How to identify olivine?

Olivine is a common rock-forming mineral recognized by its distinctive appearance and optical properties. Identifying olivine typically involves examining its characteristics in both hand samples and under a microscope.

How to Identify Olivine?

Identifying olivine relies on a combination of its physical properties observable in a hand sample and its specific optical characteristics when viewed through a petrographic microscope.

Macroscopic Identification (Hand Sample)

When examining olivine in a hand sample, look for these key features:

• Color: Olivine is famously named for its characteristic olive-green color, which can range from yellowish-green to brownish-green. This is often its most striking feature.
• Luster: It typically exhibits a vitreous (glassy) luster.
• Habit: Olivine often occurs as granular masses or disseminated individual grains within igneous rocks. Larger, well-formed crystals are less common but can be found in some volcanic rocks.
• Hardness: With a Mohs hardness of 6.5 to 7, it's harder than glass and can scratch it.
• Fracture: Olivine lacks cleavage and breaks with a conchoidal (shell-like) or irregular fracture, which is a key distinguishing feature.

Microscopic Identification (Thin Section Petrography)

For definitive identification, especially in fine-grained rocks, geologists use a petrographic microscope to examine olivine in thin sections. Its optical properties are highly diagnostic:

• Color in Thin Section: In plane-polarized light, olivine typically appears colorless to pale olive green.
• High Relief: Olivine grains will stand out prominently against other minerals in a thin section due to their high refractive index.
• Interference Colors: When viewed under crossed polarizers, olivine displays second-order interference colors. These colors are typically bright, vibrant, and rich, often described as "peacock" or "sunset" colors (e.g., bright greens, pinks, purples, blues).
• High Retardation: The vibrant interference colors are a result of olivine's high retardation, meaning light passing through the mineral is significantly delayed, leading to the pronounced color display.
• Lack of Cleavage: A crucial distinguishing feature of olivine is its lack of cleavage. Unlike many other minerals that break along flat planes, olivine does not exhibit systematic parallel fractures.
• Distinctive Fracturing: Instead of cleavage, olivine commonly shows distinctive fracturing. These fractures are often irregular or conchoidal, appearing as random cracks across the grain.
• Alteration to Serpentine: Olivine is prone to alteration, particularly in the presence of water. It commonly alters to serpentine, which can appear as green, fibrous, or platy masses often along the edges or within fractures of the olivine grains. This alteration product can be a strong indicator of olivine's presence.

Summary Table of Microscopic Features

By carefully observing these macroscopic and microscopic characteristics, olivine can be reliably identified in geological samples.