Glikin A.E. Polymineral-Metasomatic Crystallogenesis

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4.2 Isothermal Replacement in Ternary Systems 161

1995). In the Schreinemakers diagrams the breaks are completely absent (Fig.

3.8). Interpretation of the process occurring in this system quite agrees with con-

ception of continuous miscibility (see Sect. 3.4.3). It is quite unlikely that those

insignificant structural differences hamper autoepitaxial precipitation, and the

mechanism of monocrystalline replacement remains the same even after passing

the point of isodimorphism. At the same time one cannot exclude the possibility

that the observed heterogeneity of replacement (Fig. 1.6d) is caused by isodimor-

phism of the compounds.

Some systems with absence of additivity failures contain an intermediate addi-

tional compound (so-called alyotropic compound) corresponding to a singular

point detected as a maximum or a minimum of the total solubility in the Treivus

diagram without breaks in the Schreinemakers isotherms. These points have been

revealed in K(Cl,Br)–H

O and (Na,K)AlSiO

–SiO

systems (Treivus, 2000). Most

likely, at least three systems we tested exhibit similar singular points: (K,Rb)

–H

O, characterized by close solubility values of the extreme members

and a convex Schreinemakers isotherm (Fig. 3.19); (K,NH

–H

O, also char-

acterized by a convex isotherm; and Na(Cl,Br)O

–H

O, having a profoundly con-

cave isotherm. Monocrystalline isomorphic replacements, characterized by change

of mechanism from volume-deficit to volume-excess [K(Cl,Br), (K,Rb)HC

and (K,NH

series] or vice-versa [Na(Cl,Br)O

series], take place in four of

the above systems. We consider this phenomenon to result from variation of the

isotherm steepness in different sectors of the replacement trajectory (see Sect.

3.4.2). However, this might be a property of the systems forming additional com-

pounds. From the above it can be concluded that when examining systems, which

form additional compounds, it is necessary to take into account both the isotherm

shapes and the process regularities.

In general, conodes connecting corresponding compositions of liquidus and

solidus in the Schreinemakers diagrams are not parallel, indicating variation of

the distribution coefficient according to changes occurring in the solution compo-

sition [Fig. 4.1f; see also K(Cr,S)O

–H

O system in Fig.3.8]. If the conodes con-

verge at the solidus line, the composition of the newly generated crystals

corresponding to this section of the isotherm changes insignificantly, but if the

conodes converge at the liquidus line, the composition changes abruptly and

quickly. This phenomenon is referred to as a “wide fish” in the Treivus diagram.

As it was pointed out in Sect. 3.4.3, volume-deficit replacement in the region of

slowly changing solid-phase composition produces insignificant variations of

composition at the crystal surface and in the diffusion layer and, thus, does not

result in formation of inclusions. Evidently, the signs of volume-excess replace-

ment in such a case should also be expressed weakly. As a result, the most expect-

able is prevalence of dissolution accompanied by more or less uniform deposition

of material undergoing salting-out on the surface. On the contrary, in the region

of a rapid change of the solid-phase composition the volume effect of replace-

ment should be apparent.

162 4 Physicochemical Analysis of Metasomatic Crystallogenesis

4.3 Isothermal Replacement in Complex Systems

and Formation of Poikilitic Crystals

The next level of complication consists in increasing the number of system compo-

nents, thus approaching the naturally occurring systems. It can be stated that physi-

cochemical factors of salting-out processes and crystallogenic mechanisms of

replacement in quaternary or more complex systems are principally analogous to

those of ternary systems. Nevertheless, for description of processes taking place in

quaternary systems it is essential to use three-dimensional diagrams, where isother-

mal equilibria are depicted as planes, plane intersection lines, and crosspoints of the

lines. The Schreinemakers diagrams are still important for analysis of metasomatic

processes in these systems, though methods of their application in a three-

dimensional space (Fig. 4.2) have not been considered yet.

