ON THE NOMENCLATURE AND CLASSIFICATION OF SILICEOUS ROCKS

V. A. SARAEV

(Presented by the scientific seminar of the Department of Petrography)

Silicon is one of the most widespread elements in nature. Among the elements composing the Earth’s crust, it occupies second place after oxygen (26%). Silica minerals account for 14% of the weight of the Earth’s crust, and siliceous rocks play a notable role in its composition.

One of the shortcomings in the study of siliceous rocks is the existing confused and unsettled nomenclature. This is explained by many reasons: the use by man of quartz and its varieties since ancient times, the absence of a general scheme for classifying siliceous rocks, the heterogeneity of the latter, the diversity of parageneses and geological conditions of formation, and the varied character of their metal content.

Existing classifications concern either individual subgroups of siliceous rocks or some of their types developed in particular regions. Thus, P. F. Ivankin and coauthors [3] distinguished in the Rudny Altai “...10 types (formations) of quartzites, which may be reduced to four main genetic types: sedimentary quartzites, quartzite-hornfelses, leaching quartzites, and deposition quartzites.” V. M. Timofeev [16] gives a classification of quartz sedimentary rocks. M. S. Shvetsov [18], and then G. I. Teodorovich [14, 15], presented a classification of silicites. One of the first attempts was the classification of the entire series of siliceous rocks by V. A. Saraev [9].

Summarizing data on the petrography of siliceous rocks, the author of the present article gives a scheme for their classification at the level of genetic and mineral types. In it the author seeks to group the entire complex (aggregate) of rocks of silica minerals so that their place in the process of petrogenesis can be represented (Table 1). The classification of siliceous rocks has been made on the basis of the empirical approach common in the geological sciences, and was developed with consideration of the complex of characteristic features and conditions of formation of these rocks.

It should be noted that the nomenclature of both siliceous rocks and varieties of quartz and its fine-grained modifications includes many hundreds of names, distinguished chiefly on the basis of color, external textural elements of structure, aggregate state, conditions of occurrence, and other secondary features. Only a few of these names are retained as synonyms. Below are considered the more clearly differentiated siliceous rocks and some varieties of silica minerals, conceptions of which are used in Russian and foreign literature.

Table 1

Classification scheme for siliceous rocks ($\mathrm{SiO_2} > 85\%$)

Note: 1) a. Primary-sedimentary—predominantly opaline silicites; b. Diagenetized—predominantly chalcedonic silicites; c. Metamorphosed—quartz silicites.
2) Synonyms are indicated in parentheses.
3) Of mixed or unclear origin.

A. Magmatic and pegmatitic formations

Norsfil'dite (Emerson, 1915 [5])—an ultracidic granite containing 88% quartz, 9% muscovite, and 3% biotite, actinolite, ore mineral, apatite, tourmaline, and zircon.

Silexite (Miller, 1915 [5])—a rock consisting almost entirely of pure quartz; it represents the end member of pegmatitic formations and occurs in the form of dikes or veins, segregations, or inclusions in related rocks or outside them. This rock closely approaches norsfil'dite.

B. Hydrothermal-metasomatic formations

1. Secondary quartzites. According to N. I. Nakovnik [6], secondary quartzites are a formation of ore-bearing, hydrothermally altered rocks of volcanic structures, forming during the waning stage of a volcano and representing a complex of regularly associated facies. It is characterized by constant quartz (partly chalcedony and opal, passing into quartz), rutile, sulfides (or iron oxides), and by minerals that are not constant but are typical of the complex: high-alumina minerals, aluminum sulfate, and native sulfur, which disappears under regional metamorphism. The composition of secondary quartzites, or, in the terminology of V. A. Sokolov [10], of cornakinites, varies from almost monomineralic quartz (monoquartzites [6]) to quartz-sericite with a predominance of sericite. Within this formation there are observed almost pure andalusite, alunite, dickite, topaz, etc., rocks. Secondary quartzites, like greisens and skarns, constitute a specific polymineral formation with a very wide range of changes in mineral composition. A characteristic feature of these formations is the presence of alumina-bearing minerals. Two genetic types [9] of secondary quartzites are distinguished (aluminous and siliceous), characterized by a definite assemblage of mineral facies and by the distinctive nature of their geological position.

