Genesis of Högbomite from Gornaya Shoria

A. M. Kuzmin

This article considers the question of the origin of the rare mineral högbomite, which was found by K. V. Radugin along the Mrassu River, in the area of the mouth of its right tributary, the Tashelga River [1].

The crystallographic description of this mineral, its physical properties, and its composition are examined in detail in another article¹. In the present work, while drawing the reader’s attention to the description of the mineral itself, we shall be very brief.

Morphology of högbomite. At the above-mentioned deposit the mineral högbomite occurs in the form of small tabular, platy, and scaly crystals, often hexagonal, sometimes trigonal in outline. The sizes of the crystals range from 5 to 1 mm and less in cross section. Rarely, crystals as large as 5–10 mm occur. Their thickness rarely exceeds 2 mm, more often 0.5–1.5 mm. They are often found to be deformed to varying degrees.

Crystallographic measurements showed that, in addition to the principal habit form \(\{0001\}\), the basal pinacoid, there are very narrow faces \(\omega\{1121\}\), \(a\{1120\}\), \(X_6\{5.5.10.6\}\), \(K_8\{7.7.14.8\}\), \(K_5\{7.7.14.5\}\), \(X_3\{5.5.10.3\}\), \(K_4\{7.7.14.4\}\), which, taken together, appear in the form of striation.

Fig. 1. Högbomite crystal.

The geometric constants of högbomite are: \(a:c = 1:1.36785\), which are somewhat greater than the analogous constants of corundum and hematite and somewhat smaller than those of ilmenite, but are closer to corundum. Comparison of the structural X-ray patterns of högbomite with the X-ray patterns of corundum and ilmenite indicates that, in its structural features and composition, högbomite is closer to corundum than to ilmenite.

Taken all together, this made it possible to assign högbomite to the trigonal-scalenohedral \((G_3 3g_2 3PC)\) symmetry class.

Physical properties. The color of högbomite from Gornaya Shoria is yellow- and dark-brown to black-brown. The mineral in plates is translucent,

In thin sections 0.03 mm thick it is pale yellow, brownish yellow, with a noticeable greenish tint. Pleochroism is distinct: brownish yellow, pale yellow along \(N_m\), yellow, light yellow along \(N_p\); absorption \(N_m > N_p\). The refractive indices for högbomite are: \(N_m = 1.853 \pm 0.002\) and \(N_p = 1.803 \pm 0.003\), \(N_p - N_m = (-)0.050 \pm 0.003^{1)}\). Specific gravity is 3.80–3.82 g/cm³. The hardness of the mineral is \(8^{1}/_{2}\); it is appreciably higher than the hardness of zircon and lower than that of corundum²). The mineral is brittle; the fracture is uneven. Cleavage on \(\{0001\}\) is perfect or distinct. The luster of the mineral is pearly-adamantine on the cleavage plane, which brings högbomite close to diamond spar.

To conclude the description, we give a comparative table chiefly of the optical properties of högbomites from various deposits (Table 1).

Table 1

Optical properties of högbomite

From consideration of the data presented it follows that högbomites from various deposits are very close to one another. The differences that appear are, first of all, small, and the deviations depend not only on variations in composition but also on the very small size of the crystals themselves, which causes difficulties in their study.

Chemical composition. The study of the chemical composition of högbomite presents certain difficulties, which are determined by the following factors. The mineral has high hardness, and when grinding it in agate mortars it is impossible to avoid the introduction of foreign silica into the sample. During their growth, högbomite crystals often enclose crystals of vesuvianite, rutile, grains of calcite, hercynite, and corundum; in addition, the last two and magnetite often grow into it. Na—

1) Determined with the aid of the Berek compensator.
2) The hardness of högbomite from Sweden was erroneously determined by Gavelin as 6.5. The hardness of the same mineral from Switzerland was determined to be not less than 8 by cleavage [2, 3].
3) According to Ramdohr [5].
4) Determined with the aid of the Ehringhaus compensator [2].

unlike only the indicated minerals, is reflected correspondingly in the chemical analysis of the mineral. For the Gornaya Shoria högbomite, at present I have three analyses (Table 2), of which I and III are close to one another, and both samples were preliminarily treated before analysis: the first with acetic acid, and the second with dilute hydrochloric acid to remove calcium carbonate. Sample II was not preliminarily treated with acids, and comparatively large crystals were selected for the analysis.

