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 Post subject: Testing a tourmaline with 1 wt. percent lead, Achroite.
PostPosted: Fri Feb 23, 2018 12:52 am 
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Joined: Thu Sep 25, 2008 10:33 am
Posts: 833
Location: Mars PA
On my recent trip to the west coast I paid a visit to the GIA in Carlsbad California. After getting over the sunny 70 degree day that is SO different than the Pittsburgh Pa area, we focused on testing some interesting tourmaline with their laser ablation equipment. One of the tourmalines I wanted tested was a nice sized round that is an Achroite and had been shown to have 1 wt. percent lead with x-ray at the Washington University in St Louis. The water white tourmaline was also shown to contain a significant amount of manganese.

The laser ablation testing confirmed both the amount of lead in the tourmaline and its significant amount of manganese. Yes, a tourmaline with a heavy load and manganese that does not give any color to Achroite. ( Many sources ascribe the color of pale pink to Mn+2, which I am confident this is not the case. Only Mn+3 can impart the red vector caused by manganese in some tourmaline.)

I do NOT think that the lack of color in the Achroite and its lead content have a casual relationship. I also think that the tourmaline was exposed to enough radiation to have its manganese content oxidized to Mn+3 from the Mn+2, that was incorporated in the original composition of the crystallizing crystal. So why isn't the Achroite colored pink to red?

Both lead and manganese have two common oxidation states. (Pb+2,Pb+4 and Mn+2, Mn+3) Both elements are metallic and have electrons in their outer shell that are easily lost in redox reactions. Both oxidation states of lead would not produce any color in the tourmaline because of its electronic configuration while manganese can impart the previously mentioned red vector, or a yellow depending on its oxidation state and associations.

Now the stage had been set for the power of natural radiation, which probably came from an isotope of potassium, to color this tourmaline's world. Electrons are lost by both lead and manganese when their electrons receive enough energy to move into the matrix of the tourmaline. (The color of the tourmaline is probably pink at this stage in its creation.) When the radiation is removed, the electrons will come into a balance between lead, manganese and the matrix of the tourmaline to maintain electrical neutrality.

It is my contention that the large amount of lead in this Achroite affects the distribution of electrons between the matrix and the manganese ions, which causes the manganese to only be stable in the Mn+2 oxidation state. This prevents the tourmaline from retaining its pink color, that is got from being irradiated, no mater how much energy was imparted into the crystal while being irradiated.

Since I do not know the oxidation states of the lead in this tourmaline or any detailed information about its matrix, my contention can not be confirmed. But if I had this tourmaline irradiated and it became pink and then faded to colorless again after the treatment is stopped, it would lend strong support to my contention.

While I would love to know the secrets of this tourmaline, I have my reservations about ever irradiating it. I love my water white, flashy gemstone that is unappreciated by the trade, as I am told in many articles, despite being rarer then maybe even PARAIBA. Yes PARAIBA the neon star. I don't have anything like it in my collection (Other gems I have called Achroite seem to still hint at some color in their body.) and I don't want it damage as some of my other tourmalines have been during testing.


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 Post subject: Re: Testing a tourmaline with 1 wt. percent lead, Achroite.
PostPosted: Fri Feb 23, 2018 12:48 pm 
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Joined: Sun Oct 16, 2005 12:22 pm
Posts: 20625
Location: San Francisco
Can you post a picture of what you are describing, Bruce?

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