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limonite, yellow; and chalybite, white. The streak of amineral
may be either shining (e.g. argentite) or dull.
Another character depending on light is that of lustre, which
is often very characteristic in certain minerals, though it may
be considerably modified by the state of aggregation. For
example, the usual adamantine lustre of diamond is not exhibited
by the compact aggregate known as carbon ado; while earthy
masses of any mineral will be devoid of lustre. Descriptive
terms applied to the kinds of lustre are: metallic (e.g. pyrites),
adamantine (diamond), vitreous (quartz), resinous (pyromorphite),
greasy (elaeolite), waxy (chalcedony), pearly (talc,
heulandite and other minerals with a perfect cleavage), silky
(satin-spar), &c. The degrees of intensity of lustre are described
as splendent, shining, glistening, glimmering and dull, and
depend usually on the smoothness of the crystal-faces.
The phenomena of phosphorescence (q.v.), fluorescence (q.v.)
and radio-activity (q.1J.) are strikingly exhibited by some
minerals. (See FLUOR-SPAR, DIAMOND, &c.)
b. Magnetic, Electrical and Thermal Characters.-These, as far
as related to crystalline form, are discussed under crystallography
(q.'v.). Magnetite (“ lode-stone ”) is the only mineral which is
strongly magnetic with polarity; a few others, such as pyrrhotite
and native platinum, possess this character to a much less degree.
Many minerals are, however, attracted by the pole of a strong
electro-magnet, while a few (diamagnetic) are repelled.
Most minerals with a metallic lustre are good conductors of
heat and electricity; others are bad conductors. For example,
graphite is a good conductor, while diamond is a bad conductor.
Non-conductors of electricity become electrified by friction, some
positively (e. g. quartz and topaz), others negatively (e.g. sulphur
and amber). The length of time during which different
gem-stones retain their charge of frictional electricity was made
use of by R. ]. Haüy as a determinative character. For the
pyro-electrical and thermo-electrical characters of crystals
see CRYSTALLOGRAPHY. Some minerals-for example, salt,
sylvite and blende-are highly diathermanous, i.e. transparent
for heat-rays.
The specific heat and melting point of minerals are essential
characters capable of exact measurement and numerical expression,
but they are not often made use of. Different minerals
differ widely in their “ fusibility ”: the following scale of fusibility
was proposed by F. von Kobellz-I.
Stibnite (525° C.) 5. Orthoclase . (1175° C.)
2. Natrolite (965° C.) 6. Bronzite (ISOOO C.)
3. Almandine (1265° C.) 7. Quartz . (1430° C.)
4. Actinolite (12960 C.)
The melting points given above in parentheses were determined
by ]. ]oly. Stibnite readily fuses to a globule in a candle-flame,
while quartz is in fusible even on the thinnest edges before the
ordinary blowpipe.
c. Characters depending on Cohesion.-Some minerals (e.g. a
sheet of mica) are highly elastic, springing back to their original
shape after being bent. Others (e.g. tale) may be readily bent,
but do not return to their original form when released; these
are said to be pliable or fiexible. Sectile minerals (ag. chlorargyrite)
may be cut with a knife without being fractured: related
characters are malleability (ag. argentite) and ductility (e.g.
silver). The tenacity, or degree of frangibility of different
minerals varies widely: they may be brittle, tough, soft or
friable. The fractured surface produced when a mineral is
broken is called the “ fracture, ” and the kind of fracture is often
of determinative value; descriptive terms are: conchoidal (ag.
quartz, which may often be recognized by its glassy conchoidal
fracture), sub-conchoidal, uneven, even, splintery (e.g. jade),
hackly or with short sharp points (e.g. copper), &c.
In many cases when a crystallized mineral is broken it
separates in certain definite directions along plane surfaces.
