©Lin Yangchen


Beneath the surface of a grey and dead-looking rock is a universe of colour, pattern and texture that comes alive under a polarizing microscope. Those who can decode the information hidden within the Pink Panther-like tapestry of mineralcules gain access to the deepest secrets of the Earth and rule over the fashion catwalks of Paris and Milan.

I present here my analyses and photomicrographs of petrographic thin sections with my ‘Special Forces’ Olympus BHSP petrographic microscope. Reflected-light micrographs (of opaque minerals) will be captioned as such. All other micrographs are in transmitted light. Mineral identifications are dependent on my ongoing study of optical mineralogy and may not be error-free. Please contact me if you spot mistakes so I can correct them and make this a better reference for everyone.


Click individual samples:


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Accession no. R001


Locality: unknown (may not be Southeast Asia)
Sample date: unknown
Method: hand sample
Finder: unknown


Macroscopic description





Petrographic analysis


30 μm unpolished with no. 1 coverslip
Norland Optical Adhesive 61 (n = 1.56, US military specification MIL-A-3920)






Accession no. R002


Locality: unknown (may not be Southeast Asia)
Sample date: unknown
Method: hand sample
Finder: unknown


Macroscopic description


Embedded in Struers EpoFix (n = 1.578)

Suevite impact melt breccia with reddish-brown cement.


Petrographic analysis


30 μm unpolished with no. 1 coverslip
Norland Optical Adhesive 61 (n = 1.56, US military specification MIL-A-3920)






Accession no. R003



The 1815 caldera of Tambora in September 2005.


Locality: western caldera rim of Tambora volcano, Sumbawa, Indonesia
Sample date: 23 September 2005
Method: hand sample
Finder: Lin Yangchen


Macroscopic description


Embedded in Struers EpoFix (n = 1.578)




Petrographic analysis


30 μm unpolished with no. 1 coverslip
Norland Optical Adhesive 61 (n = 1.56, US military specification MIL-A-3920)



Clinopyroxenes, olivine and opaque oxides in a matrix of plagioclase flakes and fine-grained groundmass.


Plagioclase hosting melt inclusions in earlier stages of crystal growth (see Nurfiani et al. 2021).


Combined oscillatory and sector zoning of clinopyroxene.



Groundmass with opaque magnetite grains.





Accession no. R004



The crater of Barujari in September 2005.


Locality: northeast foot of Barujari volcano in the Rinjani caldera, Lombok, Indonesia
Sample date: 18 September 2005
Method: hand sample
Finder: Lin Yangchen


Macroscopic description


Embedded in Struers EpoFix (n = 1.578)

Volcanic bomb of vesicular basalt.


Petrographic analysis


30 μm unpolished with no. 1 coverslip
Norland Optical Adhesive 61 (n = 1.56, US military specification MIL-A-3920)



Plagioclase and clinopyroxenes in fine-grained groundmass with vesicles.



Microlite-rich groundmass with opaque magnetite grains.


Melt inclusions with bubbles in zoned plagioclase.





Accession no. R005


Locality: Bukit Gombak, Singapore
Sample date: 25 August 2021
Method: hand sample
Finder: Lin Yangchen


Macroscopic description





Petrographic analysis


30 μm unpolished with no. 1 coverslip
Norland Optical Adhesive 61 (n = 1.56, US military specification MIL-A-3920)






Accession no. R006


Locality: Bukit Gombak, Singapore
Sample date: 25 August 2021
Method: hand sample
Finder: Lin Yangchen


Macroscopic description


Leuco-granite dyke with fine-grained dolerite-like veins. Zircon U-Pb dated to Early Triassic, 249.9 ± 1.9 million years (Oliver et al. 2014).


Petrographic analysis


30 μm unpolished with no. 1 coverslip
Norland Optical Adhesive 61 (n = 1.56, US military specification MIL-A-3920)



Mosaic of undulose quartz and sericite flakes probably altered from plagioclase.


Close-up of vein showing pleochroic acicular mineral with zoned birefringence and straight extinction, possibly epidote.



Possible muscovite with moderate relief, high interference colours and straight extinction.



Pale green chlorite with iron-rich regions yielding Berlin blue anomalous interference colour.





Accession no. R007


Locality: Bukit Gombak, Singapore
Sample date: 25 August 2021
Method: hand sample
Finder: Lin Yangchen


Macroscopic description





Petrographic analysis


30 μm unpolished with no. 1 coverslip
Norland Optical Adhesive 61 (n = 1.56, US military specification MIL-A-3920)






Accession no. R008


Locality: Bukit Gombak, Singapore
Sample date: 25 August 2021
Method: hand sample
Finder: Lin Yangchen


Macroscopic description


Gombak norite (a plutonic basic olivine gabbro). Coarse-grained equigranular bronzite (an orthopyroxene) and white plagioclase. Zircon U-Pb dated to Late Permian, 260.3 ± 2.3 million years (Oliver et al. 2014).


Petrographic analysis


30 μm unpolished with no. 1 coverslip
Norland Optical Adhesive 61 (n = 1.56, US military specification MIL-A-3920)


Plagioclase with polysynthetic twinning, exsolution lamellae and oscillatory zoning. Pyroxene with moderate relief and moderate interference colours. No pink-green pleochroism observed. Occasional opaques and fractured olivine with no cleavage. Oliver & Gupta (2017) recorded 50% calcic plagioclase, 35% orthopyroxene, 5% olivine, 5% brown hornblende and 5% opaques.



