Composition and Structure of Tourmaline
Tourmaline (Ca,K,Na)(Al,Fe,Li,Mg,Mn)3(Al,Cr, Fe,V)6 (BO3)3(Si,Al,B)6O18(OH,F)4
is a crystalline boron silicate mineral compounded with elements such as
aluminum, iron, magnesium, sodium, lithium, or potassium. Classified as a
semi-precious stone and gemstone, tourmalines are minerals with an incomparable
variety of colors. The name tourmaline is derived the Tamil and Sinhalese word
"Turmali", which applied to different gemstones found in Sri Lanka.
In translation, the word is thought to mean 'stone with mixed colors',
referring to the color spectrum of this gemstone, which outdoes that of all
other precious stones. There are tourmalines with colors ranging from blue to
yellow and from red to green.
X = Ca, Na, K, or is vacant (large cations);
Y = Al, Fe2+, Fe3+, Li, Mg2+, Mn2+
Z = Al, Cr3+, Fe3+, V3+ (small
T = Si (and sometimes minor Al, B3+);
V = O and/or OH;
W = F, O and/or OH.
Tourmaline is a ring cyclosilicate containing a six-member trigonal
crystal system. With no other common mineral having three sides, tourmaline is
notable for its three-sided prisms. It is further differentiated from other
minerals because its prisms faces often have heavy vertical striations that
produce a rounded triangular effect.
With long slender to thick prismatic and columnar crystals that are
usually triangular in cross-section, tourmaline is often found with curved
striated faces. The crystal is hemimorphic because the termination at the ends
of crystals is sometimes asymmetrical. Such hemimorphic crystals form radial
daisy-like patterns that are common in a fine-grained granite called aplite. All
hemimorphic crystals are piezoelectric and generate electric charges in
response to applied mechanical stress. Most hemimorphic crystals are also
pyroelectric or capable of generating
an electrical potential when cooled or heated.
The compositional differences and unique structure of
tourmaline make it a mineral known for exhibiting a variety of colors. In most
cases, magnesium-rich varieties of tourmalines are brown to yellow, iron-rich
ones are black to bluish-black to deep brown, and lithium-rich tourmalines are
almost any color: blue, red, green, pink, yellow, etc. Bi-colored and multicolored tourmaline
crystals are common, demonstrating variations of fluid chemistry during crystallization.
A notable example of this property is in “Watermelon Tourmaline” or a
tourmaline crystal that is pink at one end and green at the other. Some forms
of tourmaline are dichroic, meaning they change color when observed from different directions.
An example of a multicolored tourmaline is
Fluor-liddicoatite. It is a is a rare
member of the tourmaline group of minerals, with a formula of Ca(Li2Al)Al6(BO3)3Si6O18(OH)3F.
Its properties are defined in the table below: