Bismuth oxychloride adds a shimmery pearlescent shine to mineral makeup. Bismuth starts out as a brittle white or silvery-pink metal, which is then processed with chlorine and oxygen to create bismuth oxychloride. While many mineral makeup manufacturers like its silky feel and subtle metallic sheen, some avoid using bismuth oxychloride because it is thought to cause irritation or allergic reactions in people with sensitive skin.
In its natural state, bismuth is non-toxic, so there's some speculation that any irritants in the finished product could be a result of the refining process and not the mineral itself. But since mica offers similar benefits without the risk of a reaction, manufacturers who want to play it safe simply leave the bismuth oxychloride out.
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