PREHNITE

Prehnite – Ca2(Al,Fe)Al2Si3O10(OH)2 – is a rarer inosilicate (Strunz) or phyllosilicate (Dana), typical of some more restricted parageneses. It has some economic importance as a decorative stone and gemstone.

There is a pink variety, patricianite, which contains inclusions of chlorite and native copper. Prehnite may contain Fe, Mg, Na, Ti, K, and H2O as impurities.

1. Characteristics

Crystal system: Orthorhombic pyramidal.          

Color: Colorless, gray, yellow, yellowish-green, or white.    

Habit: Crystals are rare, usually aggregated in fan-shaped, reniform, globular, granular, stalagmite-like, etc.       

Cleavage: {001} good, {110} poor.      

Tenacity: Brittle.        

Twinning: thin lamellae.       

Fracture: Irregular.       

Mohs Hardness: 6 – 6.5

Parting: No.         

Streak: White.         

Lustre: Vitreous, pearly.          

Diaphaneity: Transparent.           

Density (g/cm³): 2.8 – 2.95

 

2. Geology and Deposits

Prehnite, an alteration product of the anorthite component of plagioclase, is typically found filling tension cracks, fractures, and cavities (vesicles) in mafic and intermediate igneous rocks such as basalts, diabases, their pyroclastic derivatives, and gabbroic rocks. More rarely in granodiorites.

It most commonly occurs in graywackes and basalts that have undergone low-grade regional metamorphism.

It can be found in marls that have undergone contact metamorphism.

 

3. Mineral Associations

It is associated with the primary minerals that make up the ultrabasic, basic, and intermediate igneous host rocks, such as plagioclase, clinopyroxenes, olivine, magnetite, ilmenite, pyrite, and chalcopyrite, etc.

Among the metamorphic mineralogy that developed on these mafic protoliths are albite, epidote, zoisite, quartz, calcite, zeolites (laumontite and others), actinolite, chlorite, apophyllite, gyrolite, babingtonite, native copper, and pectolite.

In calcic rocks of contact metamorphism with albite, epidote/zoisite, garnet (grossular), and babingtonite.

 

4. Transmitted Light Microscopy

Refraction indices:  nα: 1,611 – 1,632     nβ: 1,615 – 1,642       nγ: 1,632 – 1,665

PLANE POLARIZED LIGHT – PPL

Color / Pleochroism:  Colorless, it never has color, much less pleochroism.         

Relief: Moderate.           

Cleavage: {001} good, {110} poor.           

Habits: Tabular in [001], prismatic, radiating, forming bundles and intergrowths in a checkerboard pattern. Crystal arrangements can be in a bowtie pattern, fan-shaped, radial, dendritic, reniform, globular, stalactitic, granular, or massive.            

CROSSED POLARIZED LIGHT – XPL

Birefringence and Interference Colors: Birefringence of 0.021 to 0.035, corresponding to intense second-order colors: blue, orange, red.

They are frequently anomalous, appearing blue, bluish-gray, or leather-brown. Birefringence increases with increasing Fe content. 

Extinction:  Parallel, it closely resembles the habit and colors of muscovite, but the extinction is not mottled as in micas. Checkerboard patterns may occur, resembling microcline, but with anomalous interference colors and incomplete extinction.          

Elongation sign: ES(+) or ES(-), it is not a diagnostic. 

Twins: Thin lamellar, rare, can occur in two sets, per (110) and (001). 

Zoning: sometimes zoned, with hourglass-shaped zonation. 

CONVERGENT LIGHT

Character: B(+)          

2V angle: 64 – 71º           

Alterations: It does not alters, it is a product of alteration.          

May be confused with: Cleavage and birefringence are diagnostic.

Lawsonite, pumpellyite, and epidote, also found in low-grade metamorphic rocks, show pleochroism (weak) and/or have either lower or higher birefringence.

Datolite occurs in vesicles but has higher birefringence and poor cleavage.

Chondrodite shows weak golden pleochroism and lamellar twinning.

Zeolites have lower birefringence (are gray) and lower relief.         

A vein of prehnite and calcite in XPL in basaltic rock. At the top of the image, the basaltic rock. At the bottom of the image, calcite. Between them are aggregates of prehnite that tend to be fan-shaped and exhibit intense interference colors, as well as anomalous interference colors (blue and gray).
Prehnite 01: In PPL, metavolcanic rock with a vesicle (gas bubble) filled with prehnite.
Prehnite 01: The same section as of the previous image, now in XPL. Very typical are the strong interference colors and the fan-shaped aggregates that prehnite exhibits.
A very typical trait of prehnite is the tendency to form bow-tie shaped aggregates. XPL.
Several of these bowtie-like structures can intergrow to form a roughly round prehnite aggregate with a radial structure. XPL.
Prehnite in XPL with anomalous blue interference colors. This blue color is somewhat different from the anomalous blue color of chlorite, and there is always prehnite with intense ("normal") colors associated with it.
Several fan-shaped aggregates of prehnite in XPL, exhibiting strong yellow colors and parallel extinction. Here, the prehnite fills a vein; on other occasions, it may fill cavities.
Prehnite 02: Several prehnite aggregates in PPL. They are colorless, but its relief allows to see the morphology of the aggregates.
Prehnite 02: The same section as of the previous image, now in XPL. In XPL, prehnite is very colorful, closely resembling muscovite, but the extinction is not mottled and there are anomalous colors such as blue, which muscovite never exhibits.
In XPL, prehnite in beams showing the various possibilities of interference colors. On the left, strong colors like muscovite. In the middle, anomalous yellowish-gray. On the top right, anomalous blue.
In XPL, twinning in prehnite. There are several thin, parallel lamellae running through each of the twin crystals.

5. Reflected Light Microscopy

Reflected light microscopy is clearly not the recommended analytical method for identifying prehnite. However, it is important to prepare a polished slide or section to identify the opaque minerals that occur associated with prehnite.

Sample preparation: Prehnite acquires an excellent polish without any difficulty. For this very reason, it is used in the decorative stone market for the making of ornamental items.       

PLANE POLARIZED LIGHT – PPL

Reflection color: Dark gray, the same shade as calcite in its lighter position.       

Pleochroism:  No.     

Reflectivity: Low (<<10%)        

Bireflectance: No.       

CROSSED POLARIZED LIGHT – XPL

Isotropy / Anisotropy: No anisotropy is observed.        

Internal reflections: Widespread, diffuse light, perhaps with a very subtle greenish tint.

May be confused with: many other minerals. Prehnite, when in larger crystals, can be recognized by observing the polished section under a stereomicroscope, as it presents a very typical soft greenish color. 

Prehnite 01: Prehnite in basaltic rock. In the image, the basaltic rock is at the bottom. On the upper left, a calcite grain with high relief. The rest in the images is prehnite. In PPL, a reflection color similar to that of calcite and a very good polish are observed.
Prehnite 01: The same section as of the previous image, now in XPL. There are no diagnostic features, only diffuse light reflections with a very slight greenish tint.