Table Of ContentDEVELOPMENTS IN SEDIMENTOLOGY 6
THE IDENTIFICATION
OF DETRITAL FELDSPARS
BY
L. VAN DER PLAS
Geology and Mineralogy Department
Agricultural State University, Wageningen, The Netherlands
ELSEVIER PUBLISHING COMPANY Amsterdam London New Y6rk 1966
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WITH 66 ILLUSTRATIONS AND 40 TABLES
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PREFACE
The present text was originally prepared to inform the members of a research
team about the numerous feldspar-identification methods that have been developed
in the course of time. The members of the team, developing routine methods for
the quantitative mineralogical analysis of sedimentary rocks and soils, felt the
need for a comprehensive text on feldspar-identification methods. This text had
to cover the properties of feldspars as a group of minerals, as a number of chemical
compounds and as a family of rather comparable crystalline phases. Moreover,
the text had to contain the available information on concentration techniques
and determination procedures. The manual which meets with these requirements,
grew to the present book.
Upon surveyance of the available identification techniques, it becomes
apparent that they are in most cases suitable only for the analysis of feldspars
present in igneous or metamorphic rocks. This will soon be explained. Currently
used feldspar-identification techniques have been developed to a large extent by
petrographers dealing almost exclusively with igneous or metamorphic rocks. Such
methods start with the implicit assumption that the measurements carried out on a
small number of crystals of one mineral specimen may be generalized to a large
extent to all the crystals of this mineral present in the sample. That such an as-
sumption may be made is due to the fact that samples of igneous or metamorphic
rocks may be more or less regarded as an equilibrium assemblage of minerals in
the thermodynamical sense. As soon as these methods are applied to samples
of sediments, the objections against such implicit assumptions are clearly felt.
Sedimentologists, soil scientists and a number of other specialists expect any feld-
spar concentrate to be an assemblage of an unknown number of unknown feld-
spars from an unknown number of unknown source rocks. In this text the above
aspects of the sample and the consequences this has for the identification method
receive all the interest such a characteristic deserves.
Sedimentologists, soil scientists and others are often faced with the problem
of how to make a quantitative mineralogical analysis of samples containing both
fine and coarse particles. Identification techniques for fractions smaller than 2 p
are entirely different from those applicable to fractions having a 200-p particle
size. Still, the mineralogical composition must be expressed in such a way that it
accounts for the whole sample. This implies, for instance, that the result ,of X-ray
powder analyses and those of optical analyses must be described in as fnuch the
VI PREFACE
same way as possible. The present book will give some suggestions as to the
solution of this rather difficult problem. In addition, it will show that investigators,
working exclusively with X-ray methods, speak a language entirely different from
the one used by microscopists. Microscopists have developed a rather large
“arsenal” of terms and names, whereas the X-ray worker can distinguish only
between monoclinic and triclinic phases of varying chemical composition and of
varying obliquity. This calls for a standardized nomenclature for the description
of feldspars, at least for those feldspars found in sediments.
In the last decades ideas about feldspars have changed radically. A large
number of research workers are trying to unravel the complex relationships
between structure, chemical composition, twinning pattern, physical circum-
stances of genesis, and exsolution phenomena that have been observed in the
crystalline phases of the system KA1Si308-NaAISi3Oa-CaAl2Si208. Numerous
papers have been published in addition to the increasing number of collected
lectures held during feldspar-symposia. The results of such a vast effort in this
fascinating field of mineralogy and crystallography are of the greatest importance
to the sedimentologist, the petrographer and the soil scientist. For this reason
the purely analytical aspects of a specific identification procedure need to be
discussed against the background just mentioned.
One of the most important aspects of the quantitative mineralogical analysis
of sediments and soils is the concentration of a certain mineral or a group of
minerals for better and more efficient study. Consequently, an important part of
the text is devoted to concentration techniques of feldspars. Both the specific-
gravity concentration, as well as flotation methods, are treated. Handpicking
feldspars from stained samples is also a fast moving process. Staining techniques
are treated in detail.
