Howieite is a rare sodium-manganese-iron silicate typically found as dark, fibrous, or platy masses within metamorphosed sedimentary rocks. It is most famous for its occurrence in the Franciscan Complex of California, where it forms in association with other rare silicates like deerite and zussmanite. Collectors prize it for its unique chemistry and specific geological occurrence in high-pressure, low-temperature metamorphic environments.
Is this howieite?
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 howieite with a known reference. Howieite sits at Mohs 5, softer than the next harder reference, harder than the previous one.
- 2Check the streakDrag the specimen across an unglazed porcelain plate. Streak: grayish black.
- 3Read the lusterHold the specimen under a strong light. Luster: greasy.
- 4Match the color rangeCompare against the expected color range: dark green, black.
- 5Look at form & habitCrystal system: triclinic. Typical habit: bladed.
Often confused with
Howieite vs. its common look-alikes, and how to tell them apart in the field.
How to tell apart: Deerite is the harder of the two (Mohs 6 vs. 5); streak differs, so Howieite leaves grayish black, Deerite leaves black gray; luster reads greasy on Howieite and submetallic on Deerite.
How to tell apart: Howieite is noticeably harder (Mohs 5 vs. 3); streak differs, so Howieite leaves grayish black, Zussmanite leaves white; luster reads greasy on Howieite and vitreous on Zussmanite.
Often found alongside howieite
Minerals reported to co-occur with howieite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- Na(Fe,Fe,Al,Mg)₁₂(Si₆O₁₇)₂(O,OH)₁₀
- Mohs hardness
- 5
- Density
- 3.378 g/cm³
- Colors
- Streak
- Grayish black
- Luster
- Greasy
- Transparency
- Transparent to translucent
- Crystal system
- Triclinic
- Crystal habit
- Bladed
- Cleavage
- Good on {010}, fair on {100}, poor on {210}
- Rarity
- Rare
- Uses
- Collector
- Host rock
- An essential mineral in some of the metamorphosed shales, siliceous ironstones, and impure limestones of the Franciscan Formation (Laytonville district, California, USA)
Where rockhounds find howieite
Classic worldwide localities
- Laytonville quarry, and at Covelo, Mendocino Co., California, USA
- Ward Creek, Sonoma Co., California, USA
- Panoche Pass, San Benito Co., California, USA
- Pacheco Pass, Santa Clara and Merced Cos., California, USA
- Brezovica, Yugoslavia
- Tanemaya mine, Kumamoto Prefecture, Japan
Field-hunting tip
Where it forms: An essential mineral in some of the metamorphosed shales, siliceous ironstones, and impure limestones of the Franciscan Formation (Laytonville district, California, USA). If you start seeing deerite, zussmanite, stilpnomelane in float, you are in the right ground. Typical habit: bladed. Train your eye for that shape before scanning the outcrop.
Common questions
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Find howieite on the map
RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.