2. Quartzolites. “Quartzolites” was first used by Kayser in 1929. According to Kayser [19], quartzolites are quartz rocks formed by the silicification of limestones.

According to A. N. Aleshkov [5], quartzolites consist predominantly of quartz with the presence of feldspars (feldspathic quartzolites), sericite (sericitic quartzolites), and impurities: mica, epidote, etc. They occur in the form of dikes, stocks, laccoliths, and other bodies characteristic of intrusive rocks.

A. F. Korzhinskii [4] calls quartzolites essentially quartz rocks formed in zones of rock brecciation and around fractures as a result of intensive leaching of bases and replacement of minerals.

The term “metasomatic microquartzites” [11], used by some researchers, is not suitable for this subgroup for the following reasons: 1) by right of priority; 2) the term “microquartzite” does not always reflect the structural features of these rocks—formations of medium- and coarse-grained structure can often be observed; 3) this term is contrasted with primary-sedimentary microquartzites of the third stage of alteration, or quartz silicites [18]; the introduction of the term “quartzolites” will eliminate the ambiguity in the series of siliceous rocks.

The above-mentioned authors used the term “quartzolites” for certain definite types of rocks, without noting their facies or

formational affiliation. The author of the article proposes to fix the name “quartziolites” for a quite definite subgroup of rocks and gives the following definition of them.

Quartziolites are hydrothermal-metasomatic rocks of substantially quartz composition, formed in tectonic zones and zones of near-ore metasomatism, in the near-contact parts of intrusions, at the expense of rocks of various, but chiefly carbonate, composition.

Quartziolites consist mainly of quartz (95%); among impurities (their composition depends on the original rocks) there occur carbonates, micas, garnets, diopside, epidote, graphite, hematite, pyrite. Accessory minerals are apatite, zircon, leucoxene, magnetite, rutile. By structural-textural features the following types of quartziolites are distinguished: 1) fine-grained, with preservation of the structural-textural features of the replaced rocks, with quartz grain size of 0.01–0.1 mm; 2) medium-grained, with relicts of the structural-textural features of the replaced rocks (0.1–1.0 mm); 3) coarse-crystalline, without relicts of the original rocks. The bulk density of quartziolites is 2.46–2.64. The shape of their bodies is very diverse: from veins and lenses to stock- and sheet-like deposits, linearly elongated along disjunctives. The thickness of the bodies is from tens of centimeters to 100 and more meters; their strike length is from several hundred meters to the first kilometers.

The necessity of separating substantially quartz rocks into a special “hydrothermal-metasomatic” subgroup of “quartziolites” is obvious. F. N. Shakhov [17], in a review of N. I. Nakovnik’s work Secondary Quartzites of the USSR, writes: “…in the work the author repeatedly speaks of quartzites which, having been created by the activity of hydrothermal solutions, do not enter the group of ‘secondary quartzites.’ These include, in particular, hydrothermally silicified limestones, often occurring in the fields of lead-zinc sulfide ores and skarn deposits. In the summary table… the author distinguishes no quartzites other than ‘secondary’ ones. One may suppose that he does not recognize independent formations of quartzites as the highest form of alteration developing in another setting. But there is no discussion of this question in the work, which gives the study a tinge of a certain one-sidedness.”

V. Hydrothermal formations

This group includes mainly vein rocks of substantially quartz composition, as well as formations of fine-grained varieties of quartz. The latter in the Russian geological literature bear the general name “stones” (ornamental and precious). There is no need to give a characterization of these formations, since there is an enormous literature about them, for example [2, 5, 8].

