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Selective deposit on tourmaline
  
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Roger Warin




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PostPosted: Apr 03, 2024 13:05    Post subject: Selective deposit on tourmaline  

A beautiful enigma.
This tourmaline was found in Anjanabonoina, in the Betafo District, west of Antsirabe, where you can only go on foot.
It is the Schorl species, one of the most abundant in the large group of tourmalines.
It has 2 particularities:
1) It is stubby.
2) A whitish deposit is deposited only on certain faces:
{321} and {100}. The effect is pretty and I only know the Pikes Peak microcline to present this selective deposit of albite on this green variety called amazonite.

I have 2 questions:
1) Stocky tourmalines are rather rare. These minerals are most often stretched along the c axis.
In beryl (morganite variety), the presence of Cs2+ cations slows down elongation by their size.
What is the cause for this Schorl? Are there any close substitutions for dravite (with which there is a solid solution) that are often less elongated?

2) What is the nature of the selective whitish deposit?
This amorphous deposit chooses the faces of the {321} ditrigonal pyramid and the {100} trigonal prism.
Thanks.



Schorl_Madag-01-a_R.jpg
 Mineral: Schorl
 Locality:
Anjanabonoina, Ambohimanambola, Betafo District, Vakinankaratra Region, Antananarivo Province, Madagascar
 Dimensions: 4.5 cm x 3.5 cm
 Description:
 Viewed:  1097 Time(s)

Schorl_Madag-01-a_R.jpg



Schorl_Mad-IMG_4629_R.jpg
 Mineral: Schorl
 Locality:
Anjanabonoina, Ambohimanambola, Betafo District, Vakinankaratra Region, Antananarivo Province, Madagascar
 Description:
 Viewed:  1095 Time(s)

Schorl_Mad-IMG_4629_R.jpg



Schorl_Mad_1-01_R.jpg
 Mineral: Schorl
 Locality:
Anjanabonoina, Ambohimanambola, Betafo District, Vakinankaratra Region, Antananarivo Province, Madagascar
 Description:
white deposit
 Viewed:  1089 Time(s)

Schorl_Mad_1-01_R.jpg


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Bob Morgan




Joined: 18 Jan 2018
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PostPosted: Apr 04, 2024 16:21    Post subject: Re: Selective deposit on tourmaline  

There appears to be a third particularity - it's habit.
I've looked through Goldschmidt and in other places and found no report of a large 001 face with adjoining 321 faces. In fact, there is no discussion of 321 faces in my limited library.
In the drawings I've found with 321 and 021 faces they were all terminations without an 001 face.
I'm guessing the stubbiness is due to a cessation of growth on the 001 face. The terminations of additional growth of 100 and 110 layers would account for the 321 and 021 faces.

As to the selective depositions. Perhaps the 321 faces are just rougher and more favorable to depositions of other minerals. As to what the white mineral is, probably quartz or some other mineral common in and around the pegmatite. There are yellowish crystals of betafite from around there. Perhaps a test could be done with an instrument, if it is worth the trouble.

Thanks for reporting this crystal. I hope you will submit photos to Mindat, It is important to be seen by others.
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