Chromium And Iron Game Stone 1

By Aamir Mannan
(CHROMIUM AND IRON)
The pure magnesium-aluminium garnet would presumably be colourless but such a mineral is never found in nature. All red garnets,in fact,are mainly mixtures of the almandine and pyrope 
molecules.For convenience we term those with low refractive index and S.G. 'Pyrope' and those richer in iron, having higher refractive index and S.G., 'almandine'. But it so hapens that the pyropes most used in jewellery  (from Kimberley , Bohemia or Arizona) owe their colour more to chromiun than to iron. This gives rise to a broad band in the yellow green centered near 5750 A,which swallows two of the three main almandine bands which would otherwise be 

visible.The third almandine band at 5050 A can be seen rather faintly where the green merges into the blue. As with spinel, narrow chromium lines are seldom seen in the red. 
Distinctive between pyrope and red spinel should be easy. With rare expectations, pyrope has no fluorescence even under crossed filters. Moreover, the broad central absorption band is at the yellow end of the green, 



whereas in spinal it is 350 A further towards the blue. it must be conceded that some red garnets which are clearly pyropes on the basis of their low S.G. and refractive index may contain little chromium and show merely a weak almandine spectrum.