Winonaite (WIN) meteorites are a small and rare group of primitive achondrites that bridge the gap between chondritic meteorites—primitive, unmelted solar system material—and fully differentiated achondrites, which have undergone extensive melting and segregation of metal and silicates. Winonaites are noteworthy for displaying partial melting and metamorphic features while retaining chemical and textural hints of their chondritic heritage. Studying these meteorites helps unravel the complex processes that occurred on small parent bodies during the early stages of planetary formation.

Classification and Origin

Winonaites are classified as primitive achondrites due to their transitional nature. They share chemical and mineralogical similarities with two other rare groups:

1. IAB/IIICD Iron Meteorites: These iron meteorites contain silicate inclusions that share similarities with winonaites, suggesting a genetic link or a common parent body. This connection implies that the parent body underwent incomplete differentiation, preserving zones of metal-rich and silicate-rich material.
2. Acapulcoite-Lodranite Clan: Winonaites show parallels to these small-body achondrites that have also experienced partial melting and thermal metamorphism but not to the extent of forming full-fledged magmatic achondrites.

Their chemical composition, oxygen isotopic signatures, and refractory element abundances hint that winonaites formed on a parent asteroid that underwent low to moderate degrees of heating—enough to cause partial melting, metamorphism, and chemical re-equilibration, but insufficient for producing large-scale segregation of metallic cores and silicate mantles like we see in differentiated asteroids.

Mineralogy and Composition

Winonaites primarily consist of:

• Olivine (Fo-rich): A magnesium-rich olivine, typically more equilibrated than in primitive chondrites.
• Low-Ca Pyroxene: Usually orthopyroxene or a mixture of ortho- and clinopyroxene, indicating partial melting and recrystallization.
• Plagioclase Feldspar: Often more abundant and well-formed than in typical chondrites, reflecting partial melt extraction and chemical re-distribution.
• Metal and Sulfides: Iron-nickel metal grains and troilite (FeS) persist, though their distribution and morphology have often been modified by thermal processes.
• Accessory Phases: Chromite, phosphides, and phosphates may occur in trace amounts, providing clues about redox conditions, thermal history, and partial melting degrees.

The mineral assemblage and textural changes illustrate a scenario where the precursor chondritic rock was heated enough to form low-degree partial melts and cause minerals to recrystallize, but not to completely erase its primitive characteristics.

Petrologic Features and Thermal History

Winonaites reflect partial melting and metamorphism at temperatures higher than those that produce simple metamorphosed chondrites, but lower than fully differentiated achondrites. This intermediate degree of heating allowed some melt to form and potentially migrate within the parent asteroid, redistributing elements and minerals. The result is a rock that is more equilibrated and less chondrule-rich than chondrites, yet not as homogeneous or fully igneous as diogenites, eucrites, or angrites.

Observations Under a Light Microscope

Sample Preparation

To examine a Winonaite meteorite under a light microscope, a thin section is carefully prepared. The meteorite is sliced, polished, and mounted on a glass slide. Thin sections (about 30 micrometers thick) allow light to pass through many minerals, enabling petrographic analysis using transmitted and polarized light microscopy.

Chondrule Relics and Texture

Under the microscope, winonaites often lack well-defined, intact chondrules common in ordinary or carbonaceous chondrites. However, you may still find:

• Relict Chondrules or Partial Chondrule Outlines: In some cases, subtle, partially recrystallized remnants of chondrule boundaries can be discerned. These appear as faint, rounded outlines or domains of slightly different grain sizes and mineral orientations.
• Granoblastic Texture: Most of the rock typically exhibits a metamorphic, granoblastic texture—an assemblage of equigranular crystals that have recrystallized and equilibrated, often with triple junction grain boundaries at roughly 120° angles. This texture suggests prolonged thermal metamorphism and slow cooling.

Mineral Identification in Transmitted Light

• Olivine:
  Under plane-polarized light, olivine is generally colorless with high relief. Under crossed polars, it displays moderate to high-order interference colors. Unlike in primitive chondrites, olivine grains in winonaites tend to be more uniform in composition and size, reflecting chemical equilibration.
• Low-Ca Pyroxene (Orthopyroxene):
  Pyroxenes appear colorless to pale brown in thin section. They show distinct cleavage at nearly 90° angles. Under crossed polars, they yield first-order interference colors, helping to distinguish them from olivine. Orthopyroxene grains are often more homogeneous and larger than those found in less equilibrated meteorites.
• Plagioclase Feldspar:
  Plagioclase appears clear under plane-polarized light and may exhibit polysynthetic twinning visible under crossed polars. Grains are generally more well-defined than in unequilibrated chondrites, reflecting the partial melting and crystallization processes that fostered stable, larger crystals.

Opaque Minerals and Reflective Inclusions

• Metallic Iron-Nickel and Sulfides:
  In transmitted light, opaque minerals appear black or very dark. Switching to reflected light microscopy reveals bright, reflective grains of metal alloys (kamacite and taenite) and sulfides (troilite). Their distribution can be patchy or clustered, and they often appear in association with grain boundaries or as inclusions within silicates.
• Chromite and Other Accessory Minerals:
  These small, opaque phases may appear as tiny black specks in transmitted light and slightly more reflective under reflected light. Their presence and chemistry can help decipher redox conditions and melt processes.

Interference Colors and Extinction

Under crossed polarized light, the equilibrated silicate minerals show relatively uniform interference colors and extinction patterns. Olivine and pyroxene grains are typically large enough to exhibit well-defined extinction positions when the microscope stage is rotated. This consistency contrasts with the more chaotic, fine-grained textures and compositional variability found in more primitive chondrites.

Lack of Vesicles or Flow Structures

Winonaites generally do not display vesicular textures or strong flow structures associated with igneous melts. Instead, their metamorphic signatures dominate the microscopic view. The rock’s texture and composition indicate partial melting and recrystallization occurred at relatively low degrees, without extensive melt migration or magmatic flow.

Scientific Significance

Examining Winonaite meteorites such as those similar to the known type specimen “Winona” provides a unique window into the early solar system’s intermediate stages of planetary evolution. By examining their mineralogy and microtextures under the light microscope:

• Thermal and Metamorphic History: The granoblastic textures and equilibrated minerals reflect moderate thermal metamorphism, offering clues about heating events on the parent asteroid.
• Partial Melting Processes: The presence of stable, large silicate grains and partially lost chondrule textures indicate the onset of partial melt formation and melt migration processes that were never completed.
• Link to Other Meteorite Groups: The similarities to certain IAB/IIICD irons and acapulcoite-lodranite clan meteorites suggest a shared evolutionary pathway or common parent body characteristics, aiding in reconstructing the architecture and history of these early solar system bodies.