I had some other impressions that might help.
Based on your description—minerals that are grass green, exhibit conchoidal fracture, lack cleavage, have a specific gravity around 3.11, originate from Pakistan, and show dark edges resembling extinction like tourmaline with variations in light and color along axes—the primary candidate is tourmaline, specifically green elbaite or verdelite.
However, there are other possibilities. Included at the end are some simple tests for each (RI ect.) and provide a focused list with simple tests to confirm their identity, keeping in mind the optical properties like extinction and pleochroism you mentioned.
Grok and I dumped some properties for you.
List of Minerals
- Tourmaline (Elbaite/Verdelite)
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Description: A borosilicate mineral, often grass green in its elbaite form (verdelite). It’s common in Pakistan, especially in pegmatites of the Skardu and Swat regions. Tourmaline has no cleavage, shows conchoidal fracture, and has a specific gravity of 3.0–3.2, closely matching 3.11. It exhibits strong pleochroism, with color variations (green to yellowish-green or darker green) when rotated, and straight extinction under cross-polarized light. Dark edges may appear due to pleochroism or zoning, where light absorption varies along crystal axes.
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Why It Fits: Tourmaline’s grass-green varieties, lack of cleavage, conchoidal fracture, and specific gravity align perfectly. Its trigonal crystal system and optical properties (uniaxial, negative) produce extinction parallel to the c-axis, with darker edges in certain orientations due to pleochroism or inclusions.
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Epidote
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Description: A calcium aluminum iron silicate, sometimes pistachio or yellowish-green, found in Pakistan’s metamorphic rocks (e.g., Hunza Valley). It has a specific gravity of 3.3–3.5, slightly higher than 3.11, and shows conchoidal to uneven fracture with no true cleavage (though it may appear to have one imperfect cleavage). Epidote’s pleochroism can show green to yellowish hues, but extinction is typically inclined, not straight like tourmaline.
Why It Fits: Epidote can resemble grass green, but its higher specific gravity and occasional cleavage-like parting make it less likely. Its optical properties include birefringence and inclined extinction, which may not match the straight extinction you described.
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Prehnite
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Description: A calcium aluminum silicate, often pale to medium green, found in Pakistan’s basalt cavities (e.g., Balochistan). It has a specific gravity of 2.8–3.0, slightly below 3.11, and shows conchoidal fracture with no cleavage. Prehnite lacks strong pleochroism, appearing more uniform, and has inclined extinction, which may not align with the dark-edged, tourmaline-like extinction.
Simple Tests to Confirm Identity
These tests are practical for field or basic lab settings, focusing on physical and optical properties without advanced equipment. I always use weight, size, RI, crystal faces which I thought was a long upper trigonal face not characteristic of a tourmaline crystal (but not unheard of).
- Tourmaline (Elbaite/Verdelite)
- Hardness Test: Scratch with a steel knife (hardness 5.5). Tourmaline, with a hardness of 7–7.5, won’t scratch easily. Quartz (hardness 7) may feel similar, but tourmaline’s trigonal prisms distinguish it.
- Pleochroism Check: Rotate a loose crystal or thin fragment under a polarizing filter (or two crossed filters if available). Tourmaline shows distinct color changes (e.g., grass green to darker green or yellowish). Use a bright light to observe edges darkening in certain orientations.
- Streak Test: Rub on an unglazed porcelain tile. Tourmaline gives a white streak. Colored streaks would indicate another mineral.
- Crystal Shape: Look for striated, prismatic crystals with triangular cross-sections. Pakistan tourmalines often form euhedral prisms in pegmatites.
- Specific Gravity (Field Estimate): Compare weight in hand to quartz (lighter) or epidote (heavier). Tourmaline feels balanced for its size.
- Pyroelectricity (Optional): Heat gently with a lighter (avoid cracking), then see if it attracts ash or dust. Tourmaline’s pyroelectric property is unique but requires caution.
- Epidote
- Hardness Test: Scratch with a knife. Epidote (hardness 6–7) may show slight scratching, unlike tourmaline’s resistance.
- Cleavage Check: Examine broken surfaces. Epidote may show one imperfect cleavage or parting, appearing as flat planes, unlike tourmaline’s glassy, curved fractures.
- Pleochroism Check: Rotate under a polarizing filter. Epidote shows weaker pleochroism (green to yellow) compared to tourmaline’s vivid shifts. Extinction is inclined, not parallel, if viewed under a microscope.
- Streak Test: Produces a white streak, similar to tourmaline.
- Magnetism (Optional): If iron-rich, epidote may weakly attract a magnet. Tourmaline won’t.
- Association: Epidote often occurs with quartz or garnet in metamorphic rocks, not pegmatites like tourmaline.
- Prehnite
- Hardness Test: Scratch with a knife. Prehnite (hardness 6–6.5) scratches more easily than tourmaline.
- Fracture Check: Look for conchoidal, slightly brittle fractures. Prehnite’s surfaces are less glassy than tourmaline’s.
- Color Uniformity: Rotate under light. Prehnite lacks strong pleochroism, staying uniformly green, unlike tourmaline’s color shifts.
- Streak Test: White streak, like tourmaline and epidote.
- Transparency: Prehnite is often translucent with a waxy luster, while tourmaline is transparent to translucent with a vitreous shine.
- Association: Prehnite forms botryoidal masses or radiating crystals in basalts, not prismatic like tourmaline in pegmatites.
- Consider it a little light in terms of saturation and hues.
Notes on Extinction and Optical Properties
“Edges appear to have dark areas like extinction” and tourmaline’s color variations along axes. In tourmaline, straight extinction occurs parallel to the c-axis in longitudinal sections, where the crystal goes dark every 90° under crossed polarizers (if using a microscope). The dark edges you observe are likely due to pleochroism, where light absorption increases in certain orientations, making edges appear darker, especially in green varieties. Zoning (color bands) in Pakistan tourmalines can enhance this effect. Epidote and prehnite show inclined extinction, which doesn’t match the straight, axis-aligned darkening of tourmaline.
Conclusion
Tourmaline (elbaite/verdelite) is the best match given its prevalence in Pakistan, exact specific gravity, conchoidal fracture, lack of cleavage, and optical properties like pleochroism and straight extinction. Epidote and prehnite are less likely but included for completeness. Use the hardness, pleochroism, and crystal shape tests to confirm tourmaline, as they’re distinctive and practical. If you have access to a loupe or polarizers, the pleochroism check is especially diagnostic for tourmaline’s grass-green hues.