Pyroxenes with dark bands of alteration to fibrous amphibole.


Chagrined bronzite-augite lamellae with alternating straight and inclined extinction.



Augite twin with near-orthogonal cleavage planes and inclined extinction.


Hydrothermal fibrous sericitization bounded by the outline of what appears to have been a euhedral monoclinic phenocryst, possibly a clinopyroxene. The sericite exhibits localized fan-shaped structures with low relief and straight extinction. A rare occurrence in Gombak Norite (Lee & Zhou 2009).


Sericitization of zoned plagioclase.





Accession no. R009


Locality: Singapore, near Bukit Timah Fault
Sample date: 23 August 2021
Method: hand sample
Finder: Lin Yangchen


Macroscopic description


Vacuum-impregnated with Burnham Petrographics Petropoxy 154 (n = 1.540 ± 0.001)

Weakly lithified fine-grained muddy marine sandstone of the Tengah Facies, Jurong Formation. Upper Triassic.


Petrographic analysis


30 μm unpolished with no. 1 coverslip
Norland Optical Adhesive 61 (n = 1.56, US military specification MIL-A-3920)


Blend of quartz grains, opaque hematite and silty aggregates with occasional minerals of higher birefringence.

Orange-red hematite cement. The morphology resembles that of sandstones of the Stormberg Group in South Africa (see Chima et al. 2018).


Hematite pellets surrounded by lightly chagrined quartz.





Accession no. R010


Locality:
Sample date:
Method: hand sample
Finder:


Macroscopic description





Petrographic analysis


Polished down to 0.5 μm diamond paste with no coverslip







Mineralogy texts

Craig & Vaughan
Stephen Nelson
Dexter Perkins


Databases

Common minerals in thin section
Name That Mineral
Webmineral
Mindat


Petrographic atlases

University College London catalogue
Oxford Atlas of Metamorphic Minerals
NASA lunar samples
NOVA Mineralogy
Virtual Microscope
micROCKScopic
Kurt Hollocher
Alex Strekeisen
Dexter Perkins
rockPTX
Rogers


Bibliography

[also see references within entries]

Cai, J. G. 2012. Geology of Singapore. Tritech Consultants Pte Ltd.

Chima, P., Baiyegunhi, C., Liu, K. & Gwavava, O. 2018. Diagenesis and rock properties of sandstones from the Stormberg Group, Karoo Supergroup in the Eastern Cape Province of South Africa. Open Geosciences 10:740–771.

Geological Survey of Japan, Philippine Institute of Volcanology and Seismology & Volcanological Survey of Indonesia 2000. Report of International Research and Development Cooperation ITIT Projects: Research on Volcanic Hazard Assessment in Asia. International Research and Development Cooperation Division, Agency of Industrial Science and Technology, Ministry of International Trade and Industry, Tokyo, Japan.

Geotechnical Society of Singapore & Building and Construction Authority 2019. Guidelines on Identification of Rock During Bored Piling Works.

Kumarasamy, J. Singapore Geology and its Impact on Underground Construction Works.

Lee, K. W. & Zhou, Y. 2009. Geology of Singapore 2nd edition. Defence Science & Technology Agency, Singapore.

Nurfiani, D., Wang, X., Gunawan, H., Triastuty, H., Hidayat, D., Wei, S. J., Taisne, B. & de Maisonneuve, C. B. 2021. Combining petrology and seismology to unravel the plumbing system of a typical arc volcano: an example from Marapi, West Sumatra, Indonesia. Geochemistry, Geophysics, Geosystems 22:e2020GC009524.

Oliver, G. J. H. & Gupta, A. 2017. A Field Guide to the Geology of Singapore. Lee Kong Chian Natural History Museum, National University of Singapore.

Oliver, G. J. H., Khin Zaw, Hotson, M., Meffre, S. & Manka, T. 2014. U-Pb zircon geochronology of Early Permian to Late Triassic rocks from Singapore and Johor: a plate tectonic reinterpretation. Gondwana Research 26:132–143.

Public Works Department 1976. Geology of the Republic of Singapore.

Rachmat, H. & Iqbal, M. 2000. West Nusa Tenggara Volcanoes.

Raith, M. M., Raase, P. & Reinhardt, J. 2011. Guide to Thin Section Microscopy.

Scrivenor, J. B. 1924. The geology of Singapore island. With a geological sketch-map. Journal of the Malayan Branch of the Royal Asiatic Society 2(1):1–8.

Self, S., Rampino, M. R., Newton, M. S. & Wolff, J. A. 1984. Volcanological study of the great Tambora eruption of 1815. Geology 12:659–663.

Sharma, J. S., Chi, J. & Zhao, J. 1999. Geological and geotechnical features of Singapore: an overview. Tunnelling and Underground Space Technology 14(4):419–431.

Sigurdsson, H. & Carey, S. 1989. Plinian and co-ignimbrite tephra fall from the 1815 eruption of Tambora volcano. Bulletin of Volcanology 51:243–270.

Stothers, R. B. 1984. The great Tambora eruption in 1815 and its aftermath. Science 224:1191–1198.

Wong, P. P. 1969. The surface configuration of Singapore island: a quantitative description. The Journal of Tropical Geography 29:64–74.
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