Glikin A.E. Polymineral-Metasomatic Crystallogenesis

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202 5 Metasomatic Transformation of Aggregates

Fig. 5.13

Dependencies of compositional characteristics of recrystallized mixture aggregates on

their initial total composition (Data from Table 5.2 (pages 190–192), explanations are given in the

text): (a–e) – mixtures (separated component is printed in bold): (a) NaNO

–K

, (b)

Ba(NO

)

–BaCl

·2H

O, (c) KAl(SO

)

·12H

O–NiSO

·7H

O, (d) NaCl–K

, (e) NH

Cl–

. Indices 42 and 21 – temperature modes 1 and 2 from Fig. 5.10a; numeric values in the

designation table are fractions of the powders (in millimeters)

It can be seen that if initial content of NaNO

was less than 35 wt%, this compo-

nent was contained solely in the aggregate, i.e., the layering did not occur. With

higher initial amounts of NaNO

layering resulted in obtaining identical composi-

tion of bimineral aggregate, which consisted of about 61.6 wt% of K

and

38.4 wt% of NaNO

(Fig. 5.12). Compositions of aggregates obtained via recrystal-

lization of divided monomineral powders were also close to the above values:

34.0 wt% of NaNO

and 66.0 wt% of K

, correspondingly, if K

was

initially placed in the upper part of the container; and 37.5 wt% of NaNO

and

62.5 wt% of K

, if NaNO

was initially in the upper part of the container.

If initial contents of NaNO

are high, recrystallization in this system is accom-

panied by formation of intermediate unstable macrostructures resulting from

cumulative recrystallization in the lower part of the container. They exist for about

1.5–2 months and then transform into the bimineral aggregate described above. Big

sectors (10–12 mm) highly enriched with K

are formed, when the initial

NaNO

content is about 70–80%, and when this content is about 80–90%, it results

in formation of 3–4 mm thick layers stretching in perpendicular directions and

enriched with K

, located at the distance of 5–6 mm from each other; the lay-

ers are gradually merged to form a bimineral aggregate. Contents of NaNO

ranging

from 90% to 95% cause formation of large joint crystals of K

(the aggregate

sizes are about 4–6 mm, while the sizes of individual crystal in the aggregates are

about 2.5 mm; content of K

in this zone is about 2–3%) with their longer

[001] axes located in perpendicular directions to the column vertical axis and also

a thin bottom bimineral layer. When the content of NaNO

in flat samples exceeds

80%, accumulations of diffused K

can be visible in randomly located sites of

the column. If the component contents are close to equal, formation of intermediate

net structures gradually transforming in uniformly grained bimineral aggregates is

observed (Fig. 5.14a).

Kinetic curves of layering the mixture containing 40–95% of NaNO

are repre-

sented in Fig. 5.15. Each curve can be divided into three definite parts and corre-

sponding to three following stages: incubation interval I (merges with abscissa),

active stage A, and stationary stage S. Increasing the initial contents of NaNO

results in shortening incubation interval and active stage as well as accelerating the

active stage.

It is important to note that recrystallization kinetics depend essentially upon the

structure of an initial aggregate. The differences in the process of transformation of

mixtures and various monomineral combinations (Figs. 5.11 and 5.12) are dis-

cussed above. Also, influence of primary granulometric heterogeneity of the aggre-

gates should be taken into consideration. It can be seen in Fig. 5.16 that the active

stage of bimineral aggregate formation in a mixture containing equal amounts of

NaNO

and K

abruptly slows down if the mixture is seeded with individual

large crystals (5–8 mm, 4–7 wt%).

At the later stages of process, morphology of individuals is characterized by

a high degree of idiomorphism. Crystals of K

are faceted as monohedrons

{100}, {010}, {001}, {110}, {11

0}, and {111

} and, as a rule, flattened in parallel

to {010} (crystal class 1, indexing according to Groth, 1906) that corresponds to

5.3 Recrystallization of Polymineral Aggregates 203

204 5 Metasomatic Transformation of Aggregates

their most abundant form. Crystals of NaNO

(crystal class 3

m2) are faceted with

{101

1}, and surprisingly have faces {0001} (up to 60% of crystals) and {112

1}.

The crystal idiomorphism distinctly increases in transition from monomineral sec-

tors to bimineral ones, idiomorphism of K

being always higher than that of

NaNO

. At the same time, K

crystals initially appearing in NaNO

mass have

needle-like shape elongated along [001].

