Frank Wigglesworth Clarke.

The constitution of the natural silicates online

. (page 1 of 11)
Online LibraryFrank Wigglesworth ClarkeThe constitution of the natural silicates → online text (page 1 of 11)
Font size
QR-code for this ebook

389. 62


B M DflS Ib3


T; '



















CHAPTER I. Introduction 1 5

CHAPTER II. The silicic acids 10

CHAPTER III. The silicates of aluminum 19

General relations 19

The nephelite type 21

The garnet type 24

The feldspars and scapolites 34

The zeolites 40

The micas and chlorites 51

The aluminous borosilicates 65

Miscellaneous species 74

CHAPTER IV. Silicates of dyad bases 87

Orthosilicates 87

Metasilicates 94

Disilicates and trisilicates 107

CHAPTER V. Silicates of tetrad bases, titanosilicates, and columbosilicates 113







In the solid crust of the earth the silicates are by far the most
important constituents. They form at least nine-tenths of the entire
known mass and comprise practically all the rocks except the sand-
stones, quartzites, and carbonates, and even these exceptions are com-
monly derivatives of the silicates, which break up under various condi-
tions, yielding new bodies of their own class, together with free silica
and limestone. From the geologist's point of view, therefore, the
silicates are of fundamental importance, and a study of their inner
constitution may be reasonably expected to shed light upon many
serious problems. For example, every primitive rock or eruptive
mass contains an aggregation of silieates, each one of which is capable
of undergoing chemical change in accordance with limitations imposed
by the structure of its molecules. When these changes take place
secondary compounds, alteration products, are formed, and in time the
rock becomes transformed into new substances, quite unlike those
which originally existed. A knowledge of the processes which thus
occur should be applicable to the study of the rocks and should ulti-
mately render it possible so to investigate a metamorphosed mass as to
clearly indicate its origin. These processes are dependent on chemical
structure, and the study of this with regard to the silicates is the pur-
pose of the present memoir.

From the standpoint of the chemist the problem under consideration
is one of great importance but also of great difficulty. Some of the
difficulty is real, some only apparent. At first sight the natural sili-
cates appear to be compounds of great complexity, but this difficulty
becomes much less serious after careful examination. Few of the nat-
ural silicates exist in even an approximately pure condition; many
that seem fresh have undergone traces of alteration; isomorphous mix-
tures are exceedingly common; and much confusion is due to defective
analyses. By multiplied observations these difficulties can be elimi-
nated from the problem, but others yet remain to be disposed of. The



organic chemist, to whom most of our knowledge of chemical structure
is due, deals mainly with bodies of known molecular weight, which can
be measured by the density of a vapor or by cryoscopic methods. To
the mineral chemist such knowledge is not available, for the com-
pounds which interest him are neither volatile nor soluble, and their
molecular weights can only be inferred. The simplest empirical for-
mula of a silicate is not necessarily its true formula; the latter may be
a multiple or polymer of the former; and here we find a difficulty
which is at present almost insuperable. Strong evidence can be
brought to bear upon this side of the question, but it is only partial
evidence and not finally conclusive. The case, however, is by no
means hopeless, for even the partial solution of a problem is better
than no solution at all. An approximation is some gain, and it is
possible so to investigate the constitution of the silicates as to bring
many relations to light, developing formulae which express those rela-
tions and indicate profitable lines for future research.

The problem is open to attack along several lines, and various
methods of investigation can be brought to bear upon it. First, of
course, the empirical formula of each silicate must be definitely ascer-
tained, which involves the discussion of sufficiently numerous analyses
and the elimination of possible errors due to impurity, alteration, and
isomorphous admixtures. In this work the microscope renders impor-
tant service to the analyst and makes his results much more certain.
By the aid of the microscope many supposed mineral species have
been proved to be mixtures, and the problem of the silicates has been
thereby simplified. Indeed, the final outcome of such investigation
generally indicates, for any given natural silicate, simplicity of compo-
sition, and this is what should be expected. These compounds are, as
a rule, exceedingly stable salts, whereas complex substances are com-
monly characterized by instability. The mineral silicates are formed
in nature under conditions of high temperature or are deposited from
solutions in which many reactions are simultaneously possible, and
these circumstances are strongly opposed to any great complications
of structure. Furthermore, they are few in number, only a few hun-
dred at most being known; whereas, if complexity were the rule
among them, slight variations in origin should produce corresponding
variations in character, and millions of different minerals would be
generated. That few variations exist is presumptive evidence that
only few are possible, and hence simplicity of constitution is reason-
ably to be inferred. In fact, we find the same small range of mineral
species occurring under the same associations in thousands of widely
separated localities, a few typical forms containing a few of the com-
monest metals being almost universally distributed. The longer the
evidence is considered, the stronger the argument in favor of simple
silicate structures becomes.


