might serve as additional confirmation: The strike colour of arfvedsonite has a bluish hue, while that of aegirine is clearly greenish. It can though be somewhat difficult to judge without a reference to compare with (i.e. strikes from what is surely one and the other).

[Joseph DOliveira]

Thank you both Ivan and Kakov for the additional information, your input is appreciated.

[cascaillou]

why not check specific gravity

[stef4412]

Because the density is not good for discrimination (already said elsewhere) in many cases, here too with a difference of 0.1 between arfvedsonite and aegirine, smaller than what you can measure.

[cascaillou]

well accuracy of an SG measurement depends on specimen weight versus scale accuracy

[stef4412]

Good accuracy can not be obtained if there is no laboratory equipment that would quickly become expensive. Even with an accuracy of 0.1 would be an illusion without a complicated protocol to be sure to have a pure mineral.
In the majority of cases the degree of inaccuracy is such that the method becomes misfit (in fact the density is only useful for mineral values higher than 4 or even 5 with large discrepancies to discriminate)

[cascaillou]

SG is reliably used for stone identification by gemologists. Of course a stone is never pure and often included, but it's fine since SG databases do not supply you with a single theoretical value for the pure material, but rather with broad SG ranges (which were obtained from measuring the SG of a very large number of specimens obtained from various locations all around the world, and of different quality grades).
However, let's note that a 0.01g scale isn't accurate enough for measuring SG of any stone weighing less than 3g (under 3g you need to use a milligram scale).
You also need a reliable SG database (some gemological databases are very exhaustive as to the possible SG range for each mineral)

[stef4412]

Of course yes but gemologists samples are purest than conventional mineralogical samples
And to obtain a good accurancy of the SG for a sample, the process is too complex for a normal kitchen : heat the sample in the oven to avoid errors due to residual water, weigh with a sufficiently high accuracy to be credible (scales with good accuracy are quite expensive), use liquids with calibrated densities ...

All that to obtain an accurate density at 0.1 while there has mineralogical means much more direct and simple (angle measurements, color of traces, relative hardnesses, mineralogical associations, physical characteristics ...) ...

It's just a waste of time

[cascaillou]

Gemological reference values usually cover everything from gem quality to stony quality because for many gemstones, the stony quality will still be used as carving material and gemologists need to be able to use the data for identifying such objects.

I have myself been identifying different minerals samples through gemological techniques and while I obviously didn't rely only on SG for identification (other tests were performed), the measured SG has always proven right for the mineral.

Of course, if the shape of a crystal is characteristic enough for identification, fine, but that's not always the case.