Observations conducted in thin sections (see Fig. 1.2a) used to minimize gravi-

tation influence upon the mass exchange showed that large crystals of both NaNO

and K

dissolved when placed in monomineral fine-grained mass of the other

substance, which formed druses in cavities appearing due to dissolution. Dissolution

of NaNO

crystals was approximately twice as fast in comparison with that of

. At the same time, big crystals located in fine-grained mass of the same

substance did not change significantly at least for 1 month. Development of “comb

structures” was observed in vertical section-type samples. The structures appeared

on large (5–8 mm) flat crystals of K

surrounded by NaNO

mass; the “comb

cogs” (usually about 10) had almost the same length as the crystal and were its

continuation; they were surrounded with fine-grained NaNO

mass (Fig. 5.17).

Recrystallization results in enlargement of NaNO

and K

grains, which

can reach their maximal sizes observed under stationary condition, i.e., 2–2.5 mm

and 1.5–2 mm, respectively. The size distribution of the crystal grains is multimo-

Fig. 5.14 Intermediate net structures formed in thin sections via recrystallization of finely frac-

tionated mixtures NaNO

–K

(a), Ba(NO

)

–BaCl

·2H

O (b), KAl(SO

)

·12H

O–NiSO

·7H

)

·12H

O–CuSO

·5H

O (d)

dal. Reaching the stationary stage leads to disappearance of all differences in the

multimodal distributions of the grain sizes obtained in different series of experi-

ments (Petrov et al. 1988). Coarsening the fine-grained (0.005–0.008 mm) mixtures

of NaNO

and K

(as well as other mixture discussed below) in the course of

recrystallization in thin sections proceeds with a high rate and includes an incuba-

tion interval, and the active and stationary stages (Fig. 5.18).

Other systems are characterized by various tendencies though scarcity of the

data prevents development of an unequivocal picture.

During recrystallization, mixtures of Ba(NO

)

–BaCl

·2H

O and KAl(SO

)

12H

O–NiSO

·7H

O undergo separation into layers in accordance with the above

scheme (Figs. 5.13b, c; Table 5.2), but a fraction of substance transferred to the

Fig. 5.15 Time variation of separated amount of monomineral NaNO

in the course of recrystal-

lization of mixtures containing NaNO

(40–95%) and K

5.3 Recrystallization of Polymineral Aggregates 205

206 5 Metasomatic Transformation of Aggregates

monomineral part is higher in comparison with that of NaNO

–K

mixture. The

amount of separated substance insignificantly depends upon the temperature con-

ditions of recrystallization; it is indicated by a slight divergence of plots showing

separation of Ba(NO

)

–BaCl

·2H

O mixture into layers in different temperature

modes. The fraction size does not affect or only slightly affects the separation; the

plots drawn for different fractions of this mixture coincide almost completely. Also,

there is not any significant difference in the curves drawn for KAl(SO

)

·12H

O–

NiSO

·7H

O mixture, and their absolute values do not much differ from the charac-

teristics of the two previous compounds, despite the fact that the difference in the

grain sizes is about an order of magnitude. A mixture of KAl(SO

)

·12H

O and

CuSO

·5H

O also undergoes layering with formation of a monomineral alum aggre-

gate in the upper part of the container and a bimineral aggregate in its lower part.

Unstable intermediate structures, analogous to those of NaNO

–K

, were

observed during recrystallization of Ba(NO

)

–BaCl

·2H

O, KAl(SO

)

·12H

O–

NiSO

·7H

O, and KAl(SO

)

·12H

O–CuSO

·5H

O mixtures. The net structures

formed by fine-grained aggregates of one substance grain surrounding the individuals

of the other were observed in flat samples in the fine-grained (0.01 mm) aggregates

of all three pairs of compounds (Figs. 5.14b–d); the individuals were represented by

a separated component, and the nets were formed by the other one. In the cylindrical

columns the mixture of KAl(SO

)

·12H

O and NiSO

·7H

O formed isometric spotty

segregations enriched with NiSO

·7H

O, which subsequently transformed into a lay-

ered structure (in perpendicular direction to the gravitation vector) containing 3–5 mm

thick alternating layers. Contacting of monomineral KAl(SO

)