The empirical formula of a silicate having been established, its
physical properties may next be considered, and of these the crystal-
line form and the specific gravity are the most important. From
identity of form, or complete isomorphism between two species, we
infer similarity of chemical structure, and the inferences thus drawn
are often of the highest value. On the other hand, dissimilarity of
form and identity of composition indicate isomerism, as for example in
the cases of andalusite and kyanite, and here again we obtain evidence
which bears directly upon the study of chemical constitution. From
the specific gravity the so-called molecular volume of a species may be
computed, and that datum gives suggestions as to the relative con-
densation of a molecule in comparison with others of similar empirical
composition. For instance, leucite and jadeite are empirically of
similar type, but the latter has by far the greater density, together
with superior hardness. It is therefore presumably more complex
than leucite, and this supposition must be taken into account in con-
sidering its ultimate formula.

From what may be called the natural history of a mineral still
another group of data can be drawn, relating to its genesis, its con-
stant associations, and its alterability. In this connection pseudo-
morphs become of the utmost interest and, when properly studied,
shed much light upon otherwise obscure problems. An alteration
product is the record of a chemical change and as such has weighty
significance. The decomposition of spodumene into eucryptite and
albite, the transformation of topaz into mica, and many like occur-
rences in nature are full of meaning with reference to the problem
now under consideration. Just here, however, great caution is nec-
essary. Mineralogic literature is full of faulty records regarding
alterations, and many diagnoses need to be revised. Pseudomorphs
have been named by guesses, based on their external appearance, and
often a compact mica has been called steatite or serpentine. Every
alteration product should be identified with extreme care, both by
chemical and by microscopical methods; for without such precau-
tions there is serious danger of error. Each supposed fact should be
scrupulously verified.

Closely allied to the study of natural alterations is their artificial
production in the laboratory. The transformation of leucite into anal-
cite, and of analcite back into leucite, is a case in point, and the admi-
rable researches of Lemberg furnish many other examples. Work of
this character is much less difficult than was formerly supposed, and
its analogy to the methods of organic chemistry renders its results
highly significant. Atoms or groups of atoms may be split off from a
molecule and replaced by others, and the information so gained bears
directly on the question of chemical structure. With evidence of this
sort relations appear which could not otherwise be recognized, and


these relations may be closely correlated with observations of natural

Evidence of the same or similar character is also furnished by the
thermal decomposition of silicates, a line of investigation which has
been successfully followed by several investigators. Thus garnet,
when fused, yields anorthite and an olivine ; talc, on ignition, liberates
silica; and the prolonged heating of ripidolite produces an insoluble
residue having the empirical composition of spinel. All such facts
have relevancy to the problem of chemical constitution, and their
number could easily be enlarged by experiment. As yet the field
has been barely scratched on the surface; upon deeper cultivation a
goodly crop may be secured.

The artificial synthesis of mineral species, with the allied study of
crystalline slags and furnace products, furnishes still more evidence of
pertinent utility. But here again caution is needed in the interpreta-
tion of results. A compound may be produced in various ways, and it
does not follow that the first method which is successful in the labora-
tory is the method pursued by nature in the depths of the earth. The
data yielded by synthesis are undoubtedly helpful in the determination
of chemical constitution, but they furnish only a small part of the
proof needed for complete demonstration, and their applicability to
geologic questions is extremely limited. For the latter purpose they
are only suggestive, not final.

