Lecture 9: Complex silicates: tourmaline, beryl, lapis lazuli, tanzanite, and zircon I. Tourmaline 1. Basic Data: Chemical Formula: complex (see below) Mohs' hardness 7-7.5 Crystal System Hexagonal Color Varies: see varieties Fracture Conchoidal Specific Gravity 3-3.25 Refractive Index 1.64-1.68 Luster: vitreous to resinous Interesting Property: Piezoelectric and Pyroelectric Tourmaline is pyroelectric: it develops an electrical charge when heated. This can cause crystals to collect dust in display cases, since the warmth of the display lighting is often enough to induce a charge in the crystal. Tourmaline is also piezoelectric (see quartz). This property is not as sensitive as quartz, however, and so tourmaline is rarely used for more sensitive instrumentation - however, pressure gauges to measure the force of nuclear explosions contain some parts made of tourmaline.
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Elbaite crystal courtesy F. John Barlow mineral collection, MH photo. The structure of tourmaline is exceptionally complex - and an important source of the element Boron: (Na,Ca)(Li,Mg,Al)3(Al,Fe,Mn)6(BO3)3(Si6O18)(OH)4(BO3)3-forms triangle anion (unit) that results in trigonal symmetry of tourmaline crystals. Tourmaline may display beautiful triangle zoning due to compositional variation during the crystal growth (see image below). A photograph (cover of “Mineral Science” textbook) of a thin (~ 2mm) thick) polished slice of a tourmaline crystal with zoning from Betafo, Madagascar (Harvard Mineralogical Museum). The 3-fold rotation axis (the c-axis) of the tourmaline coincides with the center of the tristar pattern.