Tourmaline: Colour, Physical Properties, Varieties

Introduction

Tourmaline is a group of complex borosilicate mineral occurring in a wide spectrum of colours. The word “tourmaline” originates from the Sinhalese. (Sri Lanka). The word ‘tourmali’ meaning a very intense or multi-coloured stone. Tourmalines occur in a wide range of colours and can show beautiful effects due to optical properties. They are very durable and have sufficient hardness. They are brittle stone. Tourmalines of some varieties occur as bicoloured or tricoloured crystals and hence are rare. Thus, tourmalines can be considered as ideal gem material. The general mineral composition of the tourmaline group can be represented as follows: X1Y3 Al6 (BO3)3Si6O18 (OH) 4 where,X=Na and /or CaY=Mg, Al, Li, Fe2+ Sometimes, Al can be replaced by other elements and OH can be replaced by Fluorine. Transition elements in the crystal lattice gives rise to the various colours in the tourmaline.

Physical properties

Tourmaline occurs in the trigonal system. It crystallizes forming three sided prisms. It has terminated pyramidal tops with prismatic crystal habit. They can be found as clusters or in association with other minerals like quartz, feldspars etc. The sides of the prisms are generally heavily striated parallel to the direction of c-axis. The common inclusions in tourmalines include hollow or mineralized growth tubes, two phase inclusions (gas-liquid). The hollow tubes if aligned along a direction can give the tourmaline crystal a cat’s eye effect when polished. Irregular thread like inclusions and flat films are also present as inclusions.

The colour of the tourmaline covers the entire spectrum of visible colours. The colour can vary from black, brown, red, orange, yellow, pink, blue, green, violet and even colourless. The crystals can also exhibit colour zoning and form bicoloured and tricoloured tourmalines. The colour zoning may be lateral or across the prism faces perpendicular to the direction of the c-axis. The crystals are normally transparent to translucent. Some varieties may be opaque. The lusture although generally vitreous can vary from metallic to dull in some varieties. The hardness of tourmaline ranges from 7.0 to 7.5, and the crystals are brittle. The specific gravity is low ranging from 2.9 to 3.3. Tourmaline has three sets of cleavage and develops fractures that may be choncoidal or uneven. The refractive index of tourmaline is fairly high ranging from 1.62 to 1.64. It is generally highly pleochroic, mostly dichroic showing pleochroism in the shades of the body colour.

Tourmaline is uniaxial negative. Sometimes the pleochroism can occur in two distinct shades giving rise to the rare variety of colour-change tourmalines. The maximum absorption of light occurs along the c-axis. Thus, if a section of tourmaline is cut perpendicular to the c-axis, it can filter of the rays of light and act as a polarizer. Some varieties of tourmaline fluoresce in short wave ultraviolet light. This can help to distinguish tourmaline from its simulants. Another remarkably characteristic property of tourmaline is pyro-electricity and piezoelectricity. Thus tourmaline can attract dust particles or small bits of paper when it is rubbed or when heated. This can be a diagnostic test (besides the inclusions and crystal forms) in the absence of standard equipment for identifying tourmaline.

Synthetics and stimulants

Tourmaline has a very complex chemical composition which is difficult to replicate in lab settings. Hence, no synthetics of tourmaline are found in market. Attempts are on to produce synthetic varieties. So far, small millimetre scale crystals were produced in laboratory but they are unstable and explode. The colour zoned and multi coloured tourmalines are difficult to imitate. Production of tourmaline synthetics are just in the experimental stages. Simulants for tourmaline are plenty since tourmalines occur in various colours and can be confused with numerous minerals. The simulants are normally based on the colour of tourmalines and are listed below. The characteristic properties used to distinguish the simulants from tourmalines are described in the types of tourmalines.

Tourmalines can be confused with the following minerals:

• Green tourmalines: emerald, peridot, garnet (uvarovite, dementoid garnet)
• Red tourmalines: ruby, spinel, garnet (almandine)
• Pink tourmalines: kunzite, pink topaz, morganite, pink sapphire
• Blue tourmaline: aquamarine, blue topaz, zircon, sapphire.

Varieties of tourmaline

The complex chemical composition and wide range of physical properties of the tourmaline group gives rise to a lot of varieties of the gem. The important ones are listed and described below:

Elbaite

Elbaite is the most widely used gem variety of tourmaline. It is also the most colourful of tourmalines. Elbaite is a sodium –lithium borosilicate. It is highly pleochroic and is frequently bicoloured. The colour zoning is observed due to changes in conditions during formation of the crystal. Elbaite is most commonly confused with beryl. The characteristic distinction is the striation patterns which are hexagonal and across the prism faces, while those on tourmalines are parallel to c-axis.