THE LEVEL OF DISCUSSION
Upon writing this treatise on feldspar identification I found it rather difficult to
determine the level of discussion. The following considerations helped to reach
a compromising decision.
The petrographic microscope is a routine instrument for sedimentologists
and all of them are assumed to be familiar with the various determination tech-
niques of minerals in thin sections. Most soil scientists are in any event familiar
with, if not experts on, X-ray powder methods because they need these in a rather
specialized way for the analysis of clay minerals. Finally, a large number of
excellent handbooks are available in practically every language which give an
elementary course on X-ray powder work, as well as optical crystallography.
Therefore, the present book begins with the assumption that the reader is familiar
with the elementary aspects of X-ray powder work and with the use of a petro-
ACKNOWLEDGEMENTS VII
graphic microscope. The text provides the reader with a more or less complete
inventory of currently used identification techniques. In discussing the practical
aspects of the various procedures it takes into account the numerous limitations
experienced by the workers who study sediments and soils.
ACKNOWLEDGEMENTS
In this preface, I feel obliged to acknowledge the stimulating discussions and
good advice received from friends and colleagues. The critical remarks of my
wife concerning the wording of the presented ideas may have led to a more under-
standable text.
I wish to thank all those who contributed to this book in its embryonic,
its preliminary and its final stage. Especially my colleague, the leader of our
research team, Dr. J. Ch. L. Favejee deserves words of gratitude for his invaluable
criticism and stimulating advice. The head of our department Professor Dr.
D. J. Doeglas made valuable comments on the preliminary text. Furthermore, I
thank Dr. P..Hartman, Dr. A. C. Tobi and Dr. A. H. van der Veen for the meticu-
lous care they showed in commenting on parts of the manuscript. Moreover,
Dr. Van der Veen helped me a great deal by critically reading the whole text in
its final shape.
The workers of the ore-dressing department of the Technical University,
Delft, The Netherlands, kindly introduced me into flotation methods and Mr.
R. van Ginkel M.I. assisted in conceiving the section on this subject. Great
help was received from the people of the Mathematical Centre of the Agricultural
University of Wageningen in doing all sorts of calculations with their mechanical
equipment. Miss A. M. G. Bakker and Mr. R. Schoorl are gratefully mentioned
for their enthousiastic collaboration in optical work and in preparing and evalua-
ting numerous X-ray powder patterns. Moreover, Mr. Schoorl kindly assisted in
preparing the indexes. Last but not least I wish to thank Mr. S. Slager A.I. for
really helping me to start the work presented here.
In conclusion it must be stated that the material in this text is the fruit
of agreeable teamwork and numerous discussions. Consequently, some of the
reported ideas sprang just spontaneously from these discussions and it is hard
to trace their parentage. The personnel of the Mineralogical Laboratory and of
other branches of our Earth Science Department co-operated in every way pos-
sible. The drawings, for instance, have been expertly made by Mr. G. Buurman
and Mr. W. F. Andriessen. The flow sheets in the final chapter have been devised
and drawn by Mr. J. Bult. Mr. Z. van Druuten carefully prepared the photographs.
Thanks are also due to the authors whose diagrams, with their kind per-
mission, were used to complete this text, while at the same time I am indebted
to the many publishers who gave permission to use such material.
VIII PREFACE
Although a book like this one could not possibly have been written without
making ample use of the results of other workers in the field, the writer wishes to
state that he alone is responsible for the way in which such results have been
reported here. Moreover, he will be grateful for any criticism on the present
text as well as for any suggestion towards the development of better methods for
the identification of detrital feldspars.
Ede (The Netherlands) L. VAN DER PLAS
Chapter 1
INTRODUCTION
Feldspars constitute a group of minerals with varying amounts of sodium, po-
tassium and/or calcium in a comparable aluminium-silicium-oxyde structure. The
potassium, sodium and calcium ions are situated in large spaces within this frame-
work.
