Amazonite

Mineralogy, Crystal Chemistry, and Typomorphism

Mikhail Ostrooumov

• Publisher: Elsevier
• Year: 2015
• ISBN:
  9780128037430
• Topic: Physical Sciences
• Subtopic: Geology & Earth Sciences

Amazonite: Mineralogy, Crystal Chemistry and Typomorphism provides in-depth coverage of the geological setting, mineralogy, chemistry, spectroscopic parameters, typomorphic features, and potential technological and economical uses associated with amazonite discovery. Featuring detailed characterization of the geological settings of amazonite, as well a full investigation of its chemical composition, structural features, and properties with respect to each genetic type of granite, this essential reference covers all key aspects of amazonite and the benefits of its discovery.

Both newcomers and seasoned professionals in the field of mineralogy worldwide will find this translation of the popular Russian text a welcome addition to the literature on amazonite.

• Presents a thorough guide to amazonite, a mineral in geologic formations that often signifies the presence of rare earth elements
• Characterizes the distinctive features of influence in the geological setting of amazonite, as well as in the relationship between deposits and specific genetic types of the granitoids
• Includes discussion of amazonite science topics, including the crystal chemical and spectrometric parameters of amazonite and the gemological significance of the amazonite
• Provides the first English translation of the popular Russian reference book on the topic

Chapter 1

Research History of Amazonite

Abstract

The rich blue-green colors of amazonite have been the object of mineralogists’ and geologists’ investigations for over 200 years. Among the many hypotheses attempting to explain the cause of this specific color, none takes into account all of the crystal-chemical features of amazonitic K-feldspar. It is clear that the color of amazonite is affected by a number of parameters, which reflect the great variation in the chemical and structural peculiarities of this variety of potassium feldspar. During the last few decades, geologists and mineralogists have discussed the use of amazonite in the exploration for deposits of rare metals and rare-earth elements, and there are differing positive and negative opinions regarding this problem.

Keywords

Amazonite; 200 years of research

Chapter Outline

1.1 Discovery and Research Conducted before the End of the Nineteenth Century 1

1.2 Studies of Amazonite in the First Half of the Twentieth Century 3

1.3 Research of Recent Decades 5

The rich blue-green colors of amazonite have been the object of mineralogists’ and geologists’ investigations for over 200 years. Among the many hypotheses attempting to explain the cause of this specific color, none takes into account all of the crystal chemical features of amazonitic K-feldspar. It is clear that the color of amazonite is affected by a number of parameters, which reflect the great variation in the chemical and structural peculiarities of this variety of potassium feldspar. During the last few decades, geologists and mineralogists have discussed the use of amazonite in the exploration for deposits of rare metals and rare-earth elements, and there are differing positive and negative opinions regarding this problem.

1.1. Discovery and Research Conducted before the End of the Nineteenth Century

In the era of qualitative-descriptive mineralogy stretching from antiquity up to the end of the eighteenth century, knowledge of feldspars—the primary rock-forming minerals—was extremely scant. In mineralogical tracts from the end of this period, feldspars were differentiated only by color, using fragmentary, qualitative, and frequently imprecise data regarding their chemical composition.

Specimens of feldspars collected by naturalists from a range of deposits in Europe and Russia served as the material basis for the first quantitative observations and generalizations obtained through the methods of chemistry and crystallography. Among the specimens discovered and researched, almost all of these traveling geologists noted the green feldspar from Chebarkul’ (Il’menskie Mountains).

Soon after the discovery of the chemical element potassium by M.H. Klaproth in 1797, G. Vokelen completed the chemical analysis of the green feldspar of Siberia and established its membership among the potassium varieties. In 1801, R.J. Haüy presented the first formulation of the composition of potassium feldspars in his famous work Mineralogy. Already known at that time were the results of J.J. Bindheim’s still earlier chemical analysis of green feldspar, the source location of which was not indicated; this feldspar contained a copper impurity that sufficed as a simple explanation for the mineral’s color. Although copper was not identified in G. Vokelen’s analysis, A. Breithaupt, and later in 1866, N.I. Koshkarov, citing the research of K.F. Plattner, likewise considered copper responsible for amazonite color. The authority of these researchers served, henceforth, as the reason for the many attempts to detect a copper impurity in amazonite. Looking ahead, we should note that it was only in 1969 that researchers discovered a perfectly distinct phase of blue-green feldspar, called plagioclase-amazonite, the color of which actually was determined to be caused by a copper impurity. It is possible that the specimens that had been analyzed in the eighteenth century represented similar plagioclases.

