Minehillite is a rare phyllosilicate mineral uniquely associated with the zinc deposits of the Franklin Mining District. It is most easily identified by its bright blue fluorescence under ultraviolet light and its characteristic micaceous, platy crystal structure.

Hardness

Mohs

Luster

Pearly on cleavage, vitreous on fractures

Streak

White

Transparency

Transparent

mineralrarePhyllosilicate family

Minehillite

Hardness: ~4
Luster: Pearly on cleavage, vitreous on fractures
Streak: White
Transparency: Transparent

Is this minehillite?

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 minehillite with a known reference. Minehillite sits at Mohs ~4, softer than the next harder reference, harder than the previous one.
2
Check the streak
Drag the specimen across an unglazed porcelain plate. Streak: white.
3
Read the luster
Hold the specimen under a strong light. Luster: pearly on cleavage, vitreous on fractures.
4
Match the color range
Compare against the expected color range: white, colorless, gray white, grayish black.
5
Look at form & habit
Crystal system: hexagonal. Typical habit: platy.

Often confused with

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

mineral

Clinohedrite

Mohs 5.5

How to tell apart: Clinohedrite is the harder of the two (Mohs 5.5 vs. ~4); luster reads pearly on cleavage, vitreous on fractures on Minehillite and brilliant, glassy; pearly on {010} on Clinohedrite.

Compare →

Often found alongside minehillite

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

All properties

Chemical formula: (K,Na)₂Ca₂₈Zn₅Al₄Si₄₀O₁₁₂(OH)₁₆
Mohs hardness: ~4
Density: 2.93 g/cm³
Colors: White Colorless Gray white Grayish black
Streak: White
Luster: Pearly on cleavage, vitreous on fractures
Transparency: Transparent
Crystal system: Hexagonal
Crystal habit: Platy
Cleavage: {0001}, perfect
Fluorescence: Dull violet under SW and LW UV
Rarity: Rare
Uses: Collector, Scientific research
Host rock: A secondary low-temperature hydrothermal mineral formed by replacement of associated minerals in a metamorphosed stratiform zinc deposit

Where rockhounds find minehillite

Classic worldwide localities

Franklin, Sussex Co., New Jersey, USA

Field-hunting tip

Where it forms: A secondary low-temperature hydrothermal mineral formed by replacement of associated minerals in a metamorphosed stratiform zinc deposit. If you start seeing microcline, wollastonite, grossular in float, you are in the right ground. Typical habit: platy. Train your eye for that shape before scanning the outcrop.

Common questions

How do you identify minehillite?+

Mohs hardness is ~4. Luster: pearly on cleavage, vitreous on fractures. The streak is white. Common colors include white, colorless, gray white, grayish black.

Where is minehillite found?+

Notable localities include Franklin, Sussex Co., New Jersey, USA.

What rocks look like minehillite?+

Minehillite is most often confused with Clinohedrite. A quick hardness test and a streak check separate the look-alikes faster than color alone.

What minerals are found with minehillite?+

Minehillite commonly co-occurs with Microcline, Wollastonite, Grossular, Vesuvianite, Margarosanite, Calcite. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does minehillite form in?+

Where it forms: A secondary low-temperature hydrothermal mineral formed by replacement of associated minerals in a metamorphosed stratiform zinc deposit. Working float back to the host body is the standard way to chase a fresh occurrence.

What is minehillite used for?+

Minehillite is used in collector, scientific research.

Find minehillite on the map

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