Tourmalinite

Tourmalinite is the name given to metasomatic rocks containing in excess of 15 % tourmaline, and can be associated with Sn-W ore deposits.

Metasomatism is a process of alteration of rocks by which their chemical composition is modified, new substances being introduced while those originally present are partly or wholly removed in solution. The agencies of metasomatism are in nearly all cases aqueous solutions; probably they were often at a high temperature, as metasomatic changes are especially liable to occur in the vicinity of igneous intrusions (laccolites, dikes and necks) where large quantities of water were given off by the volcanic magma at a time when it had solidified but was not yet cold.

Tourmalinisation is the metasomatic replacement of feldspars and micas by the mineral tourmaline. The reaction is complex but involves the circulation of hydrothermal fluids rich in both the elements boron and fluorine. This process is widespread in the vicinity of igneous intrusions, and if allowed to go to completion will form a rock of granite origin where all the feldspars and micas have been replaced, forming a rock containing quartz and tourmaline only.

Tourmalinite samples. Tourmaline (dark) + quartz (white).

Bibliography

• Deer, W.A., Howie, R.A., Zussman, J. (1998) Rock-forming Minerals.
• Optical Mineralogy : The Nonopaque Minerals by Phillips / Griffen
• E. WM. Heinrich (1956): Microscopic Petrografy. Mcgraw-hill book company,inc

Photo
Tourmaline (brown) and quartz (colourless). PPL image, 2x (Field of view = 7mm)	Tourmaline (brown) and quartz (colourless). PPL image, 2x (Field of view = 7mm)	Tourmaline (brown) and quartz (colourless). PPL image, 2x (Field of view = 7mm)
Tourmaline and quartz crystals. XPL image, 2x (Field of view = 7mm)	Tourmaline (brown) and quartz (colourless). PPL image, 2x (Field of view = 7mm)	Tourmaline (brown), with zoning, and quartz (colourless). PPL image, 2x (Field of view = 7mm)
Tourmaline and quartz crystals. XPL image, 2x (Field of view = 7mm)	Tourmaline (brown) and quartz (colourless). PPL image, 2x (Field of view = 7mm)	Tourmaline and quartz crystals. XPL image, 2x (Field of view = 7mm)
Tourmaline (brown) and quartz (colourless). PPL image, 10x (Field of view = 2mm)	Tourmaline (brown), with zoning, and quartz (colourless). PPL image, 10x (Field of view = 2mm)	Tourmaline and quartz crystals. XPL image, 10x (Field of view = 2mm)
Tourmaline (brown), with zoning, and quartz (colourless). PPL image, 10x (Field of view = 2mm)	Tourmaline (brown) and quartz (colourless). PPL image, 10x (Field of view = 2mm)	Tourmaline and quartz crystals. XPL image, 10x (Field of view = 2mm)