Vanadium in the cavity minerals of the Deccan Traps/India

 

Berthold Ottens

Klingenbrunn Bahnhof 24

D-94518 Spiegelau

Germany

 

Vanadium as a trace element in the basalt of the Deccan Traps, India has an important influence for the green color of the apophyllite and the blue cavansite and Pentagonite.

 

Introduction

Approximately 100 to 800 ppm vanadium could be detected in all basalts of the Deccan Volcanic Plateau Province within the context of geo-chemical studies. The most frequent values lie between about 200 to 400 ppm. The origin of the vanadium is not explained, but one can infer the considerable presence of a mineral containing vanadium.

Vanadium was discovered for the first time in the secondary minerals of the cavities, as one searched for the cause for the green color of the apophyllite of Poona – Pashan hill and ascertained vanadium to be the important element here. The notable cavansite findings of Poona-Wagholi around the end of the 70’s meant, that vanadium is not only worked in as an impurity but also to a considerable extent as mineral species. Although vanadium occurs, geographically speaking, practically unrestrictedly in a concentration of relatively narrow tolerance, it was not clear, why there should be only one vanadium mineral in one locally defined area of discovery. Meanwhile cavansite could be found, not only in other quarries near Wagholi, but also in localities of up to 100 km away. In Wagholi, aside from cavansite, pentagonite was also discovered. Moreover, it could be observed, that apophyllite frequently occurs in more or less green color not only in Poona but also in other localities, especially in Lonavala and Jalgaon.

It was therefore necessary to deal more intensively with the question of where the Vanadium comes from and why in some select localities the secondary mineralization of the cavities is favored for its occurrence. Another point of interest was what caused the different green colorings and the various color distributions in the apophyllite.

 

Basalt and Mineralization

The main part of the Deccan Volcanic Plateau Subprovince consists of a relatively uniform tholeiitic basalt.

weight %

SiO2 47 - 53

TiO2 1,5 - 3,5

Al2O3 13 - 16

Fe2O3 2 - 3

FeO 6 - 1,

MgO 4 - 6

MnO < 0,2

CaO 9 - 11

Na2O 2 - 2,8

K2O < 1

H2O 00,5

Figure 1. Average chemical composition

of tholeiitic basalt of the Deccan Traps

( Powar et al. 1987)

 

The world-wide average of vanadium in basalt (Prinz, 1967) is about 250 ppm. The figures may be somewhat higher in the Deccan and most frequently are between 200 and 400 ppm (fig. 2). The basalts near Poona (Ghodke et.al, 1976) with about 600 to 750 ppm must be classified as having a particularly high content. The exact vanadium content of the famous occurrences of the cavity minerals is not known because of lack of analysis, so that one can only make an inference of general conditions based on available analysis of other localities .

Since vanadium occurs with a relatively uniform concentration in all stratums of the Deccan, the cause for its presence cannot based in contamination of the rising lava through the crust but must be lie in the wide, little differentiated distribution in the magma available for the Deccan. In which minerals or glassy components, primarily separated from the lava, the vanadium was attached has not so far been determined. It is assumed (Prinz 1967), that one is dealing here with magnetite or pyroxene. An evaluation of 101 chemical basalt analysis from various regions and stratums of the Deccan shows, that the TiO2 value rises in the same proportion as the increase of vanadium, which leads us to infer bonding with a pyroxene (fig. 3).

If one makes an assessment of the rock, in which the green apophyllites occur, one comes to the conclusion, that with the exception of the quarry complex at Jalgaon-Sawda, a very porous structure with great permeability predominates. Even the breccia in the quarry of Poona – Wagholi evinces many vesicles with almost unrestricted connection to each other. It can therefore be assumed, that a high specific surface is a good prerequisite for hot solutions to mobilize as much vanadium as possible.

 

The vesicles, connected with one another have lead also to the fact, that uniform mineralization conditions and thus also similar paragenesis and minerals with similar habit and similar form could be developed within one type in one connected system. Although mining was carried out for over 20 years in the former quarries at Pashan hill, apophyllite and stilbite varied only very little with regard to their appearance and color. Similar conditions seem to exist also in the region of Lonavala, about 50 km north-west of Poona on the road to Bombay. With the exception, that there in the case of the green apophyllite the form (101) appears only in a secondary manner, the type of minerals and their habit as well as the surrounding rock are to a great extent identical with Poona-Pashan hill.

