Merwinite is a rare nesosilicate mineral typically found in high-temperature contact metamorphic zones where limestones have been altered by igneous intrusions. It most commonly appears as colorless or pale green granular masses rather than well-defined crystals. It is a prized specimen for advanced collectors specializing in skarn mineralogy.

Hardness

Mohs

Luster

Vitreous

Streak

White

Transparency

Transparent

mineralrare

Merwinite

Hardness: 6
Luster: Vitreous
Streak: White
Transparency: Transparent

Is this merwinite?

5-step field check

Run through these checks against the specimen in your hand. The more boxes tick, the more confident the ID.

1
Test the hardness
Try to scratch merwinite with a known reference. Merwinite sits at Mohs 6, softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Merwinite leaves a white streak.
3
Read the luster
Hold the specimen under a strong light. Merwinite typically shows a vitreous luster.
4
Match the color range
Compare against the expected color range: colorless, white, pale green, gray.
5
Look at form & habit
Crystal system: monoclinic. Typical habit: granular, massive, or as rare small prismatic crystals.

Often confused with

Merwinite vs. its common look-alikes, and how to tell them apart in the field.

mineral

Monticellite

Mohs 5

How to tell apart: Merwinite is noticeably harder (Mohs 6 vs. 5).

Often found alongside merwinite

Minerals reported to co-occur with merwinite. Spotting these in float or country rock is a strong cue you are in the right ground.

All properties

Chemical formula: Ca₃Mg(SiO₄)₂
Mohs hardness: 6
Density: 3.15-3.20 g/cm³
Colors: Colorless White Pale Green Gray
Streak: White
Luster: Vitreous
Transparency: Transparent
Crystal system: Monoclinic
Crystal habit: Granular, Massive, Or as Rare Small Prismatic Crystals
Cleavage: Poor
Rarity: Rare
Uses: Collector
Host rock: Contact Metamorphosed Limestone (skarn Deposits)
Typical price: $20-100 per specimen

Where rockhounds find merwinite

Classic worldwide localities

Crestmore, California, USA
Carlingford, Ireland
Kilchoan, Scotland
Fuka, Japan

Field-hunting tip

Look in contact metamorphosed limestone (skarn deposits) country, the host setting where merwinite typically forms. If you start seeing larnite, gehlenite, calcite in float, you are in the right ground. Field specimens usually show a granular, massive, or as rare small prismatic crystals habit, so train your eye for that shape before scanning the outcrop.

Common questions

How do you identify merwinite?+

Mohs hardness is 6. It typically shows a vitreous luster. The streak is white. Common colors include colorless, white, pale green, gray.

Where is merwinite found?+

Notable localities include Crestmore, California, USA; Carlingford, Ireland; Kilchoan, Scotland; Fuka, Japan.

How much is merwinite worth?+

Typical asking prices fall in the range of $20-100 per specimen. Quality, size, and provenance can move individual specimens well outside that range.

What rocks look like merwinite?+

Merwinite is most often confused with Monticellite, Larnite, Bredigite. A quick hardness test and a streak check separate the look-alikes faster than color alone.

What minerals are found with merwinite?+

Merwinite commonly co-occurs with Larnite, Gehlenite, Calcite, Spurrite, Monticellite. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does merwinite form in?+

Merwinite typically forms in contact metamorphosed limestone (skarn deposits). Working float back to the host body is the standard way to chase a fresh occurrence.

What is merwinite used for?+

Merwinite is used in collector.

Find merwinite on the map

RockHoundR shows mapped rockhounding spots, access rules, and lets you log every find.