Table 2

Chemical composition of högbomite

From the analyses presented it is clear that the leading role in the composition of högbomite belongs to aluminum oxide. Stable admixtures in the mineral are also oxides of iron, magnesium, and titanium, which, in the form \((\mathrm{Mg}, \mathrm{Fe})\,(\mathrm{Ti}, \mathrm{Fe})\mathrm{O}_3\), enter isomorphously into the composition of the mineral. A large part of the calcite and silica forms a mechanical admixture in the form of vesuvianite. The high content of calcium oxide in analysis II indicates that magnesium was not separated from calcium and that part of the calcium entered the pores of the högbomite in the form of calcite. But, apparently, a small part of it entered into the composition of the mineral. Part of the silica was introduced into the analysis during preparation of the sample.

Proceeding from what has been said, the general formula for the chemical composition of högbomite, as a complex isomorphous compound, may be represented in the following form: \((\mathrm{Al}, \mathrm{Fe})_2\mathrm{O}_3;(\mathrm{Mg}, \mathrm{Fe}, \mathrm{Ca}, \mathrm{Mn})(\mathrm{Al}, \mathrm{Ti}, \mathrm{Fe})\mathrm{O}_3\). If it is assumed that the compounds \((\mathrm{Al}, \mathrm{Fe})_2\mathrm{O}_3\) and \((\mathrm{Mg}, \mathrm{Fe}, \mathrm{Ca}, \mathrm{Mn})(\mathrm{Al}, \mathrm{Ti}, \mathrm{Fe})\mathrm{O}_3\) can form a solid solution, then in such a case the chemical composition of the mineral studied may be represented in the form of the simpler formula \((\mathrm{Mg}, \mathrm{Fe}, \mathrm{Al}, \mathrm{Ca}, \mathrm{Mn})(\mathrm{Al}, \mathrm{Ti}, \mathrm{Fe})\mathrm{O}_3\), which should best correspond to its structure and be reflected in a number of physical properties.

Genesis of högbomite. The Gornaya Shoria högbomite occurs in the form of crystals of varying size, enclosed in a granular aggregate of calcite. The latter, apparently, forms veins of varying size,

¹⁾ Chemical analysis performed by V. N. Orlova.
²⁾ Chemical analysis performed by I. P. Onufrienko.
³⁾ Chemical analysis performed by A. Krivets.

nests and veinlets in augitic oligoclase, contains rare grains of corundum and högbomite. The limestone xenoliths underwent exocontact alteration with the formation of peculiar skarns in which, in addition to calcite and högbomite, there are grains of corundum, vesuvianite, hercynite, andalusite, kyanite, diopside, rutile, and magnetite.

Calcite, which constitutes the main mass of the skarn, forms a medium- and fine-grained aggregate. Calcite in isolated grains is transparent or translucent; in the latter case it bears clearly visible glide striation. In color it is colorless, whitish, or even bluish. Yellowish or brown coloration is secondary.

Vesuvianite is present in the skarns in the form of regular isometric crystals, on which the faces \((100)\), \((110)\), \((101)\), \((111)\), \((001)\) can be distinguished. The crystals are small. Vesuvianite is predominantly transparent, greenish-yellow. In thin section it occurs for the most part in the form of regular crystals. It is noticeably zoned. The zones appear chiefly in the peripheral part of the crystal, and their number ranges from 2 to 4. Cleavage is imperfect and appears parallel to the prism faces; cleavage along \((100)\) is expressed better than along \((110)\). Vesuvianite grains in the skarn are distributed extremely unevenly. In some cases the number of vesuvianite grains is relatively large; in others it appears only in the form of rare included grains. Of all the minerals in the skarns it appears among the first. Therefore we find vesuvianite included in högbomite, hercynite, and corundum.

Diopside is leucocratic and occurs rarely, in the form of imperfect prismatic crystals.

Sphene is present in the mass of limestone in small quantity and is represented by small colorless grains of irregular form.

