Maskelynite

Maskelynite is a colorless, optically isotropic aluminosilicate glass of plagioclase feldspar composition ((Na,Ca)(Si,Al)₄O₈, typically labradorite to bytownite, An₄₅–An₈₀) formed by hypervelocity shock metamorphism at peak dynamic pressures between 25 and 45 GPa in Martian meteorites (especially shergottites), lunar rocks, ordinary and achondritic meteorites, and terrestrial impactites. First described in 1872 by the Austrian mineralogist Gustav Tschermak inside the Shergotty Martian basaltic meteorite (which fell in Bihar, India, in 1865) and named in honor of the British Keeper of Minerals at the British Museum, Nevil Story-Maskelyne, maskelynite is the classic archetype of a diaplectic glass (from the Greek diaplesso, "to destroy by striking," coined by Wolf von Engelhardt and Dieter Stöffler in 1967) or high-pressure shock-quenched dense glass.

Unlike normal thermally fused plagioclase glass—which forms above the 1 atm liquidus temperature (1,100–1,550 °C) and exhibits vesicles, flow schlieren, homogenized chemical zoning, and rounded or resorbed grain boundaries—maskelynite pseudomorphously preserves the euhedral to subhedral lath morphology, sharp grain boundaries against adjacent pyroxene and olivine crystals, preexisting cleavage traces, and fine-scale primary igneous oscillatory Na–Ca zoning of the precursor crystalline plagioclase while exhibiting full optical isotropy (total extinction at all stage angles under cross-polarized light).

Diagnostic Criteria in Thin Section and Raman Spectroscopy

In petrographic thin sections of shocked gabbros, norites, anorthosites, and Martian shergottites, maskelynite is recognized by combining transmitted-light microscopy, refractive-index refractometry, and vibrational spectroscopy:

1. Optical Isotropy and Pseudomorphous Habit: Under plane-polarized light, maskelynite forms clear, colorless, non-vesicular laths intergrown in ophitic or subophitic textures with shocked clinopyroxene (pigeonite/augite displaying mechanical twin lamellae and mosaicism). When the analyzer is inserted (cross-polarized light), the entire plagioclase lath remains dark (isotropic, birefringence Δn = 0.000) across a 360° rotation, without the characteristic polysynthetic albite twinning of crystalline plagioclase.
2. Elevated Density and Refractive Index Relative to Normal Plagioclase Glass: Because maskelynite forms under tens of gigapascals of compression and quenches or decompresses before structural relaxation can complete, its density (ρ ≈ 2.55–2.65 g/cm³ for labradorite An₅₀–An₆₀) and refractive index (n ≈ 1.540–1.565) lie intermediately between those of crystalline plagioclase (ρ ≈ 2.68–2.71 g/cm³, n ≈ 1.555–1.570) and 1-atm thermally fused plagioclase glass (ρ ≈ 2.48–2.52 g/cm³, n ≈ 1.525–1.545). Annealing maskelynite at 800–900 °C for several hours at 1 atm causes its density and refractive index to relax downward toward normal glass values or devitrify back into polycrystalline plagioclase.
3. Micro-Raman and Infrared Structural Signature: Whereas crystalline labradorite exhibits sharp T–O–T tetrahedral ring-breathing Raman peaks at 478 cm⁻¹ and 508 cm⁻¹, maskelynite displays a broad, smooth asymmetric band centered at 490–510 cm⁻¹ and 980–1,020 cm⁻¹, recording breakdown of long-range translational periodicity alongside retention of fourfold (and minor shock-compressed fivefold/sixfold) AlO₄–SiO₄ tetrahedral ring statistics.

