Tourmaline

Tourmaline is the call of a huge group of boron silicate minerals that share a commonplace crystal structure and similar physical residences – but range quite in chemical composition. The huge variety of compositions, in conjunction with trace elements and shade facilities, causes tourmaline to arise in extra colours and shade mixtures than some other mineral institution.

Name: The name tourmaline comes from turamali, a namegiven to the early gems from Ceylon.

Crystallography: Rhombohedral; ditrigonal-pyramidal. Crystals usually prismatic, vertically striated. A triangular prism, with three faces, prominent, which with the tendency of the prism faces to be vertically striated and to round into each other gives the crystals a cross section like a spherical triangle. Crystals are commonly terminated by base and low positive and negative rhombohedrons; scalenohedrons may be present. When doubly terminated, crystals usually show different forms at the opposite ends of the vertical axis

Composition: A complex silicate of boron and aluminum, whose composition can be expressed by the general formula WX3B3A13 (AlSi20 9)3- (0,0H,F)4, where W = Na,Ca. X = Al,Fe,,,,Li,Mg.

Diagnostic Features: Recognized usually by the characteristic rounded triangular cross section of the crystals and coal-like fracture of black variety. Distinguished from hornblende by absence of prismatic cleavage.

Tourmaline Occurrence

The maximum marvelous tourmaline crystals are fashioned by means of hydrothermal activity. These crystals form whilst warm waters and vapors deliver the factors had to shape tourmaline into pockets, voids, and fractures, which give an open area for crystal increase. The tourmaline crystals formed in these cavities range in size from tiny millimeter crystals to huge prisms weighing over 100 kilograms. One rich pocket of satisfactory tourmaline crystals can yield mineral specimens and gem materials well worth thousands and thousands of greenbacks. Many mineral creditors and gem hunters have end up rich with the aid of discovering simply this sort of treasure-stuffed cavities.

Species and varieties

Commonly encountered species and varieties:

Schorl species:

Brownish black to black—schorl,
Dravite species: from the Drave district of Carinthia

Dark yellow to brownish black—dravite,
Elbaite species: named after the island of Elba, Italy

Red or pinkish-red—rubellite variety,
Light blue to bluish green—Brazilian indicolite variety (from indigo),
Green—verdelite or Brazilian emerald variety,
Colorless—achroite variety

Tourmaline as an Accessory Mineral

The maximum common prevalence of tourmaline is as an accessory mineral in igneous and metamorphic rocks. It commonly takes place as millimeter-length crystals scattered thru granite, pegmatite, and gneiss. In this mode of prevalence, tourmaline hardly ever makes up a number percentage of the rock’s volume. The kind of tourmaline most usually discovered as an accent mineral is black schorl.

Physical Properties of Tourmaline

Chemical Classification	Boron silicate
Color	Black is the most common color. Also occurs in blue, green, yellow, pink, red, orange, purple, brown, and colorless. Single crystals are often zoned.
Streak	White when softer than the streak plate. Colorless when harder than the streak plate.
Luster	Vitreous
Diaphaneity	Transparent to translucent to nearly opaque
Cleavage	Indistinct
Mohs Hardness	7 to 7.5
Specific Gravity	2.8 to 3.3
Diagnostic Properties	Lack of visible cleavage, prismatic crystals with rounded triangular cross-sections that are often striated, vibrant colors, pleochroism.
Chemical Composition	(Ca,Na,K,[vacancy]) (Li,Mg,Fe+2,Fe+3,Mn+2,Al,Cr+3,V+3)3(Mg,Al,Fe+3,V+3,Cr+3)6 ((Si,Al,B)6O18) (BO3)3(OH,O)3 (OH,F,O)
Crystal System	Hexagonal
Uses	A popular gemstone and mineral specimen

Tourmaline Chemistry

Tourmaline is a complex boron silicate mineral with a generalized chemical composition of:

XY₃Z₆(T₆O₁₈)(BO₃)₃V₃W

Letters in the formula above represent positions in the atomic structure of tourmaline that can be occupied by ions listed below.