Andalusite occurs in the form of prismatic, needle-like crystals, sharply elongated along \(N_r\). Weak pleochroism is noticeable: along \(N_g\)—bluish, and along \(N_p\)—yellowish. It cannot be confused with zoisite either by refractive indices or by interference color. Judging from the entire mineral assemblage, the environment in no way favored either the formation of hydrous silicates or the formation of hydromicas, which in other högbomite occurrences are usual companions. Andalusite is occasionally found included in the peripheral part of vesuvianite grains, in this case being arranged along the zonal shells. Most of it occurs in the calcite mass. The latter circumstance evidently indicates that, with a relative abundance of alumina and a small amount of silica, the formation of anorthite was impossible.

Kyanite occurs in the form of thin-prismatic and needle-like bluish crystals which, unlike ordinary ones, have negative elongation, with extinction \(N_p:c = 28^\circ\). Pleochroism is more noticeable than in andalusite and appears unevenly in bluish tones. The unusual orientation of the optical indicatrix in kyanite is probably explained by the fact that, owing to dynamometamorphism, the andalusites, in their transformation into kyanite, underwent an incomplete change, expressed chiefly in an increase of the refractive index and in a certain rotation of the optical indicatrix of andalusite.

Rutile in the skarns appears as small drop-like formations, sometimes included in small quantity in högbomite. Separate and rare grains of it occur included in calcite. In högbomite the rutile is colored dark brown, while its grains in calcite are colored violet-brown.

Hercynite predominantly forms small granular accumulations in the skarns. Its crystals are very rarely regularly formed.

often bear contact growth striations on their surface. In thin section, hercynite is colored from dark green to deep green. In some hercynite grains, polysynthetic twinning according to the spinel law is noticeable. The character of this twinning is emphasized by finely disseminated magnetite, which is distributed predominantly along the plane of twin intergrowth. Hercynite, as is evident from its relations with the minerals adjacent to it, forms later than vesuvianite and later than the compounds \(\mathrm{Al_2[SiO_4]O}\), and during its formation captures andalusite. In relation to högbomite, hercynite apparently forms later, and therefore in individual cases overgrows it and penetrates into it as tongues.

Corundum in the skarns forms grains of various sizes, and some of them are distinguishable without a microscope. Corundum grains are often irregularly rounded in outline, macroscopically bluish in color. In thin section it is colorless. From analysis of its interrelations with other minerals it follows quite clearly that corundum forms somewhat later than högbomite and hercynite. In the skarns, corundum is distributed unevenly. Therefore, in some cases accumulations of corundum grains are rare, while in others they are relatively numerous.

Magnetite occurs in small amounts and is distributed unevenly, chiefly together with other skarn minerals. Magnetite in individual crystals and small aggregates, occurring isolated from other minerals in calcite, appears in the form of octahedra and rhombododecahedra. When occurring in contact with högbomite, however, magnetite crystals assume only cubic forms. This case is a good illustration of the fact that, under the same physicochemical conditions, depending on the surrounding environment, crystals of morphologically different habit may be produced.

Högbomite in the skarn, like many minerals, is distributed unevenly. From what has been said it follows that the mineral described forms later than vesuvianite and andalusite, simultaneously with rutile, and during its growth partly captures grains of hercynite and corundum. The latter two minerals, in greater part together with magnetite, form later than högbomite, bordering and corroding it along the periphery.

The genesis of the Mrass högbomite and the minerals accompanying it may be represented as follows (Table 3). All the minerals listed above, together with högbomite, constitute a contact-metasomatic mineral association, the formation of which was preceded by the crushing and intrusion into the Cambrian limestones of a dike of augite oligoclasites. At the contact, the limestones and limestone xenoliths undergo contact-metasomatic alteration with the formation of distinctive skarns, the composition of which is determined by the introduction into the recrystallizing carbonate rocks of relatively large amounts of aluminum, iron, and titanium; magnesium and silicon are introduced in smaller amounts.

The order of formation of the entire mineral association is divided into separate stages of mineralization, and each subsequent stage is separated from the preceding one by intermineralization crushing and the opening of pathways for the movement of solutions. Such tectonic stresses lead to the partial transformation of andalusite into disthene; relatively large crystals of högbomite undergo deformation and prove to be deformed to varying degrees. As a result of pressure, calcite undergoes repeated recrystallization, as a consequence of which the polysynthetic characteristic of deformed calcite...

slip twins prove, in the case described, to be relatively rare.

