HARMOTOME
Harmotome – (Ba,Ca,K,Na)5[Al5Si11O32].12H2O – is a tectosilicate belonging to the Zeolite Group and the Phillipsite Subgroup. It is a relatively rare mineral, characteristic of certain basic rock types. It holds no significance as an ore, serving primarily as a collector’s mineral.
There is a variety known as morvenite, which exhibits a fusiform habit formed by the interpenetration twinning of two crystals along the (100) plane.
1. Characteristics
Crystal system: Monoclinic prismatic, pseudo-orthorhombic.
Color: Colorless, gray, white, or more rarely yellow, pink, or brown.
Habit: Tabular, elongated, rarely columnar. Twinning common.
Cleavage: {010} distinct, {001} poor. Striations on {010}, // to {001}
Tenacity: Brittle.
Twinning: by {001}, {021}, and {110}, of repetition.
Fracture: Irregular, subconchoidal.
Mohs Hardness: 4 – 5
Parting: No.
Streak: White.
Lustre: Vitreous.
Diaphaneity: Transparent.
Density (g/cm³): 2.41 – 2.47
2. Geology and Deposits
Harmotome typically occurs as a secondary mineral of hydrothermal origin in cavities (former gas bubbles) within volcanic rocks such as basalts, phonolites, and trachytes.
It can also be found in certain gneisses and, occasionally, in hydrothermal veins.
3. Mineral Associations
Harmotome occurs with the rock-forming minerals of the basic rocks in which it forms, such as olivines, clinopyroxenes (augite, pigeonite), plagioclases, leucite, apatite, and others.
It is found in gneisses containing quartz, potassium feldspar, plagioclases, micas, zircon, monazite, and others.
In hydrothermal veins, it is associated with galena, sphalerite, quartz, hyalophane (a Ba-bearing variety of microcline), strontianite, barite, and pyrite.
In the secondary assemblages of volcanic rocks, it occurs with quartz (rock crystal and amethyst varieties), chalcedony (agate), clay minerals (smectites), calcite, fluorapophyllite-(K), goethite, and other zeolites such as chabazite, natrolite, and phillipsite.
In secondary assemblages within volcanic rocks, it typically does not occur—or is rarely found—alongside heulandite, scolecite, and laumontite; these are also zeolites of a typically secondary nature found in volcanic rocks, but in those of a more acidic character.
4. Transmitted Light Microscopy
Refraction indices: nα: 1.503 – 1.508 nβ: 1.505 – 1.509 nγ: 1.508 – 1.514
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: Colorless; never exhibits pleochroism.
Relief: Low.
Cleavage: It may be visible, but the literature does not report it.
Habits: Tabular, elongated.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Maximum birefringence of 0.005–0.006, corresponding to interference colors between gray and white; it does not reach straw yellow.
Extinction: Oblique, with angles between 28 and 32°.
Elongation sign: ES(+) or ES(-), it is not diagnostic.
Twins: Very frequent.
Zoning: No.
CONVERGENT LIGHT
Character: B(+)
2V angle: 43º or 79º (conflicting data in the literature).
Alterations: No information available.
May be confused with: Various other zeolites.
5. Reflected Light Microscopy
Reflected-light microscopy is evidently not the recommended analytical method for the identification of harmotome. It is not possible to recognize harmotome under reflected light.
Sample preparation: xx
PLANE POLARIZED LIGHT – PPL
Reflection color: xx
Pleochroism: xx
Reflectivity: xx
Bireflectance: xx
CROSSED POLARIZED LIGHT – XPL
Isotropy / Anisotropy: xx
Internal reflections: xx
May be confused with: xx
General Characteristics: xx