This document provides an overview of kimberlites. It discusses that kimberlites are volatile-rich, ultrapotassic igneous rocks that occur as small volcanic pipes and intrusions. They are composed mainly of olivine, phlogopite, ilmenite, garnet and pyroxenes. Kimberlites are divided into two groups based on their mineralogy and isotopic signatures. Group I kimberlites are dominated by olivine and ilmenite while Group II are micaceous and enriched in rare earth elements. Kimberlites are economically important as they are the primary source of diamonds, with only a small fraction of pipes containing gem-quality stones. They

1. Pritijita Naik
Roll No-22GEOL001

2. Contents
1. Introduction
2. Mineralogy
3. Morphology
4. Petrology
5.OCCUrRENCES
6.ECONOMIC IMPORTANCE
7.CONCLUSION
8.references

3. Introduction
The Name kimberlite was coined by Henry Carwell Lewis (1884) to a Diamondiferous
mica peridotite from Kimberley area of south Africa.
Kimberlites are found in the stable intracratonic plate and no mountain building activity.
Kimberlite is a volatile-rich, ultrapotassic, ultrabasic, igneous rock which occurs as small
volcanic pipes, dykes, and sills.
Inequigranular texture consisting of large crystal or rock fragments (xenoliths) enclosed
in a much finer grained matrix.

4. Mineralogy
 It is mainly consists of olivine, phlogopite, ilmenite, garnet, pyroxenes and
groundmass.
Olivine crystals(o) are Serpentinised, and
have started to loose their outlines
Cross-polar photomicrograph of dyke
encountered in drill core
Olivine crystals(O) have been completely
replaced by serpentine (dark grey) and are
surrounded by a zone of amorphous
serpentine which partially replaces
adjacent primary calcite crystals (cc)
Fig : a photomicrograph of kimberlite dyke in K2 East

5. Modal composition and texture
 The modal composition of the rocks of the kimberlite vary greatly.
 olivine is usually the most abundant mineral , but it may be partly, or completely, replaced by secondary minerals
(for example, serpentine group minerals).
 The abundance of phlogopite and carbonate minerals is also highly variable
 Kimberlitic rocks normally have porphyritic and/or pyroclastic textures.
 The fragmental appearance of many of these rocks is enhanced by the occurrence of xenoliths.
 Most rocks contain megacrysts that are set in a finer grained groundmass that tends to contain microphenocryst.
 The megacrysts are both xenocrysts and phenocryst, they typically consist of olivine, phlogopite, garnet and
pyroxene. They are usually set in a groundmass of serpentine group minerals and carbonate minerals ,together
with microphenocrysts of Fe-Ti oxides, micas, spinels, monticellite and apatite.

6. MORPHOLOGY
 Kimberlites occur as carrot shaped, vertical
intrusions termed “pipes”. This classic carrot
shaped is formed due to a complex intrusive
process of kimberlitic magma which inherits a
large proportion of both CO2 and H2O.
The morphology of kimberlite pipes and the
classical carrot shape, is the result of explosive
diatreme volcanism from very deep mantle-
derived source.

7. Continued…
• Three main zones are recognised :
(i) CRATER ZONE – removed by erosion
(ii)DIATREME ZONE – (1-2km) & highly brecciated in nature,
hydrothermally altered zone: rich in clay and serpentine
(iii)ROOT ZONE – dyke like in nature: hard & compact in nature,
most suitable for petrological studies

8. PETROLOGY
Historically, kimberlites have been subdivided into two distinct
varieties termed basaltic and micaceous (Wagner, 1994). It was re-
named by smith(1983) as Group I and Group II based on the isotopic
affinities of these rocks. Mitchell (1995), showed that group II
kimberlites actually shows closer affinities to lamproites than group I
kimberlites.

9. Group I kimberlite
Group I kimberlites are of co2-rich ultramafic potassic igneous rocks
dominated by a primary mineral assemblage of forsterite olivine, ilmenite,
Cr-pyrope, phlogopite, enstatite.
Group-I kimberlites exhibit a distinctive ineqigranular texture caused by
the presence of rounded, anhedral, and fragmented macrocryst(a non-
genetic term for 0.5 to 10mm diameter crystal).
They are derived from sources depleted in LREE (Light Rare Earth
Element).

10. Group II kimberlite
Group II kimberlites are ultrapotassic, peralkaline and H2O-rich.
Phlogopite is the dominant macrocryst and groundmass phase.
Derived from sources enriched in LREE.
Group I and II kimberlites are distinctive isotopically. Mitchell and Bergman
(1991) concluded that the two groups must represent different magma types .
Group II kimberlites should be separated from group I kimberlite and be
renamed as orangeites.
 Mineralogically group II kimberlite are similar to lamproites, but have
sufficient petrological differences by which they be considered separately from
these rocks as well.

11. Where do kimberlites occur?
• Kimberlite do not erupt in all areas of earth. Globally ,kimberlites all
occur below the oldest parts of continents , known as cratons.
• Cratons have thick lithospheric roots that extend down to atleast 150-
200 km, and kimberlite generation in the mantle is probably
associated with the physical barrier to mantle upwelling provided by
these deep continental roots.
• Regardless of how kimberlite form, the association of these eruptions
with deep continental roots is another of the wonderful mysteries
about how kimberlite deliver diamonds .

12. Kimberlite-Diamond field relationships
For several decades, diamond was considered to be a phenocryst in
kimberlite.
But dating of diamond inclusions showed that diamond formation
pre-dates kimberlite eruption.
Diamonds are found to be much older than kimberlite.
For eg: Diamond of Archean age are found in cretaceous kimberlites.
Diamonds are hence XENOCRYSTS in kimberlite.

13. Indian occurrence
Kimberlites occur in two spatially separate groups: Mahbubnagar cluster
that was emplaced at 1400ma and the predominantly diamondiferous
Anantapur cluster, emplaced at ~1100 Ma.
The well known Wajrakarur diamondiferous field in Anantapur district
and area of 250 sqkm, out of 12 pipes identified , pipes 2 and 5 are
considered as micaceous kimberlite and the rest are typical kimberlite ,
these kimberlites are diamondiferrous.
Maddur-Mahbub nagar sector, covers an area of about 400 sq km.

15. Economic Importance
Kimberlites are the most important source of primary diamonds.
Many kimberlites pipes also produce rich alluvial diamond placer
deposits.
Only about 1 in 200 kimberlites pipes contain gem quality diamonds.
Kimberlites are very significant rock group both academically and
economically.

16. Conclusion
Kimberlite magmas are rich in carbondioxide and water which brings the
magma quickly and violently to the mantle .
Kimberlite is a gas rich potassic ultramafic igneous rock.
Australia is currently the world’s largest producer of diamonds are low
quality and used for industrial purposes.
The crater facies kimberlite is recognised by sedimentary features.
The diatreme facies are recognised by pelletal lapilli .
The hypabyssal facies is commonly recognised by the presence of
abundant calcite.

17. References
Books;
 JOHN D. WINTER,P;443-445
 Mihir K. Bose, igneous petrology
WEBSITES;
 www.geology.com
 www.Wikipedia.com

18. Thank you