Suppose now that the empirical formula of a silicate has been accu-
rately fixed, and that a mass of data such as I have indicated are avail-
able for combination with it. Suppose the physical properties to be
determined, the natural relations known, the alteration products
observed, its chemical reactions and the results of fusion ascertained ;
what then? It still remains to combine these varied data into one
expression which shall symbolize them all, and that expression will
be a constitutional formula. To develop this, the established prin-
ciples of chemistry must be intelligently applied, with due regard to
recognized analogies. The grouping of the atoms must be in accord
with other chemical knowledge; they must represent known or
probable silicic acids; and any scheme which fails to take the latter
consideration into account is inadmissible. Not merely composition,
but function also is to be represented, and the atomic linking which
leaves that disregarded may be beautiful to see but is scientifically
worthless. A good formula indicates the convergence of knowledge;
if it fulfills that purpose it is useful, even though it may be supplanted
at some later day by an expression of still greater generality. Every
formula should be a means toward this end, and the question whether
it is assuredly final is of minor import. Indeed, there is no formula
in chemistry to-day of which we can be sure that the .last word has
been spoken.


In the development of constitutional formulae for the silicates it
sometimes happens that alternatives offer between which it is difficult
to decide. Two or more distinct expressions may be possible, with the
evidence for each so strong that neither can be accepted or abandoned.
In such cases nothing can be done but to state the facts and await the
discovery of new data, to which, however, the formulae themselves may
give clues. This sort of uncertainty is peculiarly common among the
hydrous silicates, and often rises from the difficulty of discriminating
between water of crystallization, so called, and constitutional hy-
droxyl. This difficulty is furthermore enhanced by the common
occurrence of occluded water or water in so-called " solid solution/'
and also by the adsorption of water when a mineral is pulverized for
analysis. The serious nature of the latter complication was not rec-
ognized until quite recently.

In discriminating between rival formulas one rule is provisionally
admissible. Other things being equal, a symmetrical formula is more
probable than one which is unsymmetrical. Symmetry in a molecule
conduces to stability; most of the silicates are exceedingly stable; and
hence symmetry is to be expected. This rule has presumptive value
only, as an aid to judgment, and can not be held rigidly. It expresses
a probability but gives no proof. In a problem like that of the sili-
cates, however, even a suggestion of this kind may render legitimate

There is an extensive literature relative to the constitution of the
silicates, which, however, has been well summarized by Doelter, 1
whose summary need not be duplicated here. When necessary suit-
able reference will be made to the different authorities.

i Handbuch der Mineralchemie, vol. 2, pp. 61-109, 1912.


If all the silicates were salts of a single silicic acid the problem of
their constitution would be relatively simple, but this is not the case.
Many silicic acids are theoretically possible, and several of them have
representatives in the mineral kingdom, although the acids them-
selves, as such, are not certainly known. Their nature must be
inferred from their salts, and especially from their esters, and this
side of the problem is the first to be considered.

As silicon is quadrivalent, its orthoacid is necessarily represented
by one atom of the element united with four hydroxyl groups, thus
Si(OH) 4 , or, structurally:



H O Si


i O H

To this acid, orthosilicic acid, the normal silicic esters and many
common minerals correspond. Its, normal salts, reduced to their
simplest expressions, may be typically represented as follows:

Types. Examples.

R^SiO, (C 2 H 5 ) 4 Si0 4

R,Si0 4 Mg 2 Si0 4

R ni 4 (Si0 4 ) 8 Al 4 (Si0 4 ) 3

R iv SiO 4 ZrSiO 4

Any silicate in which the oxygen atoms outnumber the silicon atoms
by more than four to one, as, for example, the compound Al 2 SiO 5 , must
be regarded as a basic salt.

By elimination of water orthosilicic acid may be conceived as yield-
ing, first, metasilicic acid, H 2 SiO 3 , and, secondly, the anhydride,
SiO 2 , thus:

Si(OH) 4 O=Si=(OH) 2 O Si O


Many salts that correspond to metasilicic acid are known, but no
esters have yet been certainly obtained. The esters first described by
Ebelmen were supposed to be metasilicates, but aU recent investiga-
tions have shown them to be ortho compounds, possibly more or less
impure. Troost and Hautefeuille, however, have described an ester
having the formula (C 2 H 5 ) 8 Si 4 O 12 , which is a polymer of a metasilicate,
but its true nature has not been determined. The simplest formula?
for typical metasihcates are as follows:

Types. Examples.