Comparison of Crystalline Plagioclase, Maskelynite, and Normal Impact Melt Glass

The Diaplectic Solid-State Transformation Model (Stöffler & Hörz)

For more than five decades, experimental and meteoritic petrologists have debated two end-member physical pathways for the formation of maskelynite—both of which operate across different pressure–temperature regimes:

In the classic solid-state diaplectic amorphization model established via shock-recovery experiments on single-crystal plagioclase by Daniel J. Milton and Paul S. DeCarli (1963), Dieter Stöffler (1967, 1972), and Friedrich Hörz, shock compression between 24 and 40 GPa drives crystalline plagioclase past its Hugoniot Elastic Limit (~5 GPa) into a high-pressure regime where tetrahedral framework bonds collapse into a disordered high-coordination state at temperatures below the zero-pressure melting point. During rapid adiabatic decompression, the disordered high-pressure phase is kinetically unable to revert to the ordered triclinic framework of crystalline plagioclase and instead freezes in the solid state as diaplectic plagioclase glass. Strong evidence for solid-state transformation in terrestrial suevite and shocked anorthosites (such as the Mistastin Lake and Manicouagan impact craters in Canada) includes the coexistence within single plagioclase grains of birefringent crystalline domains containing planar deformation features (PDFs) grading transitionally into isotropic maskelynite lamellae.

The High-Pressure Dense-Liquid Quench Model (Chen & El Goresy, 2000)

In 2000, Ming Chen and Ahmed El Goresy (followed by high-pressure diamond-anvil and shock-Hugoniot analyses by Paul D. Asimow, Thomas G. Sharp, and Oliver Tschauner) challenged the purely solid-state origin of smooth, fracture-free maskelynite in Martian shergottites (such as Shergotty, Zagami, and Tissint). They noted that because the melting curve (dT_m/dP) of calcic plagioclase has a negative or flat Clapeyron slope above 10–15 GPa, shock compression to 28–35 GPa can raise the internal temperature above the high-pressure liquidus of plagioclase while leaving adjacent high-melting-point clinopyroxene and olivine solid.

Under this high-pressure dense-melt quenching mechanism:

1. Plagioclase melts in situ at high pressure (25–35 GPa), where the silicate liquid is strongly compressed (ρ_melt ≈ ρ_crystal), preventing vesicle nucleation and suppressing convective flow.
2. Surrounding unmelted pyroxene frames confine the molten plagioclase pockets, and small smooth offshoots of maskelynite occasionally inject into fractures in neighboring pyroxenes while arrays of expansion cracks form in the pyroxene around the maskelynite when the glass expands slightly upon pressure release.
3. Because thermal diffusivity of silicate rock (κ ~ 10⁻⁶ m²/s) is orders of magnitude faster than chemical diffusivity of NaSi ↔ CaAl in viscous aluminosilicate melt (D ~ 10⁻¹⁴ m²/s), conductive heat transfer into the colder adjoining pyroxene crystals quenches the molten plagioclase into dense glass within milliseconds—well before pressure drops to 1 atm and before micrometric core-to-rim Ca–Na zoning can diffuse away.

Modern shock classifications (Stöffler et al., 2018; Fritz et al., 2017) recognize both pathways: diaplectic plagioclase glass (formed by sub-solidus solid-state amorphization at 24–32 GPa, retaining precursor cleavage and PDFs) and shock-quenched dense plagioclase glass (formed by isochoric high-pressure melting and conductive quenching while still under confining pressure, yielding smooth, fracture-free shergottite maskelynite).

Dependence of the Maskelynite Transition Pressure on Anorthite Content

Shock-recovery calibrations by Dieter Stöffler, Jörg Fritz, and Artem Chanyshev demonstrate that the threshold shock pressure P_mask required for full isotropization of plagioclase decreases linearly as anorthite content (An = Ca / [Ca + Na + K]) increases (because Al-rich CaAl₂Si₂O₈ frameworks collapse at lower pressures than Si-rich NaAlSi₃O₈ albite):

• Albite–Oligoclase (An₀–An₂₀): Requires 33 to 45 GPa for full transformation to maskelynite.
• Andesine–Labradorite (An₃₀–An₆₀, typical of Martian shergottites and gabbroic target rocks): Transforms between 26 and 32 GPa.
• Bytownite–Anorthite (An₇₀–An₁₀₀, typical of lunar highlands ferroan anorthosites): Transforms at 22 to 26 GPa.