In Poona-Wagholi one came across cavansite only very seldom in the dense and solid basalt, found beneath the actual breccia bearing cavansite, at a distance of about 50 m from this. The appropriate solution could find the way through fissures in the vesicles. Perhaps similar conditions prevailed also in Jalgaon. About 2-3 m above the horizon bearing the cavity there is a layer of about 2 m with a very crumbly structure, which originates not due to recent weathering but most probably from the emergence period of the Deccan. It was possible to exhaust the vanadium in this zone.

The fact, that in Jalgaon-Sawda the mineralization itself differs seriously even in the case of closely neighbouring cavities, explains why solutions containing vanadium found their way only under certain conditions through fissures into available cavities

Due to the comprehensive field-observations one can take the general rule to be valid, that the vanadium required for the green coloring of the apophyllite originates out of very porous and permeable rock. Whether other factors such as temperature played an important role here is not clear, but also not very probable. Finally the minerals accompanying the green apophyllite and the cavansite show the same shape of appearance as in the case of association with colorless apophyllite. It is worth noting, that to date no green apophyllite has been found in Poona-Wagholi. Colorless apophyllite, itself, however, occurs there only very seldom in small cavities of the compact basalt.

 

Vanadium containing Minerals

 

Cavansite Ca [VO)Si4O10 ] . 4 H2O

Pentagonite Ca [VO)Si4O10 ] . 4 H2O (dimorphous with cavansite)

 

Cavansite normally occurs together with heulandite and stilbite. It is crystallized after a first (main) phase of stilbite, partially however overgrown with a second generation of stilbite. Occasionally over-growths of cavansite are found in calcite and sometimes the cavansite is found ingrowing in the calcite. One very seldom observes cavansite embedded in mordenite. This shows however that the formation of cavansite was possible after heulandite as well as after mordenite in localities not connected with one another and at a distance of over 100 m from one another.

 

Chemical analysis (fig. 4) of the cavansite of Poona-Wagholi shows a content of 18.03% VO2.

 

Cavansite (orthorhombic system, class mmm) forms elongated, bladed crystals. These occur either in spherical aggregates or in tabular crystals intergrown in one another of about up to 20 mm in length. No twinning could be detected.

 

Although the author studied many samples since 1989, which differed from one another with regard to external appearance and with regard to the normal association of the common cavansite, it was only in 1999 that pentagonite (fig. 5) could be recognized on the basis of its twinning and confirmed through XRD analysis (Ottens et. Al., 2000). Multiple twins such as found in Oregon could not be observed.

 

Cavansite was not only found in the "main quarry" controlled by Dr. Bhale (18.35.71 N, 73.58.47 E) but also in some neighboring quarries near Wagholi. Mookherjee and Phadke ( University of Pune ) have reported further occurrences of cavansite in the magazine of the Gondwana Geological Society, Nagpur, India (1998) as follows:

 

     

  1. Road cut near Sutarwadi, about 10 km west of Poona

     

     

  2. In the quarries at Yedgaon Dam near Narayangaon, about 100 km north-northwest of Poona

     

     

  3. At Surli Ghat, about 15 km east of Karad in Satara District.

     

 

 

These specimens, when subjected to the XRD examination, have shown the same characteristics as the cavansite from Wagholi Therefore it can be assumed, that in future, when appropriate basalt with the relevant prerequisites is found, more cavansite or pentagonite can also be found.

 

Apophyllite KCa4 [F, OH) (Si4O10)2 ]. 8 H2O

Fluorapophyllite

Hydroxyapophyllite

 

Whereas the occurrence of hydroxyapophyllite is, as per our current knowledge, mainly restricted to the spilite basalt of Bombay, fluorapophyllite is found more or less frequently in almost all localities in the main part of the Deccan. The mostly colorless apophyllite crystals are as a rule transparent – to slightly cloudy. The color variations which occur infrequently are light pink and yellow, but more often it is green apophyllite, that is found. The green coloring varies in shade, intensity and distribution in the crystals , depending on the locality of discovery and the respective surrounding conditions of the cavities.