This property of “cleavage” (see CRYSTALLOGRAPHY) is an
important essential character of minerals, and one which is
often of considerable assistance in their recognition. For
example, Calcite, with its three directions of perfect cleavage
parallel to the faces of a rhombohedron, may always be readily
distinguished from aragonite or quartz; or again, the perfect
cubical cleavage of galena renders this mineral always easy of
recognition.
“ Hardness, ” or the resistance which a substance offers to
being scratched by a harder body, is an important character of
minerals, and being a test readily applied it is frequently made
use of. It must, however, be remembered that the hardness of
an incoherent or earthy aggregate of small crystals will be very
different from that of a single crystal. A comparative “ scale
of hardness ” was devised by F. Mohs in 182O for the purpose
of giving a numerical statement of the hardness of minerals.
Mohs's Scale of Hardness.
I. Talc. 6.
Orthoclase.
2. Gypsum. 7. Quartz.
3. Calcite. 8. Topaz. .
4. Fluor-spar. 9. Corundum.
5. Apatite. Io. Diamond.
for standards, are sucselected
softest, to diamond the hardest
is readily scratched by gypsum,
A mineral which is capable of
These minerals, arbitrarily
cessively harder from talc the
of all minerals: a piece of talc
and so on throughout the scale.
scratching calcite and itself be as easily scratched by fluor-spar
is said to have a hardness of 3%.
error in the determination of
Some care is required to avoid
hardness: it is best to select a
smooth crystal-face, cleavage-surface or fracture on which to
rub a sharp corner of the scratching mineral; the powder should
be wiped off and the surface examined with a lens to see if a
scratch has really been produced or only powder rubbed off the
corner of the mineral with which the scratching was attempted.
With a little practice a fair idea of the hardness of a mineral may
be obtained with the use of a knife or file, which will scratch all
minerals with a hardness of 6 or less. Thus iron-pyrites (H. = 6%-)
and copper-pyrites (H. = 35), apatite (H. = 5) and beryl
(H. = 7%), or gem-stones and their paste imitations may be
readily distinguished by this test. Talc and gypsum can be
readily scratched with the finger-nail.
Planes of parting, etching figures, pressure- and percussion figures
are sometimes characters of importance in describing and
distinguishing minerals. (See CRYSTALLOGRAPHY.)
d. S pecijic Gravity.-The density or specific gravity of
minerals is an essential character of considerable determinative
value. In minerals of constant composition it has a definite
value, but in isomorphous groups it varies with the composition:
it also, of course, varies with the purity of the material. It is
a character which has the advantage of numerical expression:
minerals range in specific gravity from r-or for copalite to 22'84
for iridium. The exact determination of the specific gravity
of minerals is therefore a matter of some importance. Three
methods are in common use, viz. hydrostatic weighing, the
pycnometer, and the use of heavy' liquids. The first two
methods are only applicable when a weigh able amount of pure
material can be selected or picked out; this is, however, generally
a laborious operation, since impurities are often present and
usually several species of minerals are closely associated, and in
selecting material it is often necessary to determine some other
character to make certain that only one kind is being selected.
For exact determinations the pycnometer method is usually to be
recommended, using for material the pure fragments which have
been selected for quantitative chemical analysis. With a single
pure crystal or a faceted gem-stone the method of hydrostatic
weighing is usually applicable, providing the stone is not too
small. The most ready method, however, is that afforded by the
use of a heavy liquid, and the most convenient liquid for this
purpose is methylene iodide. This is a clear, mobile liquid with
a specific gravity of 3- 3 3, a.nd by the addition of benzene, drop
by drop, the specific gravity may be reduced to any desired
amount. With such a liquid the specific gravity of the minutest
fragment, the purity of which has previously been scrutinized
under the microscope, may be rapidly determined. The liquid
is diluted with benzene until the fragment just remains suspended,
neither floating nor sinking; the specific gravity of the fragment
will then be the same as that of the liquid, and the latter may
be determined by hydrostatic weighing or, 'more conveniently, by<noinclude><references/></div></noinclude>