Franciscanite is a rare manganese vanadate mineral typically found as tiny, reddish prismatic or acicular crystals within the blueschist facies rocks of the Franciscan Complex. It is most easily identified by its distinctive bright yellow fluorescence under short-wave ultraviolet light, which sets it apart from associated manganese minerals. Specimens are highly prized by systematic mineral collectors due to the specific, limited locality from which they are sourced.
Is this franciscanite?
5-step field checkRun through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.
- 1Test the hardnessTry to scratch franciscanite with a known reference. Franciscanite sits at Mohs ~4, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: brownish red.
- 3Read the lusterHold the specimen under a strong light. Luster: vitreous.
- 4Match the color rangeCompare against the expected color range: cherry-red when fresh, darkening, brownish red with exposure.
- 5Look at form & habitCrystal system: trigonal. Typical habit: acicular to prismatic crystals.
Often confused with
Franciscanite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Franciscanite is noticeably harder (Mohs ~4 vs. 2.5-3); streak differs, so Franciscanite leaves brownish red, Eurekadumpite leaves light turquoise; luster reads vitreous on Franciscanite and pearly flakes, silky aggregates on Eurekadumpite.
How to tell apart: Garnet is the harder of the two (Mohs 6.5-7.5 vs. ~4); streak differs, so Franciscanite leaves brownish red, Garnet leaves white.
Often found alongside franciscanite
Minerals reported to co-occur with franciscanite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Mn₆(V□)(SiO₄)₂O₃(OH)₃
- Mohs hardness
- ~4
- Density
- 4.1 g/cm³
- Colors
- Cherry-red when freshDarkeningBrownish red with exposure
- Streak
- Brownish red
- Luster
- Vitreous
- Transparency
- Transparent
- Crystal system
- Trigonal
- Crystal habit
- Acicular to prismatic crystals
- Cleavage
- None
- Fluorescence
- Bright yellow under SW UV
- Rarity
- Rare
- Uses
- Collector
- Host rock
- As sparse, irregular segregations within a sheared sonolite-bearing assemblage in chert
Where rockhounds find franciscanite
Classic worldwide localities
- Pennsylvania mine, San Antonio Valley, Santa Clara Co., California, USA
Field-hunting tip
Where it forms: As sparse, irregular segregations within a sheared sonolite-bearing assemblage in chert. If you start seeing sonolite, hausmannite, braunite in float, you are in the right ground. Typical habit: acicular to prismatic crystals. Train your eye for that shape before scanning the outcrop.
Common questions
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Find franciscanite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