The potassium- and sodium-rich members of the group with a negligible
amount of calcium are known as alkali feldspars. These crystalline phases may
still have a rather varying chemical composition. Moreover, the structure of
phases with an identical chemical composition is not necessarily the same. The
various modifications of feldspars rich in potassium are known as sanidine,
orthoclase, microcline and adularia. If the amount of sodium surpasses the po-
tassium content, the phase is sometimes described as anorthoclase. Lamellar
aggregates consisting of potassium-rich and sodium-rich feldspars, caused by
exsolution, are known as perthites.
Members of this group of minerals rich in calcium and/or sodium, but with
a negligible amount of potassium, are known as plagioclases. The plagioclases are
named according to their chemical composition, notwithstanding the fact that
different modifications exist. The pure or rather pure sodium feldspars are known
as albites. Conventionally, albite must have less than 10 molecule percentage of
the pure calcium-feldspar composition. The following chemical boundaries, also
described in the next chapter, are used for the other categories. From 10 to 30%
the phase is known as oligoclase, from 30 to 50% as andesine, the next 20%
range is termed labradorite and the following bytownite. Anorthite is at least a
90% pure calcium feldspar. For an insight into the chemical composition see
Fig. 1.
As the next chapter deals with a more detailed description of the nature of
feldspars, we can leave this subject now and turn to the reasons for studying
feldspars. Sedimentologists and soil scientists faced with the identification of feld-
spars may still wonder why the identification of these minerals receives special
treatment. They may even wonder why attention should be payed to these minerals
at all; the reasons will be considered here.
Textbooks on geology and petrography leave no doubt as to the fact that
the crust of the earth comprises large amounts of these minerals. Igneous rocks
are estimated to contain on the average more than 60 volume percentages of feld-
spars. Another important group of rocks, the metamorphic rocks, are kiown to
2 INTRODUCTION
pure potassium
feldsrar
Fig.1. A triangular diagram illustrating the chemical composition of feldspars in terms of end
members. The nomenclature of the feldspar series is also shown. The numbered dots represent
the chemical analyses of specific feldspars listed with the same numbers in Tables I and 11.
be composed of, among other minerals, large amounts of feldspars. In this case
these minerals are either brought about by the recrystallization of the original
material, or as a result of such a recrystallization with the additional introduction
of potassium and/or sodium from elsewhere. The last mentioned process, so
important in metamorphism, is known as potassium and/or sodium metasomatism.
In the light of the foregoing discussion about the importance of feldspars in
igneous and metamorphic rocks, we may state without further proof that feldspars
must play a role in the composition of sediments and soils. They play this role
either as feldspar fragments or as the alteration products of these minerals. This
statement may be made irrespective of the fact that large amounts of sediments
came into being through a re-sedimentation of the erosion products of other
sediments.
Numerous papers, a few of which will be discussed later on, elucidate the
foregoing thesis. Feldspars are found in virtually every sample of a sediment or a
soil. It goes without saying that the quantity may vary. Moreover, the afore-
mentioned papers also provide evidence for the occurrence of feldspar fragments
in the whole range of possible particle sizes. Fractions even smaller than 2 ,u may
FELDSPARS EXPOSED TO TERRESTRIAL INFLUENCES 3
contain feldspars. On the other hand, feldspars have been found in sand fractions
of more than 500 p. This feature is familiar to those who deal frequently with
X-ray diffraction patterns of the clay fractions of such samples, or to those dealing
exclusively with sand fractions.
FELDSPARS EXPOSED TO TERRESTRIAL INFLUENCES
The behaviour of feldspars in sediments and soils can be explained by some
thermodynamical properties of these crystalline phases. Studies have shown that
feldspars are stable in a surrounding characterized by relatively high temperatures
and pressures. Both pressure and temperature are much higher than the values
they show at the earth’s surface. It seems that specific amounts of the substance
H2O favours also the origin of feldspars under certain physical circumstances.
On the other hand, there is enough evidence that feldspars can grow in an en-
vironment characterized by pressures and temperatures very similar to those found
on the surface of the earth. In the latter case, the concentration of specific ions
in the all-pervading lye is exceptional. We are not quite certain as yet whether
such feldspars grow as stable or as metastable crystalline phases.