The varieties of Elbaite are as follows:

• Achroite- a colourless transparent to transluscent variety
• Indicolite- pale to deep blue variety
• Paraiba- electric blue to blue green variety which is coloured by copper
• Rubellite-pale pink to reddish pink variety
• Verdelite- pale green to deep green variety
• Watermelon tourmaline- a bicoloured crystal having a red coloured core and a green coloured rim resembling a watermelon. Colour zoning can also occur laterally and the typical colours are violet-blue, green-pink and various shades of pink, green

The chrome

Dravite variety is green due to presence of chromium. Dravite is generally confused with Uvite and is not possible to distinguish between them physically. They are normally distinguished by chemical analysis.

Uvite

Uvite is a calcium-magnesium-iron-aluminium borosilicate and occurs as red, brown and green (very dark coloured which normally appear black). Uvite fluoresces yellow under short wave ultraviolet light. Uvite can be confused with schorl and Dravite. To distinguish between uvite and Dravite we have to rely on chemical analysis. Schorl is opaque compared to uvite which is translucent. Uvite even if deeply coloured and appears black may still show tinge of colour along the edge which are normally transluscent. Schorl and uvite occur in different environments and if associations are known, we can distinguish between them.

Buergerite

Buergerite is a sodium-iron-aluminium-fluorine borosilicate where OH ions are replaced by fluorine. It is normally brown coloured. It has a distinct bronze schiller. It can be confused with the brown varieties of Dravite. The point of distinction is the bronze schiller in Buergerite while Dravite will appear comparatively dull. Buergerite showing distinct bronze schiller

Schorl

Schorl is sodium-iron- aluminium borosilicate. It is always opaque and appears black in colour. It may show metallic lusture sometimes. Schorl can be confused with rutile and dravite. Rutile crystals will have a reddish tinge compared to the black schorl while dravite is transluscent compared to schorl which is opaque.

Heat treatment

Tourmalines normally are heat treated to enhance the colours. The brown red tourmalines are enhanced to give a good red colour. The greenish stones are enhanced to give deep emerald green colour. Dark, translucent stones can be heat treated to give good quality tourmalines. Dark deep coloured tourmalines also can be heat treated to give lighter shades.

Occurrences

Tourmalines generally occur in igneous and metamorphic rocks. They occur in granites, or pegmatites. Tourmaline is very resistant, hence it can be found as detrital minerals in sedimentary rocks. Tourmalines can also occur in metamorphic rocks. They normally occur as huge prismatic crystals in pegmatites or in clusters. The principal localities where tourmalines are found is the Ural Mountains in Russia. These are normally good quality blue, red or violet crystals occurring in yellow clay as a disintegration product of granites. Sri Lanka is probably the first known source of tourmaline. Tourmaline occurs in alluvial deposits in the south east part of Sri Lanka. These are normally yellow or brown tourmalines. Mogok in Myanmar also has extensive tourmalines in alluvial deposits of disintegrated granites and gneisses. These are normally red coloured tourmalines. Pink, light blue and green tourmalines are reported from pegmatitic dykes of China.

Afghanistan is an excellent source of some exceptional Elbaite, which occur as bicoloured or singly coloured blue, green, pink tourmalines. Brazil produces tourmalines in variety of colours, especially green and red. The tourmalines found here are normally colour zoned. Paraiba district in north-eastern Brazil is famous for the rare electric blue, blue-green variety of tourmaline which occurs in granitic pegmatite. Copper, a previously unknown colouring agent is responsible for the electric blue colour while iron-titanium and manganese shift the colour towards green and violet respectively.

Tourmalines are also found in Malawi, Nigeria, Zambia, Mozambique, Namibia and Tanzania. Tanzania produces yellow to orange colour change tourmalines. Zambia produces rare bright yellow tourmalines while Nigeria produces violet, yellow and watermelon tourmaline. Madagascar produces most gem quality tourmaline the most prized being the red variety and the rare colourless achroite. These deposits are from pegmatites. In India, tourmalines are found in the southern parts of Indian subcontinent.

Uses

Tourmalines occur in a variety of attractive colours hence it can be used in jewellery. Tourmalines can show colour change effect and chatoyancy. Tourmalines are faceted and polished to get out the best optical effects. They are generally cut into mixed or trap cut or cabochons. The rarer varieties of tourmaline are normally valuable as collector’s items. Tourmaline showing colour zoning and rare patterns can be cut into slices and re normally collector’s items.

References:

• O'Donoghue, M., n.d. Gems. sixth ed. s.l.: Elsevier.Read, P. G., 2005. Gemmology. 3rd ed. Berlington:Elsevierhttp://www.minerals.net/mineral/tourmaline.aspxhttps://www.mindat.org/min-4003.html