Maskelynite plays a central role in planetary mechanics—specifically in explaining how intact igneous rocks can be ejected from the surface of Mars (escape velocity v_esc = 5.03 km/s) and the Moon (v_esc = 2.38 km/s) without being vaporized or melted into unrecognizable slag. Through H. Jay Melosh's (1984) near-surface spallation theory, interference between the direct compressional shock wave and the rarefaction tensile wave reflected from the free planetary surface accelerates a thin near-surface plate of basalt above 5.03 km/s while capping its peak shock pressure at 25 to 45 GPa—matching the maskelynite stability window observed in >85% of all Martian shergottite meteorites. Furthermore, during shock compression inside the Martian atmosphere, bubbles of Martian atmospheric gas (CO₂, N₂, ⁴⁰Ar/³⁶Ar, and Xe isotopes) are shock-implanted into maskelynite and associated black shock-melt veins (as discovered by Donald D. Bogard and Pratt Johnson in EETA79001 in 1983), providing the definitive geochemical proof that the SNC (Shergottite–Nakhlite–Chassignite) meteorite clan originated on Mars.

Is maskelynite considered a valid mineral species by the International Mineralogical Association (IMA)?

No. Because maskelynite is an amorphous glass that lacks a long-range crystalline lattice, it is classified petrographically as a mineraloid or shock-metamorphic diaplectic glass variety rather than an IMA-approved crystalline mineral species, even though its stoichiometry and external crystal shape match those of plagioclase feldspar.

How can primary igneous chemical zoning survive inside maskelynite if the crystal was shocked to 30 GPa?

Coupled NaSi ↔ CaAl interdiffusion in aluminosilicate frameworks requires simultaneously breaking strong tetrahedral Al–O and Si–O bonds and has a very slow diffusion coefficient (D ≤ 10⁻¹⁴ to 10⁻¹² m²/s even at 1,500 °C). Because a hypervelocity shock pulse in a spalled meteorite boulder lasts only 1 to 50 milliseconds before conductive cooling to surrounding pyroxene quenches the grain, the diffusive migration distance (x ~ √(Dt)) is less than 0.01 to 0.1 μm, leaving 10–100 μm primary igneous zoning bands intact.

Why do Martian shergottites almost always contain maskelynite whereas nakhlites and chassignites do not?

Launch spallation dynamics and target lithology differ across the Martian meteorite suites. Basaltic shergottites were spalled from strong, fine-grained lava flows by high-energy impacts that subjected the ejecta to 28–45 GPa, converting their labradorite (An_50–60) into maskelynite. Clinopyroxenitic nakhlites and dunitic chassignites were spalled from nearer the free surface at lower peak shock pressures (14–20 GPa), below the ~28 GPa threshold needed to isotropize their sodic intercumulus plagioclase (An_25–35).

1. Tschermak, G. (1872). "Die Meteoriten von Shergotty und Gopalpur." Sitzungsberichte der Kaiserlichen Akademie der Wissenschaften in Wien, Mathematisch-Naturwissenschaftliche Classe, 65(1), 122–146.
2. Milton, D. J., & DeCarli, P. S. (1963). "Maskelynite: Formation by explosive shock." Science, 140(3567), 670–671. https://doi.org/10.1126/science.140.3567.670
3. Stöffler, D., Hamann, C., & Metzler, K. (2018). "Shock metamorphism of planetary silicate rocks and sediments: Proposal for an updated classification system." Meteoritics & Planetary Science, 53(1), 5–49. https://doi.org/10.1111/maps.12912
4. Chen, M., & El Goresy, A. (2000). "The nature of maskelynite in shocked meteorites: Not diaplectic glass but a glass quenched from shock-induced dense melt at high pressures." Earth and Planetary Science Letters, 179(3–4), 489–502. https://doi.org/10.1016/S0012-821X(00)00130-800130-8)
5. Fritz, J., Greshake, A., & Fernandes, V. A. (2017). "Revising the shock classification of meteorites." Meteoritics & Planetary Science, 52(6), 1216–1239. https://doi.org/10.1111/maps.12845