By the nineteenth century, after numerous failed attempts to detect copper in amazonite, certain researchers discarded the opinion that the presence of this element was the reason for the stone’s color. In 1876, the prominent mineralogist A. Des Cloizeaux was the first to note amazonite’s tendency to lose color under heating to the point of incandescence. “These circumstances, as well as the constant loss under direct heating observed in the analyses, serve as definite evidence that the color of amazonstone is imparted by certain organic substances”—thus Des Cloizeaux concluded from his findings in 1891. G.G. Lebedev also asserted in his Mineralogy that the green color of amazonite is not caused, as was previously thought, by an impurity of a small quantity of copper oxide. The presence of a minute quantity of organic matter in amazonite was confirmed by K.K. Matveev in 1947 in the course of specialized experiments. However, not one of these works (as well as later works, e.g., V.N. Frolovskii) contained direct evidence of the causation of amazonite color by bitumen impurity.

Thus, by the end of the nineteenth century, the first of the prominent hypotheses for the reason of amazonite color had already been discredited to a significant degree, while the organic hypothesis advanced in its place, likewise, remained to be authoritatively proven. Remaining unquestioned was only the classification of amazonite among the potassium alkali feldspars.

Dating to this period are the first investigations of another important particularity of the constitution of amazonite: its crystal structure, which was studied from a crystallographic perspective. Prior to the 1820s, all feldspathic species known at the time, including amazonite as well as labradorite, adular, and albite, were classified as monoclines. In 1801, R.J. Haüy proposed for them the general term orthoclase, derived from the mineral’s tendency to fracture along a right angle. The first measurement of the corners between the cleavage planes of feldspars, performed in 1823 by G. Rose, enabled the discovery of the triclinic symmetry of albite, labradorite, and anorthite, and separated the latter from the feldspars proper, which, in particular, included amazonite. In 1817, F. Breithaupt divided all feldspars into orthoclasic and plagioclasic, assigning amazonstone which he named amazonite, to the latter group. In 1830, that same mineralogist described a green feldspar from Greenland that did not fracture at a right angle and was thus called a microcline.

In 1866, N.I. Koshkarov in his prominent work Materials for the Mineralogy of Russia defined amazonite as a phase of orthoclase and, furthermore, citing the research of A. Des Cloiseaux, wrote: “… not all crystals of amazonstone belong without exception to the monoclinic system; on the contrary, some of them belong to the triclinic system. All crystals having deep green color and opacity belong to the triclinic system; on the contrary, the crystals of amazonstone are rather transparent and green in parts—the essence of monoclinic crystals.” These observations, remaining either unnoticed or underappreciated by researchers of that time, in essence anticipated the conclusions reached a century later regarding the connection of amazonite color to the stone’s structural particularities.

Subsequently, in his research of a specimen of amazonite from Murzinka, A. Des Cloiseaux established precisely its triclinic optical orientation and (after A. Breithaupt) distinguished definitively the microcline as an independent type of potassium feldspar. Thus, in the last quarter of the nineteenth century, the presentation of amazonite as regards the triclinic modification of potassium feldspars was firmly established in the literature.

1.2. Studies of Amazonite in the First Half of the Twentieth Century

In 1913, V.I. Vernadsky was the first to bring attention to the high rubidium content (up to 3.12% Rb₂O) in Il’menskie amazonite, noting, however, that some orthoclases are still richer in this element.

The data obtained by V.I. Vernadsky found confirmation much later—in 1935, Iu.M. Tolmachev and A.N. Filipov, having studied the chemical composition of amazonites from various deposits in the Urals, Kola Peninsula, Madagascar, and Colorado Plateau, detected in them the presence of notable quantities of rubidium, which strongly varied according to the source deposit of the stone. Regarding this as a characteristic particularity of amazonite, the authors, moreover, observed traces of lead in all specimens, but they did not find any link between the lead and color.