Fig. 4.2 Schematic representation of interrelationships between physicochemical processes proceed-

ing during a metasomatic formation of poikilitic crystals in A–B–D three-salt system occurring in the

course of metasomatic replacement of crystal A with mixed component (A,B) and with the matter of

compound D having a fixed composition: c

, c

and c

represent concentrations of the components A,

B, and D, respectively; F

and F

are the initial and final figurative point of the solution; f

represents a

projection of F

on c

plane. − ΔA is a quantity of the protocrystal matter undergoing dissolution; +

ΔB and + ΔD are the quantities of the salt components of the initial solution undergoing precipitation

4.3 Isothermal Replacement in Complex Systems 163

The most important feature of all multicomponent systems is that dissolution of

a protocrystal leads to salting-out of two or more substances that formerly were to

enrich the solution, so that the result is generation of products of combined replace-

ment. Different phases of such products precipitate in a relatively independent way

in the form of different pseudomorphs or automorphs, so that they may either grow

through each other or can be deposited in various sectors of the reaction volume. If

a system contains compounds having fixed compositions, the products are polym-

ineral pseudomorphs (replacement of alum crystal with combined K

CrO

and

aggregates – reaction Ia/5, Fig. 1.8). Their analogs in systems saturated

with compounds of isomorphic series and substances, which do not belong to this

series, are monocrystalline pseudomorphs with implanted solid inclusions, which

can be classed as poikilitic (Glikin and Sinai 1991, 2004; Glikin 1996a, 2002), or

myrmekitic formations.

The most pronounced poikilitic crystals obtained in the first experiments are

shown in Figs. 1.6e–g (Glikin and Sinai 2004). The matrix (the dark, extinguished

part) is a spongy volume-deficit monocrystalline pseudomorph of (Fe,Co)SO

·7H

after CoSO

·7H

O. The light regions are composed of randomly oriented secondary

crystals implanted into the matrix, which have probably the composition correspond-

ing to the following formula: (Fe,Co)(NH

)

(SO

)

·6H

O. The implanted crystals

precipitate in the bulk of the matrix undergoing replacement, either in its center (Fig.

1.6e) or in the periphery (Fig. 1.6f), directly in the course of replacement.

Metasomatic formation of poikilitic crystals in quaternary and more compli-

cated systems was predicted (Glikin 1996a) on the basis of experimental and

theoretical data obtained for replacement of monocrystals in ternary systems

(Glikin and Sinai 1991), and physicochemical nature of this formation involves

the following. Dissolving a protocrystal of CoSO

·7H

O results in simultaneous

salting-out of isomorphic (Fe,Co)SO

·7H

O compound and non-isomorphic

(Fe,Co)(NH

)

(SO

)

·6H

O component. Isomorphic salting-out proceeds according

to volume-deficit mechanism and, owing to decreasing solubility of compounds in

(Fe,Co)SO

·7H

O series as the content of Fe in the series increases, results

in formation of a monocrystalline spongy pseudomorph. Non-isomorphic salting-out

results in precipitation of crystals in pores of the spongy pseudomorph due to a

higher diffusion rate of the precipitating substance in comparison with that of the

substance undergoing salting-out (see Sect. 1.5). A variation in inclusion distribu-

tion depends upon the process kinetics.

As a rule, the process involves the following stages: formation of the external

spongy zone, nucleation of the double-salt crystals on the pseudomorph surface and

in inclusions within the matrix in the vicinity of the replacement front, expansion

of the spongy zone toward the centre of the pseudomorph accompanied by ingrowth

of the double-salt crystals (in a way similar to that of metacrystals into the

polycrystal pseudomorph – see Figs. 1.8b–d). Progress of the spongy zone front

toward the centre of the protocrystal is accompanied by formation of zones (from

one to three) containing crystals of the double salt. Their number depends upon a

proportion between the volume of protocrystal and that of the solution. The spongy

pseudomorph preserves both its monocrystalline nature and the initial orientation

164 4 Physicochemical Analysis of Metasomatic Crystallogenesis

of the protocrystal until the process is complete; on the contrary, orientation of the

double-salt crystals is random. The process can be accompanied by dissolution of

the inclusions previously formed in the peripheral zones with subsequent precipita-

tion of the new inclusions in the central zones of the matrix.

