Laurentthomasite: New gemstone and its comparison with grandidierite

Laurentthomasite is the mineral newly approved by IMA in 2019. It is attractive as a newly found mineral but also as good transparent and beautifully colored gemstone quality. These days, we could have a chance to test some samples. I would like to share our measured property of it.

The basic gemmological property
 Theoretically is R.I. is 1.540-1.545 (uniaxial +) but our tested samples showed 1.555-1.560 (uniaxial +). The S.G is about 2.67. The crystal system is hexagonal. And it shows strong pleochroism as blue and yellow.
The hardness is about 6.

In the comparison with Grandidierite
It is often confused with grandidierite because of same country of origin and the main blue color. However, if we can test the basic gemmological property of them, we can easily identify them.
The R.I. of grandidierite is 1.58-1.62 (bi-axial) and S.G. is 2.85~3.09. Because its crystal system is orthorhombic, it shows pleochroism as blue-green, green-blue, colorless. These properties are different enough. And the hardness is a little bit higher as 7.5.

The advanced gemmological property
Its chemical composition is ideally Mg2K(Be2Al)Si12O30. But with our EDXRF, Fe, Sc and Mn are also detected at the same level of Al and Mg.

ElementsWeight (%)
SiO274.22~79.25
K2O5.39~6.06
Fe2O33.02~4.54
MgO3.46~3.76
Al2O32.89~3.56
Sc2O32.83~3.57
MnO1.83~2.92

 And also a trace of Y, Rb, W, Ni, Zn, and Ga are detected. The FT-IR spectra can be downloaded from here.
Absorption spectra in dx file and PNG image file
Reflection spectra in dx file and PNG image file

We hope that these data may help to reduce the confusion with grandidierite and a healthy marketing of new gemstone of laurentthomasite.

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