Jacupirangite Identification Guide
How to identify jacupirangite, a dark titanaugite-magnetite pyroxenite from alkaline complexes, by its heavy dark mineralogy and field setting.
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What Jacupirangite Looks Like
Jacupirangite is a rare, dark ultramafic alkaline rock — essentially a pyroxenite dominated by titanium-rich augite (titanaugite) with abundant magnetite/titanomagnetite, plus nepheline and accessory perovskite, apatite, and biotite. It is dense, dark, and coarse to medium grained.
- Color: dark grey to black, often greenish-black
- Luster: dull to sub-metallic where magnetite is abundant; the pyroxene is glassy on fresh faces
- Transparency: opaque
- Form: massive, holocrystalline, granular; part of zoned alkaline–carbonatite intrusive complexes
Step-by-Step Field-ID Checklist
- Confirm it's a dark, heavy intrusive rock. No glassy groundmass or vesicles (not volcanic); interlocking visible crystals.
- Look for dominant black pyroxene. Stubby, blocky titanaugite grains with ~90° cleavage angles make up most of the rock.
- Test for magnetism. Abundant titanomagnetite gives a noticeable magnetic response — a strong clue.
- Note the lack of feldspar. Unlike gabbro, plagioclase is essentially absent; feldspathoids (nepheline) may be present instead.
- Check the geologic setting. Jacupirangite occurs in alkaline–carbonatite complexes, often near ijolite, nepheline syenite, and carbonatite.
Key Diagnostic Tests
- Mohs hardness: ~5–6 (pyroxene); magnetite ~6
- Streak: grey to black (magnetite component)
- Specific gravity: high, ~3.1–3.5+ owing to pyroxene and magnetite
- Magnetism: moderate to strong from titanomagnetite — useful diagnostic
- Cleavage: two pyroxene cleavages at near-90°
- Acid: no reaction unless carbonatite material is intergrown (then fizzes)
Common Look-Alikes and How to Tell Them Apart
- Gabbro: also dark and coarse, but gabbro contains abundant plagioclase feldspar (pale laths). Jacupirangite is feldspar-poor and feldspathoid-bearing, and is more strongly magnetic.
- Peridotite/dunite: dominated by olive-green olivine, often with a sugary texture and greener tone; jacupirangite is pyroxene + magnetite dominated and darker/blacker.
- Ijolite: lighter, contains abundant nepheline (greasy grey) with pyroxene; jacupirangite has far more magnetite and pyroxene and less nepheline.
- Pyroxenite (ordinary): very similar; jacupirangite is specifically the alkaline, titanaugite + magnetite + nepheline-bearing variety tied to carbonatite complexes.
- Magnetite ore: much more strongly magnetic and metallic throughout, lacking the silicate pyroxene framework.
Where Jacupirangite Is Found
The rock is named for Jacupiranga, São Paulo State, Brazil, a classic alkaline–carbonatite complex. Similar rocks occur in alkaline provinces such as the Kola Peninsula (Russia), the Magnet Cove complex (Arkansas, USA), and various East African and Canadian carbonatite complexes. It is a field rarity tied closely to carbonatite-bearing intrusions.
Frequently asked questions
How can you tell if a rock is jacupirangite?
It is a dark, heavy, coarse-grained intrusive rock dominated by black titanaugite pyroxene with abundant magnetite, giving moderate to strong magnetism. Crucially it lacks plagioclase feldspar and occurs within alkaline–carbonatite complexes alongside ijolite and carbonatite.
Jacupirangite vs gabbro — what's the difference?
Gabbro contains abundant pale plagioclase feldspar, while jacupirangite is feldspar-poor (feldspathoid-bearing) and far richer in magnetite, so it is more strongly magnetic and generally blacker and denser.
Is jacupirangite magnetic?
Yes, noticeably. Its abundant titanomagnetite produces a moderate to strong magnetic response, which helps separate it from feldspar-rich rocks like gabbro and from olivine-rich peridotite.
Where does the name jacupirangite come from?
It is named after Jacupiranga in São Paulo State, Brazil, a classic alkaline igneous–carbonatite complex where the rock was first described.