It is to be noted that the result of the process described quite agrees with char-

acter of replacements in the ternary (two-salt) parts of the quaternary system under

discussion. Replacement of CoSO

·7H

O crystal in solution of FeSO

·7H

O results

in formation of a spongy pseudomorph of (Fe,Co)SO

·7H

O, while its replacement

in (NH

)

-containing solution produces a polycrystalline pseudomorph of

(NH

)

Co(SO

)

·6H

O made of well-faceted crystals. However, this correspondence

cannot be a rule, as particular places of deposition of replacement products in com-

plex systems and some parts of the complex systems containing a lesser number of

components might be different due to alterations in diffusion characteristics of the

system caused by change in its composition.

4.4 Polythermal Processes

Variation of temperature complicates any process as it presents an additional inde-

pendent factor influencing solubility of the components. General analysis of such

apparently diversified processes requires special researches; nevertheless, a number

of important subclasses may be singled out a priori allowing a chemical scheme of

supra-elementary processes to be completed.

1. As a rule, a monotonous variation of temperature within the limits defined by

the system phase equilibria does not change the general mechanism of metaso-

matic process, while affecting its particulars.

During polycrystalline replacement of Ia type in systems with salting-out (Fig.

4.1g), the initial and final figurative points are located on different isotherms.

Effects and mechanisms of replacement stated above also take place in systems of

this type, being more profound or less expressed depending upon lowering or rais-

ing the temperature. In fact, the only limit of temperature variation consists in a

higher solubility of the substance B undergoing replacement and a lesser solubility

of the replacing substance A in the final figurative point in comparison with the

corresponding solubilities in the initial figurative point, i.e., the final point must be

located within a right-angled triangle having its apex in the point A′ and hypotenuse

at the eutonic line. It is obvious that variation of temperature can affect the volume

effect and even change its sign to the opposite.

Varying temperature in systems forming additional compounds (Figs. 4.1b, c)

affects not only the volume effect and kinetic properties of the system, but it also

changes the mechanism of the initial stage of the process, corresponding to its pro-

ceeding within the stability region of the component A and in vicinity of the eutonic

line E

. For example, if the system b is cooled sufficiently, the initial dissolution

along A′M

line is averted. Deeper cooling causes this dissolution to transform into

4.4 Polythermal Processes 165

replacement, as it is the case in the system c. On the contrary, in the system c, the

initial replacement transforms into dissolution as the temperature rises. In the

course of formation of an insoluble additional compound (a modification of

the system c), changes in temperature affect only kinetic properties of the process.

If a process occurs under non-isothermal conditions, replacement can take place

even in salting-in systems (type II, see Sects. 1.2 and 1.4). The scheme of the proc-

ess is shown in Fig. 4.1h; it results in formation of negative products (Sinai and

Glikin 1989, 1991). It is obvious that this process can occur under a wide range of

conditions and when the slope of the trajectory ranges from 0° to 90°. Examples of

the process products are shown in Fig. 4.3.

Non-isothermal processes of crystal formation in the systems containing isomor-

phic components were discussed in detail in a series of articles (Glikin 1995a, b;

Kryuchkova et al. 2002; Glikin et al. 2003, 2007) and are summarized in Sect. 3.3

with reference to some illustrative examples. Two of the most important regularities

are as follows. First, it is a regular suppression of a metasomatic component com-

bined with growth or dissolution under conditions of increased supercooling or

overheating. Second, it is a metastable heterogeneous equilibrium existing between

the crystal matter and a supercooled solution containing isomorphic substances dis-

solved in any “foreign” ratio.

If a system contains components with negative thermal gradient of solubility,

this significantly increases diversity of the process modifications, but does not

change it in essentials. If the isotherms are “inversed” (systems with inversed solu-

bility gradients of all the components), all the processes, described above as taking

place at lowering temperature, should occur at increasing temperature. For exam-

ple, a system in which negative products can be formed only with elevation of

temperature is depicted in Fig. 4.1j; principally it is similar to the process occurring

