Geological Guides in Locating Semiprecious
Beryl Aquamarine Minerals
Shri C.S. Dwivedi,
Petrologist,
Dept. of Mines & Geology, Udaipur
During the Course of investigation for aquarmine, heliodor, bery1 etc. in Todaraisingh
area of district Tonk (Raj.) centain lithological, structural and morphological controls
were established which acted as guide lines in search and exploration of semiprecious
bery1 in this area. Some of the geological guides enumerated below could help
investigation of these minerals on other parts of the state as well.
1. Bery1/ aquamarine mineralization is found associated with pegmatitic intrusions
in schistose country rock. A distinct lithological and structural control is seen in
mineralization of this semiprecious variety as Aravalli schists (Less commonly
gneisses intruded by pegmatites along foliation planes concordant pegmatites) act
as ideal host rocks for mineralization of semiprecious bery1.
2. Pegmatites of smaller dimensions with medium size quartz, feldspar crystals
favoured bery1 mineralization.
3. Intergrowth type pegmatites with partial zoning are favored in comparison to
distinctly zoned pegmatites with separate outer intermediate and border zones.
4. pegmatites with pink feldspar (microcline) are usually not found favorable sites
for semiprerecious bery1 mineralization.
5. pegmatite’s with coarse bery1 crystals and largemica block do not act as good
host for its mineralization.
6. Semiprecious variety of bery1 is found in pegmtites having small concentration of
tourmaline. Transparent gem variety is seldom seen associated with pegmatites
having rich tourmaline concentration.
7. Usually small medium dimensional pegmatites of not too course in again size
having smoky quartz, light pinkish white feldspar (plagioclase) microcline
perthite with subordinate small mica flakes act as ideal hunting grounds.
8. Transparent fracture less fractureless semiprecious crystals are of frequent
occurrence at the contact of schist with pegmatites. Here again gem variety
favours quartz in preference to feldspar in partially zoned pegmatites the core
portion specially interface of quartz and feldspar but more nearer to quartz are
consider preferred locales.
9.
Ordinary biotite schist again is not a good host. It is the recrystallised biotite
schist of dark green colour (Chlorite) in contact with small pegmatites bodies
which pay rich dividends.
10. Semiprecious crystals are usually found not far from the schist (about a meter or
so) and may some times be seen even enclosed in the schist itself.
11. In Todaraisingh area semiprecious bery1 is seen in pockets of highly contorted
foliated compressed, recrystallised biotite chlorite schist pockets copiously
injected by small medium size, medium grained blocky (Intergrowth) type of
pegmatites with partial zoning emplaced along foliation planes of schist. There is
a zone of felsparound central quartz core in these blocky of pagmatites which
along with intergrowth type of pegmatites contains semiprecious bery1
mineralization.
12. Highly contorted, folded compressed recrtstalised batiste chlorite schist forming
tight small oval packets appear ideal hunting ground and is probably due to
prevention of beryllium bearing emanations from being escaped, there by
concentrating ‘Be’ vapours to from colored, transparent, featureless, flawless gem
variety bery1 in these pockets. The ferrous ions responsible for imparting colour
to the gem variety is possibly derived from the schist components and as closer to
the schist components.
13. In Todaraisingh area muscovite- microcline-per-trite-pegmatites, (Blocky) show
semiprecious bery1 development at the interface of central block quartz zone
surrounding microcline with less frequent occurrence in the succeouter essive
zone. Petrographic studies of the area suggest that bery1 crystallization took place
after the formation of microcline-perthite zone and before crystallization of quartz
core. These inferences could act as favourble guides in locating small suitable
blocky pegmatites which may yield semiparecoius bery1 aquamarine, heliodor
and other transparent varsities if bery1.