• X = Ca, Na, K, [] ([] = vacancy)
• Y = Li, Mg, Fe⁺², Fe⁺³, Mn⁺², Al, Cr⁺³, V⁺³
• Z = Mg, Al, Fe⁺³, V⁺³, Cr⁺³
• T = Si, Al, B
• V = OH, O
• W = OH, F, O

The complex formula and many substituting ions produce the large number of minerals in the tourmaline group. The International Mineralogical Association has recognized 32 different tourmaline minerals based upon the chemical composition of solid solution series end members. These minerals are listed in the table below.

Optical Properties

Property	Value
Formula	Na(Mg,Fe,Mn,Li,Al)3Al6 [Si6 O18](BO3 )3(O,OH,F)4
Crystal System	Hexagonal (trigonal), 3m
Crystal Habit	Striated hexagonal prisms
Cleavage	{110} and {101}
Color/Pleochroism	Blue, red, green, yellow, black, brown. (Schorl, the most common tourmaline, is black)
Optic Sign	Uniaxial (-)
Refractive Indices omega = epsilon =	1.631-1.650 1.635-1.675
Max Birefringence	0.017-0.035
Extinction	Longitudial sections show parallel extinction and are length fast.
Unit Cell	a = 15.8-16.0 Å c = 7.1-7.25 Å Z = 3
Physical Properties	H = 7 G = 2.90-3.22
Distinguishing Features	– In hand sample: columnar crystals with rounded triangular cross sections. – Concentric color zoning maybe observed in end sections. – Deep color may mask interference colors. – In plane polarized light, tourmaline crystal experiences maximum absorption perpendicular to polarizer (and c-axis). This means that they appear darkest when the long grain is oriented perpendicular to the polarizer. (Strongly Dichroic).
Occurrence	Tourmaline is a characteristic mineral in granitic pegmatites. It is an accessory mineral in granite, granodiorite, and related felsic rocks. Tourmaline is also a common mineral in schist, gneiss, quartzite, and phyllite. Also found in metasomatically altered limestone and dolomite in contact metamorphic zones.
References	Minerals in Thin Section (2000) Perkins, Dexter and Henke, Kevin. Introduction to Opticial Mineralogy (1991) Nesse, William. Introduction to Mineralogy (2000) Nesse, William.
Editors	Data sources: Emma Anderson (’05), Clementine Hamelin (’13), Olivia Leadbetter (’19).

Uses

• Tourmaline is a shamanic stone, imparting safety at some stage in ritual work. It can be used for scrying, and become traditionally used to factor out a cause of trouble or an offender, and to signify an amazing direction wherein to transport. To use Black Tourmaline in purification and safety rituals, create a ring of eight small herbal tumblestones for a protection circle or region upon an altar.
• Black Tourmaline is an first-rate grounding crystal for meditation and recuperation, and a small variety of Black Tourmalines are double-terminated, ideal for frame layouts. The linear glide of energies is greater equally in both instructions alongside the crystalline shape.
• Black Tourmaline soothes panic attacks, in particular those resulting from darkish or limited areas, or in places wherein the atmosphere is frightening. It additionally allows control fears of doctors or dentists.
• Wear this crystal for safety against moaners, whiners, complaining buddies or emotional vampires who burden you with their troubles however do nothing to enhance their situations.
  In the place of work, maintain Black Tourmaline for a couple of minutes whilst you need to awaken your thoughts or when you want complete concentration. It gives an incredible guard towards environmental pollution, electromagnetic smog, and radiation related to cell phones, computers, and different electronic gadget.
• It is a professional aid stone for miners and smartphone workers.
• In industry, Tourmalines are noticeably valued as electrical tuning circuits for undertaking television and radio frequencies. They are used for his or her durability given that high frequencies can be handed via them with out shattering, as many crystals do. [Mella, 110]
  Tourmaline strengthens the experience of smell, and in that respect, can also enhance the notion of pheromones which produces an aphrodisiac effect.

References

• Dana, J. D. (1864). Manual of Mineralogy… Wiley.
• Mindat.org. (2019): Mineral information, data and localities.. [online] Available at: https://www.mindat.org/ [Accessed. 2019].
• Smith.edu. (2019). Geosciences | Smith College. [online] Available at: https://www.smith.edu/academics/geosciences [Accessed 15 Mar. 2019].
• Wikipedia contributors. (2019, April 26). Tourmaline. In Wikipedia, The Free Encyclopedia. Retrieved 00:52, July 5, 2019, from https://en.wikipedia.org/w/index.php?title=Tourmaline&oldid=894281255