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A
~rRANSAC1'IONS of THE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
NEW METHOD OF USING THE PHYSICAL CHARACTERISTICS
OF MINERALS FOR THEIR IDENTIFICATION.
(Read 16th August, 1920.)
By C. J. Gray.
During my fifteen years' service in the Zululand and Natal Mines
Departments, I examined and identified very numerous mineral
samples brought or sent to me by prospectors. Rapid work was
Systematic chemical or blowpipe examination was
necessary.
impossible. Out of that experience, the method of identification now
. placed before you has arisen.
While practice gives facility in making close conjectures, it is often
necessary, and still more often advisable, that memory be assisted.
It is easy to forget that some comparatively rare mineral, or unusual
form of a common mineral, closely resembles the sample, and therefore
easy to neglect its possible presence when making a confirmatory test.
In such a case, that test's failure throws the invest.igator into a state
of uncertainty and causes loss of time, while apparent success may
lead to incorrect identification. The test may be applied to decide
between two minerals while the sample may be neither, but a mineral
which answers the test like one of the two. The less intimate and
extensive the investigator's knowledge of minerals is, the more he needs
aid in the matter.
Several tables designed to give such aid are published. Some go
but a short way and leave the enquirer to distinguish without system
between a d(;·zen or more possible minerals.
Others are more
thorough, but my experienc~ with them has not been satisfactory. To
work through them takes much time, and they sometimes fail when
applied to prospectors' specimens, which generally are not wellcrystallised and unweathered museum specimens. Such tables have
certain inherent defects. Failure to make accurately one of the
required distinctions prevents further progress in identification. Lack
of knowledge or equipment may readily cause such failure or it may
be unavoidable because the particular characteristic is. obscured in the
specimen. Those tables prescribe rigidly which characteristics must
be relied upon in the course of identification, though others may be
Iuore clearly marked or more readily determined.
I consider that the most marked and most easily determined
characteristics of the specimen examined should be used, and that
others should be sought for and used when necessary only. The scheme
of examination should be adapted to the particular specimen and
USING PHYSICAL CHARAC'rERISTICS OF METALS FOR IDENTIFICATION.
115
should not be rigid. ':rhe investigator should be allowed to shorten his
work by first using those distinctions which he considers most likely to
be useful. He should be allowed to use those tests which he can
apply. He should be aided but not fettered.
Several years ago I thought out a means of identification in
accordance with my views. It was a series of perforated sheets or
sieves, each one of which when placed upon an index shef\t cut out
those minerals which had not the particular characteristic with regard
to which the sheet was perforated.
':1'he very incomplete set then prepared proved useful, and
prompted and aided by Mr. J. S. Hedges and others, I revised and
added to the sheets.
No claim is made that every mineral can be determined exactly
by use of the set of sheets alone, but such use, will rapidly so reduce
possibilities that almost invariably l;he exact identification will be clear
tJ a man with a good knovdedge of minerals, while reference to
descriptions of a few minerals in a mineralogy, and perhaps one or two
special tests, will resolve any doubt "vhich another less skilled may
retain.
As Mr. Hedges has suggested, even the ignorant man could settle
the identification by comparison, if he had access to a mineral collection containing the few minerals with which use of the sheets indicated
that his specimen could possibly correspond.
Several books have been consulted for the data used in preparation of the sheets. Those to which I am most indebted are E. S.
Dana's" Textbook of Mineralogy" and F. M. Endlich's" Manual of
Qualitative Blowpipe Analysis."
A couple of examples will show how the sheets are used.
The first example is a mineral of which the characteristics are:
submetallic lustre; bla.ck colour; black streak; brittle; hardness slightly
above 6; infusible; specific gravity 5·8; crystalline form undefined.
':11he key sheet is first laid down and then in register upon it sheets
1.1, 11.1, IlL 1, IV.2, V.lO, VI.7, VII. 12, corresponding to the characteristics observed. One number only on the key sheet, 94, remains
in view. On reference to the list of minerals it is found that the
number is that for Columbite-'rantalite, the mineral being one of those
corresponding in composition to a mixture of Columbite and Tantalite.
The same result vi 0uld be obtained' if sheets IV.2 and VI. 7 were
omitted or V.3 used instead of V.lO. .
The second example is an almost colourless crystal, square round
the middle with a square pyramid at each end. Its other characteristics,
which can be readily determined without breaking the crystal, are:
adamantine lustre; colourless streak; hardness greater than knife
steel; and greater specific gravity than an ordinary stone. The sheets
to be used are therefore 1.2, 11.5, 111.6, V.3 and VII.2, with VIII.5 in
addition if the crystalline system is recognised as tetragonal. If sheet
VIII.5 is used the numbers remaining are 72 (Cassiterite) and 359
(Zircon). The absence of colour makes it improbable that the crystal
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TRANSACTIONS OF 'l'HE GEOLOGICAL SOCIETY OF SOUTH AFRICA.
is tinstone, and on further examination it is found that the crystal has
not the high specific gravity of tinstone, and it is therefore identified
as Zircon.