 

In all the cases, which were examined, the green coloring is to be traced to the presence of vanadium. Rosman reported in 1974, that intensive green apophyllite from Poona – Pashan hills evinces a vanadium content of about 1.600 ppm. In a series of analysis (Ottens, 2000) conducted on green apophyllite from various occurrences, one was able to confirm vanadium as the cause of the color. In these analysis zones of the same crystals with varying coloring were also included. The results are shown in the figure 6. The general rule applies, that an increasing green-intensity can be explained with an increasing content of vanadium . The highest values measured were ascertained for blueish-green apophyllite from Jalgaon-Swada with about 3.000 ppm.

 



The apophyllite of most of the localities show a uniform distribution of color within a crystal. A characteristic feature for some localities is the more intensive coloring in the central zone of the crystal as compared with the zones on the c-axis at a distance from the center-point (fig. 7). Here it could be determined, that the relatively colorless "termination zones" are poor in vanadium or are totally free of vanadium. The color distribution does not correspond to the zonal growth of these crystals. The cause therefore cannot based in decreasing content of vanadium in the solutions. An additional study ( Fehr, 1998 ) showed, that the vanadium-ions were absorbed as a result of various conditions in varying amounts in the crystal lattice. Although the apophyllite structure is apparently suited for vanadium to be worked in as an impurity, the variations of color distribution make it clear, that there were no uniform conditions for absorption in the lattice.

 

It has not been clarified, whether other elements in connection with vanadium have had an influence on the coloring. Rosman mentions a manganese content of 350 ppm, whereas the iron content lay only at 70 ppm. In further studies other possible trace elements were not determined. The extent to which the vanadium content exerts an influence on the forms or habit of the apophyllite was also examined. With the exception of a platy habit, found relatively seldom in the Deccan, green apophyllite crystals were regularly found with cube, prismatic and prismatic-pyramid habit as well as very seldom also in spherical aggregates. All types of habit were observed both in the case of colorless as well as light and more intensive green apophyllite. No evidence could be found of any influence of vanadium on the forms and. habit (fig. 8)

 

It was also possible to ascertain, through numerous observations , whether the association of green apophyllite with other cavity minerals differs from those apophyllites, that are colorless (fig. 9). One could determine that there are no recognizable differences between colorless and green apophyllite with regard to the associated minerals. It is only noticed, that in particular the intensively green colored crystals do not reach the same size as colorless ones. Whether this is a coincidence due to local conditions or whether it represents a dependency is not clear.

 

In spite of a lack of evidence of whether other secondary minerals in the cavities of the Deccan contain vanadium, it seems to be certain that vanadium occurring as VO4+ in solution is mainly worked into apophyllite. For the sake of completeness one must point out, that the coloring of dark- to olive green, non-transparent apophyllite and heulandite of Jalgaon as well as of Shakur has not developed through vanadium but through green seladonite, which sits on worm-shaped quartz and is overgrouwn by apophyllite or heulandite.

 

Conclusion

In so far as cavansite or pentagonite is discovered to some extent in large crystals and with conspicuous frequency in new localities, they will probably not escape the trained eye of the runners and the dealers. The disadvantage is however, that the greatest prospects for V-containing minerals exist in porous basalt, which however, due to its low stability is not quarried specifically as construction material. More recent findings may therefore be limited foremost to areas such as road-or–well construction. Should vanadium occur in any other chemical compound in the Deccan, it is certain to be noticed due to the distinctive coloring of its minerals. Due to the narrow toleration of crystallization-conditions in tholeiitic basalt of the Deccan the probability of other vanadium-minerals from this region is very low.

Ascertaining the origin of the vanadium would give much information, especially regarding the minerals it is associated with. At this point one must point out, that it is also unclear how a significant mobilization of molybdenum, of which only 1 ppm is detected in the basalt, was carried out, in order to develop significant quantities of powellite (up to 1 kg in one cavity with some crystals weighing up to 100 g ) in the lava flows which are sometimes in places greatly distanced from each other and which differ also in so far as age is concerned.

 

Further studies are necessary in order to ascertain the important general conditions under which vanadium is absorbed in the apophyllite lattice. It is also interesting to know, whether vanadium was worked into only apophyllite or also into other minerals. Aside from the routine observation of the large range of cavity minerals of the Deccan there is therefore also sufficient requirement for studying other existing material.

 

 

 

References

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