G. Sedimentary formations

Among sedimentary siliceous rocks two subgroups are distinguished. The first of them is composed predominantly of clastic grains of quartz, and the second of silica of authigenic origin. For the first group we find a quite established nomenclature [16]. The second subgroup of rocks Soviet geologists call siliceous. However, for the designation of siliceous rocks there is no single generally accepted term, as there is for the designation, for example, of limestones, which causes well-known inconveniences. The author adheres to the terminology proposed by G. I. Te-

by Odorovich [13] of the term “silicites.” To use L. V. Pustovalov’s [7] term “silicolites” for this purpose would be incorrect not only because the term “silicites” was introduced into the literature earlier, but also because the term “silicolites” was proposed by Wadsworth [5] to designate both siliceous and quartz rocks. Some researchers use the term “silicilites.” However, according to Grabau [20], who first proposed this term, a silicilite is a quartz rock regardless of genesis.

Chemical silicites

Geyserites form subaerially at the outlets of hot springs, especially geysers in volcanic areas. These are tuff-like, flowstone, strongly porous or relatively dense masses of opal with a botryoidal or reniform surface. The silica is represented by a variety of opal, the water content of which ranges from 9 to 13%.

Marshallites are products of the physical and chemical weathering of fine-grained quartzites and silicified carbonate rocks. Marshallite is a loose accumulation of the finest and coarser grains of quartz—“mealy quartz.”

Chrysoprase is an apple-green, translucent variety of chalcedony, usually with a fibrous and sometimes with a fine-grained structure. Its color is due to dispersed particles of hydrated nickel silicate. Chrysoprase is rather brittle. It is usually found as a secondary formation in veinlets and filling cracks in serpentinites.

Hyalite is a colorless and transparent, glass-like stone (a variety of opal). It often occurs as crusts with a botryoidal, globular, or reniform surface, and also as stalactites. Sometimes it is faintly colored, usually in blue, green, greenish-yellow, or yellowish tones. Gradual transitions occur into translucent milky or white varieties. Hyalite is usually characterized by a higher refractive index and a lower water content in comparison with many opals.

Organogenic silicites

Diatomites consist of opal, mainly of the remains of diatom algae. These are light (bulk density 0.4–0.85), finely porous (70–90%), soft (smearing the fingers), white, light-gray, or light-yellowish rocks that adhere strongly to the tongue.

Radiolarites consist of the remains of radiolarians, known from all systems. A modern deep-water oceanic siliceous or clayey-siliceous radiolarian mud is widely distributed. Dark-gray and black crystalline radiolarites of layered and thin-layered texture have been described. The layering is explained by alternation of radiolarite with subordinate interbeds of microcrystalline silicite with recrystallized remains of radiolarians, or by the presence of interbeds of clayey radiolarite. Hard and dense silicites, differing from jaspers by a low content of radiolarians or by the complete absence of their various remains, are assigned to jasper-like rocks, or jasperoids.

Spongolites consist mainly of sponge spicules (opaline or

recrystallized); their color is varied: from white through gray, greenish-gray and grayish-green varieties to black.

Phthanites are hard black silicites, consisting of finely and cryptocrystalline silica, shaly or microlaminated, with a significant content of carbonaceous matter, clayey and ferruginous (pyrite, etc.) admixtures. Organic remains are very scarce or absent. The author, with a certain degree of conditionality, assigns cherts, tripoli, and opokas to this same type.

Chert is a rock consisting of chalcedony and microcrystalline quartz, with a characteristic splintery fracture. Cherts occur in deposits of various geological systems as layered beds in carbonate sequences and contain admixtures of various material. In the fresh state chert has a bluish-gray or bluish color, but under prolonged atmospheric action it becomes covered with a dull film. Cherts are often cut by a network of cracks filled with redeposited quartz and colored by limonite or hematite. The structure is extremely fine-grained, homogeneous. Organic remains are scarce and to a considerable extent presumed (spherical shells of radiolarians, sponge spicules).