R'aSiO, Na 2 Si6 3 -

R n Si0 3 MgSi0 3

R iU 2 (Si0 3 ) 3 Al 2 (Si0 3 ) 3

R iv (Si0 3 ) 2 Zr(Si0 3 ) 2

The last two examples, Al 2 (SiO 3 ) 3 and Zr(SiO 3 ) 2 , are salts not
actually known, but theoretically possible.

By the coalescence of two molecules of orthosilicic acid and suc-
cessive elimination, molecule by molecule, of water, a series of disilicic
acids may be produced, thus:

Si(OH) 4 Si=(OH) 3 0=Si OH

-H 2 -H 2 O -H 2
Si(OH) 4 Si=(OH), Si=(OH) 8 0=Si OH

The first of these new acids, ortho disilicic acid, H 6 Si 2 O 7 , is a sex-
basic acid of which several esters are known. It is therefore well
established, and a number of minerals appear to be salts of it. The
second acid is a polymer of metasilicic acid, dimetasilicic, and its
formula is H 4 Si 2 O 6 . The third compound, metadisilicic acid,
H 2 Si 2 O 5 , is represented by no esters, but among its salts are the
minerals mordenite, ptilolite, milarite and petalite. By removing
the last molecule of water the group Si 2 O 4 would remain, a multiple
of SiO 2 .

By a similar process, that is, by the elimination of water from three
or four molecules of orthosilicic acid, a series of trisilicic and quadri-
silicic acids may be theoretically developed. These higher acids
offer many possibilities for isomerism, just as we know to be the case
among the hydrocarbons. For the present, however, only the tri-
silicic series need be considered, for above that series the long chains
of atoms would presumably be unstable. At all events the higher
series are at present unnecessary for the interpretation of known



The trisilicic acids are important and develop as follows :

Two isomers. Two isomers.

SiE=(OH) 3 0=Si OH Si=(OH) 3 0=Si OH

A A i A


Si=(OH) s


Si=(OH) 3
H 8 Si 3 10

Si=(OH) 2 Si= O SMOH),

A i A A

Si==(OH) 3 Si=(OH) 3 0=Si OH Si^(OH) 3

H 6 Si 3 9 H 6 Si 3 9 H 4 Si 3 8 H 4 Si 3 8

Still another acid is possible to complete the series, H 2 Si 3 O 7 , to
which, however, no known minerals correspond. 1 The first acid
of the series, orthotrisilicic acid, has several representatives in the
mineral kingdom. The second and third, the trimetasilicic acids,
are polymers of metasilicic acid and make, with the similar acids of
the previous series, four of the same general formula, ?iH 2 SiO 3 . To
these acids the four known modifications of magnesium metasilicate
may perhaps correspond. The fourth and fifth acids are most impor-
tant, for they represent the feldspars and appear also in some micas,
the scapolites, and several other species. Their isomerism is most
suggestive and possibly accounts for such pairs of minerals as ortho-
clase and microcline, or eudidymite and epididymite, although the
latter case is doubtful. The simple name trisilicic acid may be
assigned to them, for in abundance their salts outrank all the other
acids of the series.

Now, by including the quadrisilicic .acids for the moment, ignoring
isomers, and tabulating the several compounds, 2 some interesting
relations appear:

Dehydration derivatives of orfhosilicic adds.






" Fifth

H 4 Si0 4 .

H 9 SiO,

Si0 2

H 6 Si 2 7

H 4 Si 2 O 6

H 2 Si 2 O 5 ....

SioO 4

HoSLO.n -

H 6 Si 3 9

H 4 Si,0



H IO Si 4 O n . .




H 2 Si 4 O 9


1 This acid is assumed by Tschermak to be the acid of albite.

2 This form of tabulation has also been employed by Tschermak, Zeitschr. physikal. Chemie, vol. 53,
p. 350, 1905.


This table can be extended indefinitely, with the result that in each
vertical column every member below the first differs from the one
preceding it by the addition of H 2 SiO 3 . Furthermore, the first
anhydride in each series is either H 2 Si0 3 or a multiple thereof. That
is, we have a number of homologous series, quite similar to those
with which organic chemistry has made us familiar. The final anhy-
dride in every series will be a multiple of SiO 2 , and that fact seems to
shed some light upon the possible differences between quartz glass,
tridymite or cristobalite, and quartz. The commonest associates of
quartz are the trisilicate feldspars, to which quartz may be related in
respect to its molecular magnitude. Tridymite and cristobalite, with
lower specific gravity, are less condensed than quartz and may belong
in the disilicic series. The still lighter quartz glass is perhaps the
simplest molecule of all, SiO 2 . This is hardly more than pure specula-
tion, but the observed relations are certainly suggestive. The denser
forms of silica are surely polymers of Si0 2 .