Fig. 4.3 Combined pseudomorphs after NaNO

: positive pseudomorphs formed by NaCl

(central parts of the picture) located inside the negative ones, which are surrounded with K

aggregates: (a–c) products of replacement obtained at various degrees of supercooling ΔT, °C:

a − ΔT = 8; b − ΔT = 15; c − ΔT = 25

166 4 Physicochemical Analysis of Metasomatic Crystallogenesis

with temperature decrease shown in chart h. If isotherms intercross (combination

of components with direct and inversed solubility gradients – Fig. 4.1k) the vicis-

situde of the process can probably change. Thus, in this case, abrupt decrease of

temperature causes the protocrystal to dissolve at the initial stage of the process

without accompanying precipitation of any new formations (trajectory A′

T >

);

then replacement starts according to an ordinary scheme presented in Fig. 4.1a. If

the temperature decrease is sufficiently slow, an ordinary replacement correspond-

ing to the trajectory A′

T >

may occur.

2. Oscillating temperature regime involves a series of recrystallizations of mono-

mineral aggregates in solutions caused by cyclic alternations of the crystal

growth and dissolution stages, which were shown (Shubnikov 1918; Punin 1964,

1965) to be accompanied by gravitational separation of solution into layers. In

general, if the aggregate has polymineral composition, redistribution comprises

a significant proportion of metasomatic components (Glikin 1991, 1995b;

Glikin and Petrov 1998) that makes it rather different from the particular process

involving monomineral formations. Recrystallization experiments are given

some attention in Chapter 5, so at present only their main physicochemical

aspects are pointed out.

Variations of temperature define the states of the system varying within the range

limited by eutonic points E

and E

of two isotherms corresponding to the upper

and lower values of temperature (e.g., Figs. 4.1h, i). Generally, the trajectories of

the solution compositions are more or less arbitrary distributed between these

points, while in a particular quasi-equilibrium case they coincide with the eutonic

lines. It is possible to distinguish systems with ordinary (Fig. 4.1h and similar) and

inversed (Fig. 4.1i) eutonic lines.

Local metasomatic reactions of different types occur in the systems. Mutual

influence of crystals formed by different phases consists in salting-out or salting-in

reactions, which can accelerate or inhibit both growth and dissolution of individuals

induced by temperature variations. These reactions are induced by alterations in

concentration of the second salt component in the vicinity of one individual, while

its neighboring individual consisting of the second component also undergoes dis-

solution or growth under the action of temperature variations.

If the trajectory diverges from the quasi-equilibrium state, it falls within the

stability region of one of the phases. When the eutonic line has an ordinary shape

(Fig. 4.1h), this is the region of a slowly growing phase, while the rapidly growing

phase appears to be unstable or metastable under discussed conditions. On the

contrary, if the eutonic line is inversed (Fig. 4.1i), this is the region of a rapidly

growing phase, while the slowly growing one appears to be unstable or metastable.

The first exemplary system is likely to be more unbalanced.

Recrystallization (see Sect. 5.3) in the systems with the inversed eutonic line

may be interpreted as an alternating proceeding of direct and inverse reactions of

replacement (Fig. 4.1i). Elevation of temperature makes the system to transfer from

to E

state and brings about a replacement of A with B, while lowering the

temperature causes the reverse change and B is to be replaced with A.

4.5 Common and Different Features of the Processes 167

It is worth mentioning that influence of temperature variation upon the kinetic

properties of the process and thereby the morphology of metasomatic reaction

products is in agreement with the concepts set forth in Sect. 1.5.3.

4.5 Common and Different Features of the Processes

Proceeding in Systems Containing either Isomorphic

or Non-isomorphic Components

In spite of the fact that replacement products produced in eutonic systems and in

the systems containing isomorphic components are radically different in composi-

tion, being polycrystalline and monocrystalline, respectively, still they have much

in common both in their properties and in their physicochemical and crystalloge-

netic nature.

Physicochemical nature of salting-out of isomorphic substances is quite similar to

that of non-isomorphic compounds, as in both the cases the substance of the protoc-

rystal dissolves making the solution supersaturated with dissolved matter, which,

consequently, crystallizes out. One of the differences is formation of monocrystalline

and polycrystalline products. Monocrystalline pseudomorphs allow visualization of

some features of the volume-deficit and volume-excess processes as implanted inclu-

sions and autoepitaxial accretions. Attributes of volume effect cannot be discerned in

polycrystalline products, because they are hidden in their friable constitution.

