Maskelynite Identification Guide

What Maskelynite Looks Like

Maskelynite is a naturally occurring glass formed when plagioclase feldspar is transformed in place by intense shock pressure (from meteorite impacts or within shocked meteorites) without melting and flowing - a process called diaplectic glass formation. It preserves the original crystal's outline but has lost its crystalline structure. It is typically colorless to gray, glassy, and found as small grains within a rock or meteorite rather than as standalone specimens.

• Color: colorless, white, gray, sometimes brownish
• Luster: vitreous (glassy)
• Transparency: transparent to translucent
• Form: retains the lath/grain shape of the original feldspar; isotropic glass internally

Step-by-Step Field ID Checklist

1. Context first - maskelynite is found in shocked rocks: meteorites (especially shergottite martian meteorites), impact-crater rocks, and impactites. Context is the biggest clue.
2. Look for feldspar-shaped grains that appear glassy and clear rather than crystalline.
3. Confirm it is glass, not melt - maskelynite keeps the original grain shape (it did not flow), unlike true melt glass.
4. Petrographic confirmation needed - under crossed polarizers it is isotropic (stays dark/extinct), unlike crystalline plagioclase which shows birefringence and twinning.

Key Diagnostic Tests

• Optical isotropy: under a polarizing microscope it remains dark on rotation (no birefringence), the definitive test versus crystalline feldspar.
• Mohs hardness: similar to feldspar glass, roughly 5.5-6.
• Refractive index: lower than crystalline plagioclase of the same composition (a measurable shock signature).
• Composition: plagioclase chemistry (Na-Ca aluminosilicate) confirmed by microprobe.
• No cleavage: as a glass it shows conchoidal fracture, not the two feldspar cleavages.

Common Look-Alikes and How to Tell Them Apart

• Plagioclase feldspar (unshocked): shows cleavage, twinning, and birefringence; maskelynite is isotropic with no cleavage.
• Volcanic/melt glass (obsidian, tektite): these form by melting and flowing; maskelynite preserves the original grain shape and occurs in shocked, not erupted, rock.
• Fused glass (lechatelierite): that is shocked quartz glass; maskelynite is feldspar-composition glass.
• Ordinary mineral inclusions: require thin-section optics or microprobe to confirm composition and isotropy.

Where It Is Typically Found

Maskelynite occurs in meteorites (notably martian shergottites and shocked ordinary chondrites) and in terrestrial impact structures and impactites worldwide. It is essentially a microscopic-to-grain-scale phase, identified in thin section rather than collected as hand specimens.