Tourmaline

Black tourmaline - Schorl

Black tourmaline from Brazil - Schorl

Tourmaline is a group of minerals, a crystalline boron silicate with elements such as aluminium, iron, magnesium, sodium, lithium or potassium.
Is a semi-precious stone and a gemstone with a variety of colors.

Chemical composition of tourmaline group
Tourmaline group is one of the most complicated groups of silicate, the composition varies widely becayse of isomorphous replacement and formula is generally formulated as:
XY3Z6(T6O18)(BO3)3V3W
X = Ca, Na, K and vacancy effect - imperfection in its crystalline form
Y = Li, Mg, Fe 2+, Mn 2+, Zn, Al, Cr 3+, V 3+, Fe 3+, Ti 4+, vacancy effect
Z = Mg, Al, Fe, Cr 3+, V3+
T = Si, Al, B
B = B, vacancy effect
V = Oh, O
W = OH, F, O

Some tourmalines, especially pink to red gems are altered by heat treatment to improve color.

Characteristics:
Category: Cyclosilicate
Formula (Ca, K, Na,) (Al, Fe, Li, Mg, Mn)3 (Al, Cr, Fe, V)6 (BO3)3 (Si, Al, B)6 O18 (OH, F)4
Olenite - 2,39% Na; 25,20 Al; 17,49% Si; 3,36 B; 0,10 H; 51,46% O
Schorl-(F) - 0,04% K; 1,74% Na; 0,05% Mg; 0,09% Ti; 0,69% Mn; 16,84% Al; 0,13% Zn; 12,38% Fe; 16,11% Si; 3,15% B; 0,32% H; 47,04% O; 1,40% F
Schorl - 2,18% Na; 15,37% Al; 15,91% Fe; 16% Si; 3,08% B; 0,38% H; 47,08% O
Crystal system trigonal
Color: most common black but can range from colorless to brown, red, orange, yellow, green, blue, violet, pink
Crystal habit: parallel and enlongated, acicular prisms
Fracture: uneven, small conchoidal, brittle
Hardness: 7 - 7,5 Mohs
Luster vitreous, resinous
Streak white
Transparent to opaque
Specific gravity 3,06
Density 2,82 - 3,32
Optical properties: double reflective, unaxial negative
Pleochroism - moderate to strong; red tourmaline: dark red, light red; green tourmaline: dark green, yellow green; brown tourmaline: dark brown, light brown; blue tourmaline: strong dark blue, light blue
Fluorescence: inert to very weak pink, red to violet in long and short wave
Special characteristics:
Magnetism - opaque black schorl and yellow tsilaisite are idiochromatic tourmaline with high magnetic susceptibilities due to high concentration of iron and manganese
Can have very weak, weak or medium fluorescence in short wave with blue and purple/violet color
Most common activator Cr; other: Fe, Mn
Cr peak at 691, 697, 707 nm
Fe 700 - 750 nm
Mn 560 - 570 nm

Tests:
Magnetism - opaque black schorl and yellow tsilaisite are idiochromatic tourmaline with high magnetic susceptibilities due to high concentration of iron and manganese
Hardness of 7 - 7,5 Mohs
Streak white

Varieties: most known are Elbaite, Dravite and most common is Schorl (brown, dark)
Schorl - most common variety of tourmaline, sodium iron; 95% or more of all tourmaline in nature; light brown, light gray, dark brown, black or deep gray NaFe2+3Al6Si6O18(BO3)3(OH)3OH;
 2,18% Na; 15,37% Al; 15,91% Fe; 16% Si; 3,08% B; 0,38% H; 47,08% O;
Schorl-(F) - pale brown, pale grayish blue, dark black; brown streak, 7,5 Mohs; 0,04% K; 1,74% Na; 0,05% Mg; 0,09% Ti; 0,69% Mn; 16,84% Al; 0,13% Zn; 12,38% Fe; 16,11% Si; 3,15% B; 0,32% H; 47,04% O; 1,40% F
Dravite - brown tourmaline is the sodium magnesium rich tourmaline; NaMg3Al6Si6O18(BO3)3OH
Uvite is a calcium magnesium tourmaline;
Elbaite is a lithium tourmaline elbaite is the most transparent and colorfull form of tourmaline, can have green; Na(Li1.5Al1.5)Al6Si6O18(BO3)3(OH)3OH
Adachiite - CaFe 3 Al 6 (Si 5 AlO 18) (BO 3) 3 (OH)3 OH
Bosiite - NaFe 3(Al 4 Mg 2) Si 6 O18 (BO 3) 3 (OH) 3O
Chromium dravite - has Cr 6 in formula
Feruvite - has Fe 3 in formula and Mg
Fluor buergerite - has Fe 3 and F in formula
Fluor-dravite; Fluor-elbaite; Fluor-liddicoatite; Fluor-schorl; Fluor-tsilaisite; Fluor-uvite (has Ca and F in formula)
Foitite - has Fe 2 and aluminium
Lucchesiite - Ca(Fe)3Al6Si6O18(BO3)3(OH)3O
Olenite - NaAl3Al6Si6O18(BO3)3(OH)3O - is a rich aluminum tourmaline with hardness of 7 Mohs and light pink color
Oxy-chromium-dravite; Oxy-dravite; Oxy-foitite; Oxyte-schorl; Oxy-vanadium-dravite
Povondraite
Rossmanite - has LiAl 2 in formula
Tsilaisite - NaMn3Al6Si6O18(BO3)3(OH)3OH
Vanadio-oxy-chromium-dravite
Vanadio-oxy-dravite
Achroite - colorless variety of elbaite tourmaline
Buergerite - rare tourmaline from Mexic
Indicolite - blue variety of elbaite tourmaline
Liddicoatite - uncommon member of tourmaline, calcium analogue of elbaite, containing calcium instead of sodium
Rubellite - pink to red variety of elbaite tourmaline
Watermelon tourmaline - variety of elbaite tourmaline with green color outside and red in the inside

Most common associations: quartz, microcline, albite, lepidolite, beryl, spodumene, biotite, cookeite, calcite

Distinguishing similar minerals:
Beryl - striations much finer and horizontal and usually more hexagonal in crystal form
Apatite - crystal lack striations, softer 5 Mohs
Epidote - softer 6-7 Mohs and different crystal habits