Loosening the substance structure and its impregnation with solution in the course

of metasomatic replacement is demonstrated by volume-deficit monocrystalline

replacement. This is an example of an extreme case, because the above-mentioned

transformations occur in the most consolidated, i.e., monocrystalline, solid object,

which, at the same time, preserves its monocrystal state. Structure loosening and its

impregnation with the solution in the course of polycrystalline replacement proceed

in a similar way, but probably more efficiently. This mechanism can be the reason of

rock permeability for metasomatic solutions (an appropriate term may be “autoper-

meability”), which does not really depend upon the natural porosity, fissuring, and

intercrystal boundaries of the rock.

On the contrary, monocrystalline volume-excess replacement prevents this reac-

tion and forms a barrier for the permeability. In the course of polycrystalline

replacement an excessive substance precipitates in the rock cavities, thus blocking

ways for solutions.

Polymineral products in the form of monocrystalline and polycrystalline pseu-

domorphs also have a common physicochemical nature. In both cases the crystal

dissolution results in salting-out of two or more substances. The difference is as

follows: the monocrystalline products contain hard inclusions implanted within a

mass of solid crystal (this may seem to be paradoxical, because formation of

such inclusions as well as introduction of fluid inclusions is not considered in

traditional theories), while the polycrystalline products result from ordinary growth

of polymineral aggregate.

168 4 Physicochemical Analysis of Metasomatic Crystallogenesis

Pathways of monocrystalline and polycrystalline replacements also have some

common, and some different stages.

An equilibrium diagram of a eutonic system contains a number of regions (Fig. 4.4)

differing in the mode of processes proceeding and separated from each other with

four lines intercrossing in the figurative point F. These lines are isoconcentration

lines aa′ and bb′ (isolines of the solution compositions containing equal contents

of the components A and B, respectively), the isotherm FE

, and the slant line FE

forming a 45° angle with aa′. Domains of direct crystallization and dissolution

coincide for both types of the systems, while regions of metasomatic replacement

overlap only partially. At the same time, in a eutonic system the regions of direct

crystallization and dissolution are characterized by the presence of a metasomatic

component, and termination of metasomatic process is also possible due to super-

cooling (like in systems with isomorphic components – see Sect. 3.3).

1. The region circumscribed with aF and Fb half-lines is a domain of simultaneous

growth of phases A and B. Supersaturation can be calculated for each phase as

a difference between a corresponding coordinate, i.e., abscissa or ordinate,

respectively, of the initial figurative point F and the eutonic point E

determined

at a specified temperature T. The sequence of substance precipitation in this

region is rather complicated and involves metasomatic stages comprising a com-

bination of growth of one of the substances and dissolution of the other. Such a

combined case is depicted by z−x′ trajectory, provided the compounds have been

Fig. 4.4 The main domains and processes of crystal formation in a ternary system containing

immiscible components A and B. Explanations are given in the text

4.5 Common and Different Features of the Processes 169

previously deposited as indicated by x–z line (Treivus 1977). However, physical

nature of metasomatic component in this process is not related to the isomorphic

exchange of matter between the crystal and solution.

2. a′Fb′ region is a domain of simultaneous dissolution of the phases A and B,

where a metasomatic component may also occur, although the nature of the

component differs from that of the system containing isomorphic components.

This may happen, for instance, if the trajectory of varying solution composition

(line F–F′) reaches a certain isotherm and further proceeds along this isotherm

F′–E

, where dissolution of the crystals in phase B continues accompanied by a

simultaneous growth of the phase A according to the mechanism of volume-

deficit polycrystalline replacement.

3. Eutonic lines are functionally similar to the equilibrium trajectories Fe

in the

systems containing isomorphic substances (Fig. 3.7).

4. E

Fb′ region, which represents metasomatic replacement at elevating tempera-

ture, is similar to a

Fb′ region in Fig. 3.9, the only difference being that such a

replacement produces polycrystalline products. The replacement is volume-

deficit, somewhat similar to the isothermal replacement corresponding to the

isotherm FE

, but the deficit is greater.

