Thin Section Preparation

A thin section is a slice of rock ground down to approximately 0.03 mm thickness and mounted on a standard glass petrographic slide measuring 27 mm × 46 mm. At this thickness, most minerals become transparent, allowing optical properties to be examined under a petrographic microscope using transmitted polarized light. Thin sections are one of the most productive tools for studying rocks because they reveal mineral composition, crystal habit, optical properties, and textural relationships simultaneously. ^[1]

Preparation Steps

Cutting and Mounting the Rock Chip

A chip of rock slightly smaller than the slide is cut from the hand sample. If the rock is porous or friable - a condition common in loosely cemented sandstones or weathered material - it is impregnated with epoxy before grinding. Impregnation is accomplished by immersing the chip in liquid epoxy, placing it in a vacuum chamber, and then breaking the vacuum; atmospheric pressure forces the epoxy into the pores, strengthening the sample and preventing collapse during grinding. ^[1]

Grinding and Cementing

One side of the chip is ground smooth and flat, starting with coarse abrasive and finishing with 600-grit abrasive for routine work. The smooth face is then cemented to a clean microscope slide using epoxy, a UV-setting adhesive, or another suitable cement. Once the cement has fully cured, the excess rock is trimmed away with a diamond saw and the exposed surface is ground down to the target thickness of 0.03 mm. Machines are available to speed this process, but final grinding to exactly the right thickness is typically done by hand. The correct final thickness is confirmed by monitoring the interference colors of a known mineral in the section - quartz is the standard reference, which shows first-order white at 0.03 mm. ^[1]

Staining and Finishing

If mineral identification requires extra contrast, the thin section can be stained to distinguish K-feldspar from plagioclase, to reveal carbonate minerals, or to highlight other phases. The section is then completed by either cementing a coverslip over the surface or polishing the surface to a mirror finish. A polished surface (without coverslip) is required for reflected-light work and electron microprobe analysis; a coverslip protects the surface for standard transmitted-light petrography. ^[1]

Grain Mounts

A grain mount is an alternative to a thin section when loose mineral grains rather than intact rock need to be examined. Grains are separated, crushed if necessary, sieved to a working size fraction (-100/+170 or -140/+200 mesh work well), and cleaned to remove fine dust. A few dozen grains are sprinkled on a microscope slide and covered with a coverslip. A drop of immersion oil placed at the edge of the coverslip is drawn under by capillary action, immersing the grains for optical property measurement. ^[1]

Master UPSC Geology Optional

Ex-ONGC Geologist & Rank Holder

Learn the exact analytical answer-writing patterns needed for UPSC Optional from an AIR 2 & AIR 25 holder.

1-on-1 Personalized Mentorship

Elite Batch (Strictly 10 Seats)

Targeted Strategy for AIR 1-100

Bilingual Conceptual Lectures

Join Us

Offline in Delhi