Tripoli, in outward appearance, is similar to diatomites, but consists of the finest globular particles of opal. The bulk density is somewhat greater than that of diatomites. Between tripoli and opokas there are transitional varieties with an opoka-like appearance. In tripoli and rocks transitional from it to opokas the bulk density ranges from 0.5 to 1.82; in opokas, 1.1–1.82. The specific gravity of diatomite powder is 2.0–2.21; of opokas, 2.23–2.32. The absorptive properties of diatomite are high, and those of opokas are lower.

Opokas are light, more or less hard opaline silicites, ringing when struck, consisting of gelatinous or indistinctly microglobular opal, finely porous, and sticking to the tongue.

Cryptogenic silicites

This type of silicites includes [18] formations of obscure or mixed origin.

Jasper is a massive, fine-grained, dense quartz rock with a relatively high amount of admixtures, chiefly iron oxides, the content of which may reach 20% and more. The coloration of jaspers is in red and green (to black) tones of varying intensity. Jaspers may be variegated. The pigment is finely disseminated hematite, goethite, clay, chlorite. The fracture is usually even and smooth, gradually passing into conchoidal. The structure is fine-grained. Jaspers occur as very large bodies or entire formations of sedimentary or metamorphogenic origin and form large-blocky outcrops. Interbeds of jasper, alternating with beds of black or reddish fine-grained hematite, are part of jaspilites—metamorphosed rocks associated with Precambrian iron formations.

Itabirite is a quartz–hematite rock with fine fissility or shaliness and characteristic relatively large hematite crystals. The name “jaspery onyx” denotes finely banded jaspers in which light and dark bands alternate. Velvety-black jasper, used for millennia to determine the gold content in its alloys by means of streak color, is called lydite, and also touchstone, or basanite.

Siliceous shales are fairly widespread

rocks of Precambrian–Cambrian strata. They most often have a bedded texture (from thin- to thick-bedded), expressed in the alternation of the color (gray tones predominate) of different interbeds. A characteristic feature is schistosity. They consist mainly of quartz. Impurities are calcite, dolomite, sericite, chlorite, biotite, carbonaceous matter, pelite, and pyrite. Grain sizes are 0.01–0.1 mm.

Microquartzites are dense rocks of lithoid appearance, ranging in color from white tones to black, bedded, microgranular, and jasper-like. They occur as interbeds from several centimeters to 2–5–20–40 m thick in Riphean–Cambrian carbonate deposits. These rocks are close to chert.

Novaculite is a white, uniformly grained rock without signs of bedding, composed entirely of finely crystalline quartz. When studied with optical and electron microscopes, novaculite reveals aggregates of randomly oriented polyhedral blocks or grains of quartz with rare straight, but somewhat curved boundaries, so that, overall, such aggregates somewhat resemble soap foam.

Flints are chiefly siliceous concretions, as well as accumulations of bizarre shape. Flints consist of very dense microcrystalline silica. On fresh fracture, flint concretions usually have a light-gray or dark color, sometimes with a brownish or bluish tint. The forms of the concretions approach ellipsoidal or disk-shaped, with the planes of flattening oriented parallel to the bedding of the enclosing rocks. The sizes of the concretions are 1–30 cm, sometimes up to 1 m or more in diameter. Flint is characterized by a coarse conchoidal fracture with smooth, variously oriented surfaces and sharp edges between them; however, all transitions are observed to a splintery or irregularly serrated fracture, which is characteristic of chert. The mineral composition and structures of flints are analogous to chert and microquartzites.

It is necessary to point out that in Soviet geological literature the term “quartzites” is widely used. Analysis has shown that this term is used mainly for metamorphosed quartz sandstones, as well as for rocks of dense structure and substantially quartz composition, regardless of genesis. The author of the present article accepts the latter definition. The nomenclature and classification of clastic quartz rocks are quite clear and do not call for additional analysis.

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