Harmotome is a barium-rich zeolite typically recognized by its distinctive, cross-shaped (cruciform) penetration twins. It is most often found as secondary mineral deposits lining cavities in basaltic rocks or within low-temperature hydrothermal veins.

Hardness

4.5

Mohs

Luster

Vitreous

Streak

White

Transparency

Transparent to translucent

mineraluncommonZeolite Group family

Harmotome

Hardness: 4.5
Luster: Vitreous
Streak: White
Transparency: Transparent to translucent

Is this harmotome?

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 harmotome with a known reference. Harmotome sits at Mohs 4.5, 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: vitreous.
4
Match the color range
Compare against the expected color range: white, gray, yellow, red, brown.
5
Look at form & habit
Crystal system: monoclinic. Typical habit: cruciform, striated, twinning common.

Often confused with

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

Often found alongside harmotome

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

All properties

Chemical formula: Ba₂(Si₁₂Al₄)O₃₂·12H₂O
Mohs hardness: 4.5
Density: 2.41-2.47 g/cm³
Colors: White Gray Yellow Red Brown
Streak: White
Luster: Vitreous
Transparency: Transparent to translucent
Crystal system: Monoclinic
Crystal habit: Cruciform, striated, twinning common
Cleavage: {010} distinct, {001} poor
Rarity: Uncommon
Uses: Collector, Scientific research
Host rock: Of hydrothermal origin, in cavities in basalts, phonolites, trachytes; in gneisses, and in some ore veins

Where rockhounds find harmotome

Classic worldwide localities

Glen Riddle, Delaware Co., Pennsylvania, USA
Sing Sing, near Ossining, Westchester Co., New York, USA
St. Andreasberg, Harz Mountains, Germany
Idar-Oberstein, Rhineland-Palatinate, Germany
Silberberg, near Bodenmais, Bavaria, Germany
Sarrabus, Sardinia, Italy

Field-hunting tip

Where it forms: Of hydrothermal origin, in cavities in basalts, phonolites, trachytes; in gneisses, and in some ore veins. If you start seeing zeolites, calcite, leucite in float, you are in the right ground. Typical habit: cruciform, striated, twinning common. Train your eye for that shape before scanning the outcrop.

Common questions

How do you identify harmotome?+

Mohs hardness is 4.5. Luster: vitreous. The streak is white. Common colors include white, gray, yellow, red.

Where is harmotome found?+

Notable localities include Glen Riddle, Delaware Co., Pennsylvania, USA; Sing Sing, near Ossining, Westchester Co., New York, USA; St. Andreasberg, Harz Mountains, Germany; Idar-Oberstein, Rhineland-Palatinate, Germany; Silberberg, near Bodenmais, Bavaria, Germany.

What rocks look like harmotome?+

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

What minerals are found with harmotome?+

Harmotome commonly co-occurs with Zeolites, Calcite, Leucite, Hyalophane, Strontianite, Quartz. Spotting any of these in float or country rock is a useful trip signal.

What kind of rock does harmotome form in?+

Where it forms: Of hydrothermal origin, in cavities in basalts, phonolites, trachytes; in gneisses, and in some ore veins. Working float back to the host body is the standard way to chase a fresh occurrence.

What is harmotome used for?+

Harmotome is used in collector, scientific research.

Find harmotome on the map

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