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Above: samples of fine crystals of the zeolites stilbite and chabazite with the related silicate mineral apophyllite. Here we see:
- Stilbite , a monoclinic zeolite, an hydrated Na Ca aluminosilicate,
notable for its typical crystal habit, as sheaf-like
masses of penetration twins. Material from Pune is commonly
pale "pink", as in these examples (though in some lights, including these photographs,
the hue is more a pale orange than a delicate pink).
- Chabazite, a trigonal zeolite, an hydrated Ca aluminosilicate,
occurring here as white to clear
crystals with vitreous lustre (top sample, at lower right).
- Apophyllite, a tetragonal mineral group, an hydrated K Ca silicate (e.g., hydroxyapophyllite).
The crystals may appear to be cubic, with cube and octahedral forms,
but cleavage and lustre reveal a different symmetry.
In its detailed structure, apophyllite is an
unusual sheet silicate composed of layers of 4- and 8-membered rings.
The upper sample displays, on the right side, a large
tetragonal prism of apophyllite
with a pale green colour.
A notable occurrence of vugs lined with green apophyllite is described
by Makki and Makki (2008).
The green hue of apophyllite from the Pashan Hills is said
to be due to a high content of vanadium (Ottens, 2003, p.76).
This sample is probably fluorapophyllite (cf. green samples in
Ottens, 2003; Wilson, 2011, p.19), though this has not been confirmed by analysis.
- Chloritic matrix material (dark grey, altered basalt host rock). Such zeolite-rich mineral assemblages are typical of regions with amygdaloidal (cavity-filled) basalts and altered basalts and tuffaceous sediments, notably including the Deccan, New Jersey and Nova Scotia. Basaltic sequences in other parts of the world also display zeolites, including Scotland (e.g., the Glasgow district, and the islands of Mull and Skye) and Australia (Victoria and New South Wales). Zeolites also form in certain alkaline rocks, including the Palabora carbonatite in South Africa, and sundry syenites and associated albitites and alkaline ultramafic rocks in the western U.S.A., Greenland, India, Russia and elsewhere.
At top is sample 1335, 537.12 g, the crystal aggregate not appreciably magnetic, on a base which is a thin crust of basalt host rock, bulk magnetic susceptibility at least 2x10-3 SI units. The host rock is dark basalt on which is deposited a thin layer of microcrystalline silicates, upon which the coarser crystals evidently grew into an open space such as a large vug. There is delicate pale-pinkish stilbite, plus pale green apophyllite and clear chabazite.
Below is sample 1334, 399.74 g, largely radiating rosettes of pale pinkish stilbite, growing upon white to colourless tetragonal prismatic to equant (pseudocubic) apophyllite. The specimens are 11x7.5x5.5 cm and 16x5x5 cm in size, respectively. They were purchased from a street vendor in Bombay (Mumbai) in January 1991, precise locality unknown.
"Rock of the Month #142, posted for April 2013" ---
Zeolites and apophyllite from Pune (Poona)
in western India. This coastal area is part of the
vast expanse of the Deccan Traps, flood basalts
that covered much of the region in
one of the largest known episodes of flood basalt volcanism,
near the end of the Mesozoic era.
The basalts are well described in the literature,
and were recently reviewed by
Vaidyanadhan and Ramakrishnan (2008, pp.733-784).
The basalt-related occurrences were noted at an early date, and
described over the years (e.g., Medlicott and Blanford, 1893).
A zonation of zeolite species was proposed long ago
(e.g., Sukheswala et al., 1974)
and subsequently investigated by
Jeffery et al. (1988) who studied the relatively small
(15,000 km2!) area of the Deccan
north and south from Bombay (Mumbai),
including Pune (to the southeast) and Nasik (to the northeast)
of the great city.
They studied occurrences of minerals in the basalt,
finding the eight most frequently encountered minerals to be
heulandite, then stilbite and (progressively less common)
mordenite, apophyllite, mesolite, epistilbite,
calcite and chabazite. No clear-cut zonation was recognized.
The zeolites are chemically rather pure (James and Walsh, 1999).
Parthasarathy et al. (2001) identified the rarely-seen
silica polymorph moganite in association with zeolites,
within a chalcedonic silica forming
amygdaloidal and vein fillings in Deccan basalt.
The fine mineral specimens have often been recovered from roadstone quarries, the
operations of which may expose cavities in the lava flows encrusted by
mineral specimens. Zeolites of the Deccan are reviewed by Ottens (2003).
The region is world-famous for its mineral specimens, including the
blue zeolite cavansite (Kothavala, 1991; Praszkier and Siuda, 2007),
green apophyllite (Kothavala, 2003) and other species.
Many can be found at the Gargoti Museum in Nasik (Ramakrishnan, 2007)
and other institutions around the world.
Selected references, in chronological order
Medlicott,HB and Blanford,WT (1893) Encyclopedia of Indian
Geology. 2nd edition, revised and rewritten by Oldham,RD and
republished by Cosmo Publications, New Delhi, in 1994. Volume
2, 255-543.
Sukheswala,RN, Avasia,RK and Gangopadhyay,M (1974)
Zeolites and associated secondary minerals in the Deccan Traps of
western India. Mineral.Mag. 39, 658-671.
Jeffery,KL, Henderson,P, Subbarao,KV and Walsh,JN (1988)
The zeolites of the Deccan basalt - a study of their distribution. In
`Deccan Flood Basalts' (Subbarao,KV editor), Geol.Soc.India
Memoir 10, 393pp., 151-162.
Kothavala,RZ (1991) The Wagholi cavansite locality near Poona,
India. Mineral.Record 22, 415-420.
James,S and Walsh,JN (1999) Zeolites from the Deccan basalts:
chemistry and formation. In `Deccan Volcanic Province'
(Subbarao,KV editor), Geol.Soc.India Memoir 43(2), 803-817.
Parthasarathy,G, Kunwar,AC and Srinivasan,R (2001)
Occurrence of moganite-rich chalcedony in Deccan flood basalts,
Killari, Maharashtra, India. Eur.J.Mineral. 13, 127-134.
Kothavala,RZ (2003) Recollections of mineral collecting and
dealing in India. Mineral.Record 34, 135-154.
Ottens,B (2003) Minerals of the Deccan Traps, India.
Mineral.Record 34, 1-82.
Praszkier,T and Siuda,R (2007) The Lonavala quarry, Pune
district, Maharashtra, India. Mineral.Record 38, 185-189.
Ramakrishnan,M (2007) Gargoti mineral museum.
J.Geol.Soc.India 69, 403.
Makki,S and Makki,MF (2008) Green apophyllite from Bidkin,
Aurangabad, India. Mineral.Record 39, 267-273.
Vaidyanadhan,R and Ramakrishnan,M (2008) Geology of India.
Geol.Soc.India, Bangalore, 2 volumes. Vol.1, xxx+1-556,
Vol.2 xxix+557-994+14 plates, plus map.
Wilson,WE (editor) (2011) Private mineral collections in Italy.
Mineral.Record 42 no.1, supplement, 144pp.
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