Table 3

Sequence of mineral precipitation in the högbomite skarn

Within each stage, the formation of minerals is regulated by its geochemical features. Thus, the first mineral association of the deposit described is geochemically determined by the joint presence of Ca, Mg, Al, Si, and Ti, which leads chiefly to the formation of silicates. At this stage diopside (leucodiopside), titanite, and relatively abundant vesuvianite are formed. In the presence of an excess of aluminum and a deficiency of silica, andalusite is formed.

At the next stage, in the presence of Ca, Al, Mg, Fe, Ti, and Si, andalusite continues to form, binding the remaining silica; its disappearance leads to the deposition of alumina in the form of corundum. At the same time högbomite and rutile begin to form; the former, as a complex compound, binds Al, Ti, Mg, and Fe, while an excess of Ti leads to the formation of rutile. Fe and Al are bound in the form of hercynite.

At the third stage, with falling temperature, the formation of hercynite ends, the deposition of corundum continues, and iron in the form of magnetite replaces hercynite and continues to accumulate in large quantities at the last—the fourth—stage of formation of the entire mineral association. At individual points in the basin of the Tashelga River, magnetite is concentrated in industrial quantities.

The genesis of högbomites from Ruotivara is closely connected with the origin of iron-ore deposits, which are one of the products of differentiation of a calcareous-alkaline magma. The magmatic iron ores of the deposits themselves are spatially associated with anorthosite or monzonitic rocks. Pleonaste and corundum occur in ores rich in silicates, and in anorthosites and other silicate rocks poor in ores. Högbomite, together with corundum and spinel, develops only in ores poor in silicates or free of them. For example, in magnetite-spinelids, magnetite-högbomitites, and magnetite-corundites [2].

Under these conditions högbomite is accompanied by magnetite, ilmenite, pleonaste, corundum, hydrargillite, and rarely pyrrhotite. In ca-

as secondary and accidental constituents, magnesite, amphibole, and chlorite occur together with högbomite. Studying the relations of the indicated minerals, Gavelin says that there is a primary högbomite, which forms as a result of the solidification of the primary ore melt, and a secondary, later högbomite, which forms by replacement in primary pleonaste of ferric iron by titanium oxide [2].

Virginia högbomite was found in emery rocks, which are a black, dense aggregate of green spinel (hercynite), titanomagnetite, and corundum. Similar aggregates occur adjacent to granites in mica schists, which are cut by granitic pegmatites. The paragenesis of the minerals of this deposit is as follows: magnetite I—corundum-spinel, högbomite—magnetite II. Högbomite is more often manifested in the form of euhedral crystals, which are older than magnetite II, but at the same time cryptocrystalline högbomite is encountered, enclosing spinel grains in the form of a rim. Such rims of högbomite are regarded as reaction borders around the spinel. Cryptocrystalline högbomite grades into crystalline högbomite II. This second generation remained unexplained for T. Watson, since it is unclear whether it arises only by alteration of spinel, or whether it develops in the reaction border between crystalline högbomite and spinel [6; 7].

The genesis of the Swiss högbomite is somewhat different. In the basin of the Tessino River, högbomite was encountered in grammitic marbles containing actinolite-chlorite lenses of various sizes. In these metamorphic rocks the following minerals are found: calcite, actinolite, chlorite, spinel, ilmenite, högbomite, corundum, clinozoisite, epidote, apatite, zircon. Spinel, ilmenite, and högbomite intergrow with one another and in places form relatively large accumulations. The spinel is a green pleonaste in the form of rounded-angular grains. Ilmenite is polysynthetically twinned and forms individuals elongated parallel to the surface of tabular högbomite [4].

Comparing the genesis of the Gornaya Shoria högbomite with that of Lapland, Virginia, and Switzerland, it may be noted that the högbomite from Gornaya Shoria, in its origin, will be closer to the origin of the högbomite from Ruotivaara. This similarity consists above all in the fact that, both in the Ruotivaara area and in the area of the Tashelga River, we have magnetite deposits.

The albitic oligoclasites should be regarded as an analogue of the sjenites from Ruotivaara and as one of the extreme members of the plumasites.