·12H

O and

CuSO

·5H

O columns resulted in formation of the “upper” substance aggregates

Fig. 5.16 Formation of bimineral aggregate in a 50%:50% mixture of NaNO

–K

of homo-

geneous structure (1) and that containing big crystals of both substances (2)

(K-Al-alum) in the mass of the “lower” substance. When KAl(SO

)

·12H

O was

arranged in the lower zone, large (up to 2–4 mm) joints of CuSO

·5H

O crystals were

formed in its bulk; and if the arrangement was inversed, the lower CuSO

·5H

O mass

became broken into blocks divided by veins composed of KAl(SO

)

·12H

Behavior of NaCl–K

and NH

Cl–K

mixtures (Figs. 5.13d, e; Table

5.2, pages 190–192) and mixtures of KCl and K

reveals other regularities.

During recrystallization of a fine-grained fraction under low-temperature condi-

tions, mixtures of NaCl and K

do not practically undergo any separation into

layers, while elevation of temperature (and increase in the amplitude of temperature

fluctuations; see Fig. 5.10a) or increasing the amount of the fraction increases the

tendency of the separation. At elevated temperature and increasing the amount of the

fraction, the plot shapes become similar to those of the layered systems. Under

Fig. 5.17 A “comb-like” structure on a big crystal of K

surrounded by NaNO

5.3 Recrystallization of Polymineral Aggregates 207

208 5 Metasomatic Transformation of Aggregates

low-temperature conditions the mixture NH

Cl–K

undergoes insignificant

layering; elevated temperatures produce alterations similar to those occurring in the

layering systems. Mixture of equal contents of KCl and K

(78 experiments)

does not undergo layering, but predominance of KCl (80–90%) results in accumula-

tion of potassium dichromate in the form of big crystal joints chaotically distrib-

uted within the container and undergoing no changes for a long time in contrast to

the morphologically similar but unstable joint crystals of NaNO

(95%)–K

mixture. When a monomineral mass of K

is placed over a mass of KCl, big

stable joint crystals of K

are also formed in the mass of KCl; the crystals grow

bigger in the narrow bordering zones of the both substances. It is to be noted that recrys-

tallization of the above mixture was accompanied by mutual epitaxial growth of

components, including formation of KCl incrustations all over big K

crystals.

Coarsening the grains, which was determined in the series of mixtures in the

fine-grained preparations (Fig. 5.17), includes three stages (similar to the coarsening

in a NaNO

–K

mixture). As a result, the aggregate acquires a stationary state.

Coarsening of crystals in KCl–K

mixture proceeds with the least extent and

the rate in comparison with coarsening processes taking place in NaNO

–K

Ba(NO

)

–BaCl

·2H

O, and KAl(SO

)

·12H

O–CuSO

·5H

O mixtures.

Fig. 5.18 Temporal evolution of grain size in the course of recrystallization of mixtures contain-

ing equal amounts of finely fractionated components

R, mm

day

Recrystallization of coarse-grained (1–2 mm) ordered NaCl–KCl (1:1) aggre-

gates was studied in nine experiments in flat samples.

Amplitudes of the tempera-

ture fluctuations ranged from ambient conditions to 50–55°C and the period varied

from 20 to 60 min. The main peculiarity of the process appeared to be formation of

fine-grained fraction consisting mainly of KCl surrounding individual initial crys-

tals and the whole aggregate (Fig. 5.19). Newly formed crystals precipitated at the

beginning of the process; subsequently they become coarser and sometimes intergrown.

The process terminates when an aggregate consisting of coarse and fine grains

acquires a stationary state (or extremely sluggishly altering state).

Thermogradient recrystallization of aggregates was observed in four binary powder

mixtures: KAl(SO

)

·12H

O–NiSO

·7H

O, KAl(SO

)

·12H

O–CuSO

·5H

O, KCl–

, (NH

)

Ni(SO

)

·6H

O–CuSO

·5H

O (three of them were tested under thermo-

oscillatory conditions) and one ternary mixture NaNO

–K

–(NH

)

Ni(SO

)

·6H

The columns contained equal amounts of components, and the experiments were

carried out using the temperature mode shown in Fig. 5.10b.

Fig. 5.19 Exemplary recrystallized aggregate of KCl–NaCl surrounded by fine-grained precipi-

tated crystals, which also fill up the intercrystalline cavities:

(a–d) various parts of the sample (45 periods of temperature oscillations)

Mr. A. V. Talyzin performed the experimental investigations.