Experiments have shown that feldspar fragments behave rather differently
under leaching conditions. Their behaviour in this case depends on numerous
conditions. Some of these are enumerated here:
(1) the pH of the surrounding liquid phase,
(2) the nature of the ions in the liquid phase,
(3) the concentration of ions in the liquid phase,
(4) the temperature of the liquid phase,
(5) the particle size of the feldspar fragments,
(6) the chemical composition of the feldspar fragments,
(7) the modification of the feldspar fragments,
(8) the presence of exsolution phenomena such as perthites.
A comparable behaviour has been demonstrated to exist through the study
of feldspar fragments in soils under the influence of different climates. It is known,
for instance, that the fine fraction present in some soils from arctic regions is still
rather rich in feldspars. Soils of about the same age found in humid tropical
regions have fine fractions that are devoid of feldspars, even if the parent material
contains this mineral in large quantities.
The analysis of these and comparable examples gives an important insight
into the aspects of feldspar alteration during the weathering of rocks, sediments
or the parent material of soils. It has been established that slight differences in the
physical circumstances, i.e., in climate, play an important role. It is self-evident
that the time factor may not be neglected. In general, we can state that feldspars
are rather susceptible to alteration under the influence of the factors fo&d fre-
4 INTRODUCTION
quently in humid tropical climates. The alteration of feldspars in moderate climates
or even in arctic environments depends on numerous factors. The amount of
water percolating through the soil profile is a variable property; the pH of this
percolating water may be either low or high. The soil may well be frozen for a
large part of the year. The type of vegetation, whether a forest or a tundra vege-
tation, is also of influence. Soil scientists will grasp the reason for the reluctance
of definite statements immediately. One may simply not say, for instance, that
“feldspars only fragmentate in arctic soils”. First of all, arctic soils do not exist
as such. The different soils found in arctic surroundings may even have properties
similar to those found in the humid tropical regions. This is due to the differences
observed in the digesting of organic material found in some of such soils.
If the surface conditions are such that the feldspars are severely attacked,
the sodium, potassium and calcium ions are rather rapidly washed away, whereas
the silicon- and aluminium-rich compounds tend to remain in situ for some time.
The resulting residue is therefore enriched in such compounds as silica, aluminium
silicates, silicium hydroxide and aluminium hydroxide. For example, the genesis
of some bauxite deposits on granites, basalts or comparable rocks is thought to be
favoured by such or similar processes.
Sedimentologists may wonder why the soil-forming processes and the deg-
radation of feldspars in the parent material of soils has been emphasized. The
answer is simple. At present, mineral fragments of sand or silt size are formed by
mechanical and thermal weathering in only subordinate amounts. As examples
we may take the formation of such fragments by the mechanical action of glaciers
or by the nocturnal chilling of rock surfaces in desert regions. It is assumed that
the greatest amount of sand, silt and clay particles is presently formed by the
influence of a vegetation. Subsequent erosion of these soils or debris will produce
a considerable load of small particles in the gullies, in the small rivers and in the
larger rivers in such an area. The irregular supply of water, due to seasonal
variations, often leads to floods in the lower plains with all the consequences a
sedimentologist can envisage. The effects of wind erosion after the retreat of
glaciers or after the harvesting of large areas producing cereals must also be con-
sidered. A third aspect is the accelerated erosion, one of the consequences of
turning primeval landscapes into arable land. Enormous amounts of sand and
clay have been made available in the last few thousand years through soil erosion.
It is only recently that man has organized his struggle against this type of erosion.
In order to stress not only the role of vegetation in the formation of sedimentary
rocks, but also the influence of such a vegetation on feldspars as well, soil-forming
processes have been considered here in some detail.
As soon as soils are washed down by rain and are transported by rivers or
by the wind, the resulting sediment may have a feldspar content different from
the parent material of the original soil. The alteration of feldspars in such soils
before erosion started may explain this. One step further, we may envisage a