5. E

region of metasomatic replacement at decreasing temperature is analogous

to the region aFa

depicted in Fig. 3.9, but differing in formation of polycrystal-

line replacement products. As it is in the systems with isomorphic components,

the line of 45° divides this region into two parts (if the coordinates are expressed

in volume units). Replacement reactions are volume-deficit within the region

FE (the deficit is lesser than it is for isothermal replacement corresponding to

the adjacent isotherm FE

), and volume-excess within EFE

region.

6. A particular case, similar to that discussed for systems containing isomorphic

components, results in termination of metasomatic process, when the tempera-

ture lowers, if ordinates of the dots F and E

coincide; supercooling, which is

necessary for such termination, must ensure equal solubilities c

of the sub-

stance undergoing replacement in the initial figurative point and in the eutonic

point corresponding to the new value of temperature (Fig. 4.4). The system

remains in metastable equilibrium, if supersaturation with B component corre-

sponding to FE

interval is less than the width of the metastable region.

7. For the systems under discussion, a′Fb region and the region above the eutonic line

are forbidden, because the compositions falling within their boundaries are

unattainable for any combination of salting-out processes and temperature varia-

tions at the initial figurative point F. There is no such prohibition for systems con-

taining isomorphic components. The reason for such a difference is as follows: In

a eutonic system, the only permissible movement of the composition figurative

points is that along the isotherm and toward the eutonic line, which limits the

admissible domain, while in the systems with continuous miscibility the movement

is permissible in both directions without limits within the range of the isotherm.

The nature of isomorphic replacement processes may be interpreted as a particular

case of replacement in systems containing phases of fixed compositions that may

170 4 Physicochemical Analysis of Metasomatic Crystallogenesis

form an infinite number of additional compounds. The diagram of a system with

infinite miscibility of solid components may be represented as a set of infinitely

narrow regions divided by extremely close eutonic lines. The process may be rep-

resented as a continuous series of successive elemental stages, similar to the dis-

crete replacements with formation of a number of additional compounds mentioned

before (see Sect. 3.2). Essentially, only a continuous change of the solid-phase

composition (that is the reason for the reaction to be unconditionally considered as

incongruent) makes this process different from the discrete one, shown in Fig. 4.1b.

This approach may be fruitful in estimation of permissible divergence of composi-

tion trajectories of the solid and liquid phases in the course of direct crystallization,

in discussion of polymodal distribution of spontaneously formed crystals in accord-

ance with their isomorphic compositions, and for analysis of other crystallogenetic

features of systems containing isomorphic components. However, this approach

requires a solid theoretical basis taking into account the phase rule.

4.6 Some Methods of Estimation of Reaction Volume Effect

Physicochemical analysis is used to estimate phase relations in various systems,

and in general it cannot provide reliable information about structure and shapes of

the products formed in the course of certain processes. Nevertheless, discovering a

correlation between the process mechanism and volume effect can help in revealing

some structural-morphological properties of the products. An illustrative example

is formation of monocrystalline pseudomorphs having a spongy structure or pos-

sessing autoepitaxial accretions that can be quite confidently predicted on the basis

of solubility data.

Volume effects in reactions of non-isomorphic salts can be directly derived from

their diagrams.

In the simplest diagram containing only two regions (Fig. 4.1a), the distances

between the initial and final figurative points in the ordinate and abscissa axes are

confined between the dash lines and correspond to the quantities of dissolved pro-

tocrystal substance and the newly generated phase. Obviously, such replacement

results in excessive volume of the new formation if the angle between the isotherm

(or a tangent to the curved isotherm in the point of interest) and the coordinate axis

of the new formation does not exceed 45° (A′A

trajectory), and in deficit of its

volume, if the angle exceeds 45° (A

E).

This means that if the amount of substance undergoing replacement is great

enough to induce a sufficient change in the solution composition, a considerable

curvature of the isotherm may be the reason for an essential change of volume

effect at different stages of the replacement process. Thus, if the isotherm is con-

cave, as shown in Fig. 4.1a, the process results in formation of box-like pseudo-

morphs, which were observed in the course of KCl and KBr replacements in

solution of K

, as well as during replacement of CuSO

·5H

O crystals in

solution (reactions Ia/11, 14, 22; see Table 1.1, Fig. 1.9). Formation of a