PHILLIPSITE
Phillipsite – K(Ca,Na)[AlSiO]·12H2O – is a rarer tectosilicate from the Zeolite Group, Phillipsite Subgroup. It has no economic importance beyond being a collector’s mineral.
“Phillipsite” is not actually a mineral, but a generic term referring to a group of minerals formed by Ca-phillipsite, K-phillipsite, and Na-phillipsite, which form series with each other. Of the three, Na-phillipsite is the rarest.
1. Characteristics
Crystal system: Monoclinic prismatic, pseudo-orthorhombic.
Color: White, colorless, reddish-white, light yellow. More rarely pink.
Habit: Columnar, tabular, radiating aggregates.
Cleavage: {010} good, {001} poor. Striations // to (001).
Tenacity: Brittle.
Twinning: It always forms twins by {100} and {110}. A typical twin is that of three columns perpendicular to each other, each with 4 crystals, with a generally pseudocubic shape.
Fracture: Irregular.
Mohs Hardness: 4 – 5
Parting: No.
Streak: White.
Lustre: Vitreous.
Diaphaneity: Transparent.
Density (g/cm³): 2.2
2. Geology and Deposits
Phillipsite occurs in cavities (vesicles) and fractures of basic volcanic rocks such as alkaline basalts, hyalobasanites, tephrites, phonolites, and nepheline syenites.
It can be authigenic, filling pores in sediments (marine limestones) and sedimentary rocks.
It also occurs authigenically in salt lakes and hot spring deposits.
A detailed approach to the occurrence, forms, and other aspects of phillipsite can be found in the book “Zeolites of the World” by Rudy Tschernich, available for download on the internet. Detailed information on the various ways phillipsite occurs can be found on the website of the Natural Zeolites Commission: http://www.iza-online.org/
3. Mineral Associations
It occurs associated with the constituent minerals of the host rocks, such as plagioclase, olivine, nosean, nepheline, and melilite.
Typical secondary minerals in these rocks are quartz, chalcedony, clay minerals (celadonite), calcite, fluor-apophyllite, pyrite, and zeolites such as analcime, natrolite, mesolite, gmelinite-Ca, paulingite-Ca, gonnardite, gismontine, thomsonite-Ca, and chabazite-Ca.
4. Transmitted Light Microscopy
Refraction indices: nα: 1.483 – 1.504 nβ: 1.484 – 1.509 nγ: 1.486 – 1.514
PLANE POLARIZED LIGHT – PPL
Color / Pleochroism: Colorless, can be pale white or pale yellow.
Relief: Moderate.
Cleavage: (010) good, parallel to the stretch.
Habits: Columnar, tabular, twinned. Twinned structures can produce pseudo-orthorhombic, pseudo-tetragonic, or pseudo-cubic symmetries.
CROSSED POLARIZED LIGHT – XPL
Birefringence and Interference Colors: Maximum birefringence of 0.009, corresponding to colors between dark gray, light gray and white, with a maximum first-order straw yellow.
Extinction: Oblique, 11 to 30º.
Elongation sign: ES(+)
Twins: Always twinned on {100} and {110}.
Zoning: No.
CONVERGENT LIGHT
Character: B(+)
2V angle: 60-80º
Alterations: No information available.
May be confused with: Other zeolites and minerals from the paragenesis
5. Reflected Light Microscopy
Reflected light microscopy is clearly not the recommended analytical method for identifying phillipsite. However, it is important to prepare a polished slide or section to identify the opaque minerals that occur associated with phillipsite.
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