In the foregoing discussion the silicic acids have been represented
by " chain" formulae, analogous to the formulae of the aliphatic hydro-
carbons. But "ring" formulae of several types are also possible, and
some authorities prefer them. For example, one type is as follows:

(OH), (OH), (OH),

Such formulae can be extended indefinitely, but no matter how many
silicon atoms are introduced into the ring the saturated compound
will be a metasilicic acid, H 2 SiO 3 . The successive anhydrides will
correspond empirically but not structurally to some of the acids of
the previous scheme, although none can be equivalent to the higher
orthoacids. This limitation makes the ring system less general than
the linear or chain system of expressions. Such acids as H 4 SiO 4 ,
H 6 Si 2 O 7 , and H 8 Si 3 O 10 are impossible under it.

By the coalescence of two or more rings, such as is common among
the aromatic hydrocarbons, still more complex acids are conceival ,le,
thus :




or H 4 Si 3 O 8 , isomeric with the important trisilicic acids of the chain


(OH) 2 =Si Si Si Si=(OH) 2


or H 2 Si 4 O 10 , a polymer of the disilicic acid H 2 Si 2 O 5 , and so on indefi-
nitely. Here again the limitation holds that the acids with a higher
oxygen ratio than appears in the formula H 4 Si 3 O 8 are excluded from
the scheme. With triple linkings of oxygen only one silicic acid is
immediately possible, namely,


H O Si- O Si O H


or H 2 Si 2 O 5 , another isomer of the disilicic acid in the chain series.
Two such rings, however, may be linked together by an oxygen
atom, thus:


H O Si O Si O Si O Si O H

v v

or H 2 Si 4 O 9 , an acid which corresponds to no known compounds. All
possible acids which appear in the ring formulae are included in the
chain system, at least so far as their empirical formulae are considered.
It is evident, therefore, from what has been already demonstrated,
that the chain system is the most complete and general. It is not
necessary, in the present state of knowledge, to go beyond it, although
this conclusion should only be held tentatively. It is possible that
some of the simpler rings may help to interpret some cases of
isomerism. 1

So far, then, there are only a moderate number of silicic acids
whose salts appear to need consideration in interpreting the natural
silicates. They are:

Orthosilicic acid ..H 4 SiO 4

Metasilicic acid H 2 SiO 3

Orthodisilicic acid H 6 Si 2 7

Dimetasilicic acid H 4 Si 2 6

Metadisilicic acid H 2 Si 2 5

Orthotrisilicic acid H 8 Si 3 10

Trimetasilicic acid H 6 Si 3 9 (two isomers)

Trisilicic acid H 4 Si 3 O 8 (two isomers)

i Ring formulae, like some of those given in the text, are used by Vernadsky, Zeitschr. Kryst. Min., vol.
34, p. 37, 1901.


Many other acids are theoretically possible, and one of them,
H 8 Si 4 O 12 , is perhaps represented by Troost and Hautefeuille's ester'
(C 2 H 5 ) 8 Si 4 O 12 . Salts of such acids may occur in the mineral kingdom^
but so far as present evidence goes the probability of their existence
is very small.

If the natural silicates were simple normal salts of a few silicic acids
the problem of their constitution would not be difficult. But rela-
tively few of the known species are of this description; the greater
number are double salts, and even triple replacements are not uncom-
mon. Furthermore, there are acid and basic salts to be interpreted,
and the latter class offers the most serious difficulties. A basic meta-
silicate, for example, may have the same empirical composition as an
orthosilicate, so that its ratios, studied apart from other evidence, tell

1 3 4 5 6 7 8 9 10 11

Online LibraryFrank Wigglesworth ClarkeThe constitution of the natural silicates → online text (page 1 of 11)