Selenium allotropy?
Different sources seem to describe the allotropes of selenium and their production differently.
All sources seem to agree on the existence of red amorphous selenium, prepared by precipitation from chemical reactions, and several monoclinic forms
of selenium, prepared by crystallization from carbon disulfide. There is also the metallic, stable form.
What is less clear, is the form Wikipedia calls "black selenium".
Wikipedia presents a pretty clear image of two allotropes
-Grey, made by heating the other forms to 180 °C
-Black, vitreous allotrope forms when the red form is rapidly melted Apparently the form usually sold as beads.
On the other hand, this page: http://selenium.atomistry.com/selenium_allotropy.html
Seems to claim that black and vitreous selenium are different things by listing the following:
-Grey Crystalline (Metallic) selenium. made by heating other forms near their melting points.
-"Vitreous selenium" made by cooling molten selenium "in not too protracted manner" reddish-brown, amorphous
-Almost black material of the same composition as grey selenium obtained by slow cooling of molten selenium.
This forum thread gives yet another, conflicting view, listing red, amorphous and black forms.
So what is the truth, is vitreous selenium black selenium, or is black selenium just a darker form of grey selenium? Or somehing else?
[Edited on 27-1-2018 by CaCl2]
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