5.3 Recrystallization of Polymineral Aggregates 209

210 5 Metasomatic Transformation of Aggregates

The main result was common for all the mixtures and consisted in separating the

mixtures into a series of zones located in perpendicular direction to direction of

coinciding vectors of the temperature and gravity gradients. Zonal structures of all

investigated systems included a central cavity having a thickness of about 5–10 mm,

sharp contacts between the selvage zones, and asymmetric configurations of the

upper and lower selvages in respect to the cavity (Fig. 5.20). The process of separa-

tion into layers takes about 2–3 months, and after that the column is not percep-

tively changing for at least the same period.

In KAl(SO

)

·12H

O–NiSO

·7H

O mixture (Fig. 5.20a) the cavity A is delimited

by the crystal druses (3–6 mm) of both substances. Zone B having the thickness up

to 50 mm and containing large (up to 10–12 mm) zonal idiomorphic crystals of

Fig. 5.20 Columns obtained after recrystallization of mixtures in a thermogradient mode.

Explanations are given in the text

NiSO

·7H

O bound with KAl(SO

)

·12H

O crystals having the sizes up to 4 mm is

located lower. A fine-grained zone C is located below the zone B; it is enriched with

NiSO

·7H

O and is gradually transformed into a bound primary aggregate.

The cavity A in KAl(SO

)

·12H

O–CuSO

·5H

O mixture (Fig. 5.20b) is delim-

ited by crystal druses (8–10 mm) of both substances growing upward from the zone

B and also containing crystals of Cu

(OH)

. Below that cavity are the following

zones: C, which is a monomineral aggregate of CuSO

·5H

O having the thickness

up to 3–4 mm; D, which is a monomineral aggregate of Cu

(OH)

having the

thickness of about 2–12 mm; E, which is practically a monomineral aggregate con-

taining phenocrysts of CuSO

·5H

O; F, which is a series of vertical tapered out

monomineral veins having the length up to 20–35 mm. These veins look like a

continuation of the zone E and are surrounded by a fine-grained bound aggregate

of KAl(SO

)

·12H

O and CuSO

·5H

O (the initial ratio 1:1). The upper zones rep-

resent gradual transition from a zone similar to B to the bound primary aggregate.

The cavity in KCl–K

mixture is delimited by large (4–6 mm) crystals of

and by relatively symmetric upper and lower zones enriched with the same

substance and having sharp contact zones with a primary aggregate.

The cavity A in CuSO

·5H

O–(NH

)

Ni(SO

)

·6H

O mixture (Fig. 5.20c) is

delimited by crystal druses (up to 8–10 mm) made up of the substances composing

the system and (NH

)

Cu(SO

)

·6H

O. The borders between the selvages and the

main volume are sharp and their compositions are distinctly asymmetric. The upper

selvage is enriched with (NH

)

Ni(SO

)

·6H

O (B

), whereas the lower one with

(NH

)

Cu(SO

)

·6H

O and CuSO

·5H

O (B

). The upper and lower parts of the

main mass are represented by a bound primary aggregate enriched with correspond-

ing substances. The upper part contains a druse grown above the massif in a free

part of the solution and consisting of crystals of (NH

)

Ni(SO

)

·6H

O, CuSO

·5H

and (NH

)

H(SO

)

In ternary NaNO

–K

–(NH

)

Ni(SO

)

·6H

O mixture, the cavity is deline-

ated from above and below by crystal druses composed of (NH

)

Ni(SO

)

·6H

(up to 8–10 mm) with insignificant amounts of NaNO

and K

crystals. On

either side of the druse boundaries there are located a strong porous macrocrystal-

line aggregate made up of (NH

)

Ni(SO

)

·6H

O. After the aggregate there are

zones of dense macrocrystalline aggregates of K

containing up to 20% of

NaNO

, which form sharp contact zones with the main bound mass. These borders

exist even at the upper zone, but at the lower zone they have wedges intruding into

the main bound mass and gradually disappearing in it.

The results discussed in this section show a variety of transformations of indi-

viduals and phenomena of matter redistribution occurring during recrystallizations

in solutions. However, they have a number of regularities that can be traced despite

the incoherence of the experimental data.

5.3 Recrystallization of Polymineral Aggregates 211