On the basis of results obtained from his own research in 1938, V.M. Goldschmidt with collaborators classified the presence of rubidium in amazonite as a necessary condition for the phenomenon of the green color of potassium feldspar. Still later, in 1954, V.M. Goldschmidt, abandoning his previous hypothesis, proposed that amazonite color might be caused by thallium atoms or ions activated by natural radiation.

In 1939, N.P. Kapustin identified a correlation between the intensity of color of a given amazonite and its rubidium content; unfortunately, this discovery was accepted by certain researchers (K.K. Zhirov et al., G.S. Pliusnin) as little more than a curiosity. It should be noted that N.P. Kapustin measured the intensity of color in terms of its maximum reflection (or transparency). For that period, this methodology was fully acceptable, so this finding was considered unquestionable experimental evidence. Subsequently, Kasputin’s work was cited more than once as the evidential basis for a correlation between the color of amazonite and its rubidium content. From the point of view of contemporary understanding of the nature of mineral color, the role of rubidium as a center or precursor site of color is unlikely; however, the presence of rubidium in amazonite is not accidental and, as will be shown below, does hold a place in the aggregate of reasons that determine the amazonitization phenomenon as a whole.

At this time, progress quickly developed on new aspects of the amazonite subject: geological, genetic, and applied.

The sources of new interest in amazonite can be dated to the nineteenth century. Precisely during this period of intensive development of the Il’menskie amazonite mines, a close connection was noted between amazonite and topaz, along with rare black minerals (columbite, fergusonite) as well as rare and specific minerals such as cryolite and chiolite. Among the miners in the Il’menskie region, the prospecting significance of amazonite was well known as a reliable indicator of semiprecious stones.

From the first decades of the twentieth century, scientific research began of the Il’menskie Mountains. In 1928, E.O. Kopteva-Dvornikova described the mineral composition and characteristics of the internal texture of topaz-amazonite veins, noting in particular that in the transition zone from graphic granite to the central, interfolded amazonite and quartz, in the majority of cases albite is widespread, and there are often concentrations of rare-earth metals and minerals with volatile components.

The first synthesis of the geology, mineralogy, and genesis of amazonite was produced by A.E. Fersman [23]. In the classification he developed for granite pegmatites, amazonites are registered among the first four types together with the minerals fluorine, beryllium, and boron, and rare elements such as uranium, niobium, tantalum, and yttrium. He established the definitive characteristics of the composition and geological position of these pegmatites (see Chapter 5.2). According to Fersman’s hypothesis, amazonite in pegmatites crystallized from melting solution in one of the late (pegmatoid) geophases, that is, at temperatures of 600 to 500 °C. Furthermore, he indicated that not infrequently “the formation of amazonstone occurs in earlier or later phases—the graphic granite and pneumatylitic phases, for which formation temperatures are respectively 700–600 °C and 500–400 °C.”

A new point of view regarding the genesis of amazonite, fundamentally distinct from that mentioned above, was proposed and developed by A.N. Zavaritsky [6]. In his description of the amazonite mines of the Ilmen reserve, he devoted special attention to the character of the distribution of the variously colored sections of microcline, emphasizing the occurrence of a gradual transition from the usual potassium feldspars to amazonite, while noting that the crystals of the latter possess a bright color only on a side facing a cavity or a vein of quartz. In his opinion, this results in the impression of a subsequent change in the color of a feldspar under the action of residual dissolution, feldspar’s own version of the amazonitization process. Based on the experimental data obtained previously by N.P. Kapustin, A.N. Zavaritsky interpreted amazonitization as the process of ionic metasomatic substitution of part of a microcline’s potassium ions by rubidium ions.

In 1940 on the southwest shores of Lake Balkhash, the geologists S.I. Letnikov and B.S. Dmitrievsky described a previously unknown genetic type of amazonite-containing rock: amazonitic granite. This discovery and especially ...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• Preface
• Introduction
• Chapter 1. Research History of Amazonite
• Chapter 2. Geological Setting of Amazonite
• Chapter 3. Associated Minerals and Geochemistry of Amazonite
• Chapter 4. Morphology, Crystal Chemistry, and Properties of Amazonite
• Chapter 5. Color and Genesis of Amazonite
• Chapter 6. Significance of Amazonite
• Conclusion
• References
• Index