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mining word of uncertain origin, possibly connected with the old German Schor (refuse), in allusion to the occurrence of the mineral with the See also:

WASTE (O. Fr. wast, guast, gast, gaste; Lat. vastus, vast, desolate)

waste of the See also:

TIN (Lat.- stannum, whence the chemical symbol " Sn "; atomic weight =117.6, 0=16)

tin-mines. The German See also:

VILLAGE

village of Schorlau may have taken its name from the mineral. It has been suggested that the See also:

SWEDISH

Swedish See also:

FORM (Lat. forma)

form skorl has possible connexion with the word skor, brittle. Tourmaline crystallizes in the See also:

RHOMBOHEDRAL

rhombohedral See also:

DIVISION (from Lat. dividere, to break up into parts, separate)

division of the hexagonal See also:

SYSTEM

system. The crystals have generally a prismatic See also:

HABIT (through the French from Lat. habitus, from habere, to have, hold, or, in a reflective sense, to be in a certain condition; in many of the English senses the French use habitude, not habit)

habit, the prisms being longitudinally striated or even channelled. Trigonal prisms are characteristic, so that a transverse See also:

section becomes triangular or often nine-sided. By See also:

COMBINATION (Lat. combinare, to combine)

combination of several prisms the crystals may become sub-cylindrical. The crystals when doubly terminated are often See also:

HEMIMORPHIC

hemimorphic or See also:

PRESENT

present dissimilar forms at the opposite ends; thus the hexagonal prisms in fig. I are terminated at one end by rhombohedral faces, o, P, and at the other by the basal See also:

PLANE

plane k'. Doubly-terminated crystals, however, are comparatively rare ; the crystals being usually attached at one end to the See also:

MATRIX

matrix.

It is notable that prismatic crystals of tourmaline have in some cases been curved and fractured transversely; the displaced FIG. I. fragments having been cemented together by deposition of fresh mineral See also:

MATTER

matter. Tourmaline is not infrequently columnar, acicular or fibrous; and the See also:

fibres may radiate from a centre so as to form the so-called " tourmaline suns." Crystals of tourmaline present no distinct cleavage, but break with a sub-conchoidal fracture; and whilst the See also:

general lustre of the mineral is vitreous, that of the fractured See also:

SURFACE

surface is rather pitchy. The hardness is slightly above that of See also:

QUARTZ

quartz (7). The specific gravity varies according to chemical See also:

composition, that of the colourless varieties being about 3, whilst in schorl it may rise to 3.2. Tourmaline has a See also:

GREAT

great range of See also:

COLOUR (Lat. color, connected with celare, to hide, the root meaning, therefore, being that of a covering)

colour, and in many cases the crystals are curiously parti-coloured. Occasionally, though rarely, the mineral is colourless, and is then known as achroite, a name proposed by R. See also:

Hermann in 1845, and derived from the See also:

Greek axpoos (uncoloured). Red tourmaline, which when of See also:

FINE

fine colour is the most valued of all varieties, is known as See also:

RUBELLITE

rubellite (q.v.). Green tourmaline is by no means uncommon, but the See also:

BLUE (common in different forms to most European languages)

blue is rather rare and is distinguished by the name indigolite, generally written indicolite. See also:

Brown is a See also:

COMMON

common colour, and See also:

black still more common, this being the usual colour of schorl, or common coarse tourmaline. Thin splinters of schorl may, however, be blue or brown by transmitted See also:

LIGHT

light.

The See also:

double See also:

refraction of tourmaline is strong. The mineral is optically negative, the See also:

ORDINARY (med. Lat. ordinarius, Fr. ordinaire)

ordinary See also:

INDEX

index being about 1.64, and the extraordinary 1.62. Coloured tourmalines are intensely pleochroic, the ordinary See also:

ray, which vibrates perpendicular to the See also:

PRINCIPAL

principal See also:

AXIS (Lat. for " axle ")

axis, being much more strongly absorbed than the extraordinary; hence a slice cut in the direction of the principal or optic axis transmits sensibly only the extraordinary ray, and may consequently be used as a polarizing See also:

MEDIUM

medium. The brown tourmaline of Ceylon and Brazil is best adapted for this purpose, but the green is also used. Two plates properly mounted form the See also:

INSTRUMENT (Lat. instrumentum, from insincere, to build up, furnish, arrange, prepare)

instrument used by opticians for testing spectacle-lenses, and are known as the " tourmaline See also:

TONGS (O. Eng. Lange, M. Eng. tonge, cf. Du. tang, Ger. Zange, from base tang, to bite, cf. Gr. SaKVeiv)

tongs." In See also:

order to secure the best colour-effect when used as a gem-See also:

stone, the tourmaline should be cut with the table parallel to the optic axis. It was in tourmaline that the phenomenon of pyroelectricity was first observed. On being heated in See also:

PEAT (possibly connected with Med. Lat. petia, pecia, piece, ultimately of Celtic origin; cf. O. Celt. pet, O. Ir. pit, Welsh peth, portion)

peat ashes its attractive See also:

POWER [WILLIAM GRATTAN] TYRONE (1797-1841)

power was observed by the Dutch, in the See also:

early See also:

PART

part of the 18th See also:

CENTURY (from Lat. centuria, a division of a hundred men)

century; and this curious See also:

CHARACTER (Gr. xapareri7p, from xap&crew, to scratch)

character obtained for it the name of aschtrekker, or ash-drawer. J. R. See also:

HAUY, RENE JUST (1743-1822)

Hauy first pointed out the relation of pyroelectricity with hemimorphism. Tourmaline is also piezoelectric, that is, it becomes electric by pressure. If a crystal be subjected to pressure along the optic axis, it behaves as though it were contracting by reduction of temperature.

The mineral may also be rendered electric by See also:

FRICTION (from Lat. fricare, to rub)

friction, and retains the See also:

charge for a See also:

long See also:

time. Tourmaline is a boro-silicate of singularly complex composition. Indeed the word tourmaline is sometimes regarded as the name of a See also:

GROUP

group of isomorphous minerals rather than that of a definite See also:

SPECIES

species. Numerous analyses have been made, and the results discussed by a large number of authorities. In the view of S. L. Penfield and H. W. See also:

Foote all tourmaline may be derived from a boro-silicic See also:

ACID (from the Lat. root ac-, sharp; acere, to be sour)

acid of the See also:

formula H2nB25i4O21. It is believed that the See also:

HYDROGEN [symbol H, atomic weight x-oo8 (0=16)]

hydrogen is present as hydroxyl, and that this may be partially replaced by See also:

FLUORINE (symbol F, atomic weight 19)

fluorine. The tourmaline acid has probably the constitution H1s(B•OH)2Si4O19. Nine atoms of hydrogen are replaced by three of See also:

ALUMINIUM (symbol Al; atomic weight 27.0)

aluminium, and the remaining nine in part by other metals.

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