1£ the investigator does not recognise the crystalline system, he
must do without sheet VIII.5 and make further and more careful
determinations. He finds that the hardness is between 7 and 8, and
the specific gravity 4·6 and uses sheets V.lI and VII.lO. The only
number remaining is then 359, which represents Zircon.
In the examples given, the sheets have been used almost in the
order of numbering, but that is not, necessary. They may be used in
any order which is considered most convenient or likely to be effective.
I should like to make the following notes with regard to the preparation of the various sheets and their use.
Index ·List.
The index list of 361 minerals contains all common minerals and
many that are rare. A prospector is not likely to find that, anyone
of the minerals which he comes across in the field is not included.
Proba.bly any particular undetermined specimen is one of those minerals
named in larger type, as those are the most common. When complete
0'::- approximate identification has been made by the sheets, the composition of the mineral, as given in the list, will suggest appropriate
confirmatory blowpipe or chemical tests.
I.-Lustre.
'rhe lustre of a, mineral sample is often hard to name exactly.
Sometimes it is even difficult to decide whether the lustre is metallic
or non-metallic. Though the sheets are prepa.red so that several
minerals appear on both, it is safest to consider a doubtful lustre as
non-metallic. That precaution is particularly necessary if the mineraI
i., not quite opaque.
II.-Oolour of Mineral.
Like lustre, colour should, if possible; be observed on a fresh
surface. Tarnish colours, such as the blues and greens often seen on
iron pyrites, must not be used.
No sheet is given for yellow or brown minerals; as those colours
are so common and so frequently due either to partial decomposition
or to impurity that they cannot be safely used for distinction.
As the apparent colour of some minerals with metallic lustre
depends on the brightness of the light and the position and polish of
suria,ces, lead grey and steel grey minerals with metallic lustre appear
both on the sheet "White, light grey, or colourless" and the sheet
"Bla,ck, nearly bla,ck, or dark grey_"
I1I .-Oohesion.
Five sheets are induded under the head of cohesion, viz., distinct
cleavage, brittle, flexible, malleable, pulverulent or earthy, and sectile
USING PHYSICAL CHARACTERISTICS OF METALS FOR IDENTIFICATION.
117
malleable. A sample with distinct cleavage will at the same time be
brittle, flexible, malleable or seetHe, and will therefore be represented
by perforations on two sheets.
. As some forms of minerals usually cleavable do not readily show
cleavage, the absence of observed cleavage is not a distinction which
can be relied upon, therefore a sheet of non-cleavable minerals is not
given.
IV.-Hardness.
The first three sheets are for use when lack of time or of a scale
of hardness preYE,nts more accurate determination. It is sometimes
advantageous to use them in a first reduction of the number of possible
minerals by rapid and easy tests only, as, if the number then remaining is small, a,ccurate determination of the hardness may be
unnecessarv.
Though, in order to lessen dange,r of error, several minerals which
occur in earth:! form appear on the sheet, "Hardness 0 to 1," it is
safest not to use the hardness sheets when the mineral is pulverulent
or earthy.
V.-Effect of Heat.
The different effects of heat upon which the sheets are based are
mostly ascertained in the flame formed by a mouth blowpipe.
Two or more sheets may sometimes be used for the same· sample.
For instance, if the sample intumesces, it will be found both on one of
the fusibility sheets and on sheet VI.3.
VI.-Specific Gravity.
The first two sheets, like some of the hardness sheets, are for
preliminary use, or for use when exact determination is not, convenient
or practicable.
VI I.-Crystallization.
Only the practised mineralogist will use these sheets frequently.
A prospector's sample seldom shows well developed crystals and few
prospectors know sufficient crystallography to recognise a particular
crystal's system, The more skilled must beware of the pitfalls of
pseudomorphs, mimetic crystals, twins, and distorted or partially
developed crystals. When, ho"wever, the crystalline system is known,
the sheets will be very useful, and even when the investigator is not
quite certain, they may assist him to decide between the few possible
minerals left by other tests. Thus, if he doubts whether the sample
is isometric or tetragonal, but is sure that it is one or the other, he
excludes from further consideration those minerals for which there are
no perforations on either sheet VIII.1 or sheet VIII. 5.
N.B.-The index list of minerals, the Keysheet and the 66 perfora,ted
sheets used in identificatiop are not reproduced,