Tilasite is a rare calcium magnesium arsenate fluoride mineral often found in metamorphosed ore deposits. It typically occurs as small, prismatic, yellowish to colorless crystals that can be difficult to distinguish from other similar species without mineralogical testing. It is most famous for its occurrence in the classic mining localities of Långban, Sweden, and the Franklin Mining District in New Jersey.
Is this tilasite?
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 tilasite with a known reference. Tilasite 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: white.
- 3Read the lusterHold the specimen under a strong light. Luster: resinous, vitreous on cleavages.
- 4Match the color rangeCompare against the expected color range: violet gray, green, gray, pink brown.
- 5Look at form & habitCrystal system: monoclinic. Typical habit: massive, prismatic.
Often confused with
Tilasite vs. its common look-alikes, and how to tell them apart in the field.
Often found alongside tilasite
Minerals reported to co-occur with tilasite. Spotting these in float or country rock is a strong cue you are in the right ground.
All properties
- Chemical formula
- CaMg(AsO₄)F
- Mohs hardness
- 5
- Density
- 3.77 g/cm³
- Colors
- Streak
- White
- Luster
- Resinous, vitreous on cleavages
- Transparency
- Translucent
- Crystal system
- Monoclinic
- Crystal habit
- Massive, prismatic
- Cleavage
- On {101}, good; partings on {133}, {102}, {011}
- Fluorescence
- Pink-orange under SW UV
- Rarity
- Rare
- Uses
- Collector
- Host rock
- Typically in metamorphosed manganese or zinc deposits containing arsenic
Where rockhounds find tilasite
Classic worldwide localities
- Sterling Hill, Ogdensburg, Sussex Co., New Jersey, USA
- White Tail Deer mine, Bisbee, Cochise Co., Arizona, USA
- Monticello tin district, Sierra Co., New Mexico, USA
- Långban, Värmland, Sweden
- East flank of Pizzo Cervandone, Alpe Devero, Val d’Aosta, Piedmont, and at Val Strona, Ossola, Italy
- Urupskoye copper deposit, northern Caucasus Mountains, Russia
Field-hunting tip
Where it forms: Typically in metamorphosed manganese or zinc deposits containing arsenic. If you start seeing manganoan diopside, manganberzeliite, svabite in float, you are in the right ground. Typical habit: massive, prismatic. Train your eye for that shape before scanning the outcrop.
Common questions
How do you identify tilasite?+
Where is tilasite found?+
Is tilasite safe to handle?+
What rocks look like tilasite?+
What minerals are found with tilasite?+
What kind of rock does tilasite form in?+
What is tilasite used for?+
Find tilasite on the map
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