Recently, högbomite deposits have been found at two points in the northern part of the Kuznetsk Alatau, in the basin of the Kiya River. In its character, the högbomite from these deposits, disseminated in coarse-grained marble, corresponds exactly to the Mrassky högbomite both in external form and in the assemblage of its accompanying minerals.

The new finds of högbomite give grounds to suppose that this mineral in the Kuznetsk Alatau is more widely distributed than could have been thought, and the association of högbomite with corundum in a number of deposits gives grounds to believe that högbomite in the Kuznetsk Alatau may be one of the guide minerals in prospecting for corundum deposits.

Conclusions. In its morphological properties, högbomite belongs to the trigonal-planaxial type of symmetry; it occurs in tabular crystals of hexagonal outline; its geometrical constant, \(a:c—1:1.36775\), brings högbomite close to the crystals of the corundum—ilmenite group. This affiliation of högbomite with this group is also indicated by the fact that, along with forms whose indices are no greater than 4, there are

forms with indices not only greater than 5, but greater than 10 are found, which must be recognized as especially characteristic of minerals of this group.

Högbomite, as follows from consideration of the physical and chemical properties of the mineral, is a complex isomorphous compound of variable composition of the type $(Al, Fe)_2O_3 : (Mg, Ca, Fe'', Mn'') (Al, Fe, Ti) O_3$ or, more correctly: $(Mg, Ca, Fe, Al, Mn) (Al, Fe, Ti) O_3$.

On this basis, högbomite cannot be regarded as some variety of corundum containing one or another impurity. Högbomite differs from corundum in the following properties, namely: the hardness of högbomite is somewhat lower in comparison with corundum, and at the same time the mineral is more brittle than corundum, ilmenite, and other minerals of this group. In contrast to corundum, högbomite has a higher refractive index and greater birefringence, which, as should be expected for isomorphous mixtures, depends on the amount of isomorphous impurities of the components of the composition—pyrophanite, geikielite, and others, whose refractive index is higher than that of corundum. Hence the luster of högbomites is always adamantine. Further, högbomites have a clearly expressed perfect cleavage on $(0001)$, and, as studies show, it can in no way be regarded as parting caused by twinning. This perfect cleavage, pearly-adamantine luster, and tabular-platy habit of the crystals determine the character of the so-called diamond spar.

The occurrence of högbomite in Gornaya Shoria until recently was the fourth known. New finds of this mineral are known from Bohemia and Karelia. It must be supposed that högbomite is a more widely distributed mineral than we know.

It apparently occurs in corundum deposits, i.e., where, owing to special genetic conditions, alongside an excess of alumina there is an excess of titanium salts of magnesium and iron. At least the brown, dark-brown, and black-brown corundums mentioned in the literature, which at the same time have distinct cleavage on $(0001)$, an adamantine luster, and contain in their chemical composition $Fe_2O_3 > 4\%$, $MgO > 1\%$, $TiO_2 > 2.74\%$ [3, pp. 1769, 1770, 1775], are most probably högbomites.

LITERATURE

1. Kuzmin A. M. Högbomite from Gornaya Shoria. Scientific-technical conference of the Tomsk Polytechnic Institute named after S. M. Kirov. Publ. Tomsk Polytechnic Institute, Tomsk, 1942.

2. Gavelin A. Über Högbomit. Bull. of the Geolog. Institut of the Univers. of Upsala. XV, 1916.

3. Hintze C. Handbuch der Mineralogie. I. Bd. 1907—1908.

4. Mittenholzer A. E. Beitrag zur Kenntniss der Metamorphose in der Tessiner Wurzdzone Sweiz. Miner. u. Petrog. Mitt. Bd. XVI, 1936.

5. Randohr P. Die Erzmineralien und ihre Verwachsungen, Berlin, 1935.

6. Watson T. L. and Steiger G. Titaniumbearing corundum spinelliae (rock emery). Journ. Wash. Acad. Sci., 8. 1918.

7. Watson T. L. Hoegbomit from Virginia. Amer. Miner. 10, No. 1, 1925.

1. The article “Högbomite from Gornaya Shoria” was accepted for publication by the journal Izvestiya Zap. Sib. Otd. AN SSSR, 1960. ↩