Epidote in Basalt

Description

Epidote in basalt refers to the mineral epidote occurring within a basaltic rock matrix. Basalt is a fine-grained, dark-colored extrusive igneous rock composed primarily of plagioclase feldspar (typically labradorite or bytownite), pyroxene (augite), and often olivine. When basalt undergoes hydrothermal alteration, epidote (a calcium aluminum iron sorosilicate mineral) forms as a secondary mineral. Epidote typically appears as greenish to yellowish-green crystals, often acicular, prismatic, or granular, disseminated throughout the basalt or filling amygdules (gas vesicles) and veins. The basaltic matrix may retain its original texture (e.g., porphyritic, vesicular) but will show signs of alteration, such as a duller luster, a greenish hue due to chlorite, and the presence of other secondary minerals like albite and quartz. The overall appearance can range from a dark, slightly greenish basalt with subtle epidote specks to a rock heavily replaced by epidote, giving it a distinct pistachio-green color.

How to Identify

Color

The epidote itself is typically pistachio-green, yellowish-green, or olive-green. The basaltic matrix will be dark gray to black, often with a greenish tint due to associated chlorite.

Luster

Epidote crystals exhibit a vitreous to resinous luster. The basaltic matrix will have a dull to sub-vitreous luster.

Texture

The basalt matrix is typically fine-grained, aphanitic, or porphyritic. Epidote may occur as fine-grained disseminations, radiating aggregates, prismatic crystals, or granular masses within the basalt, often filling amygdules or veins. The overall texture can be massive or amygdaloidal.

Crystal Form

Epidote often forms elongated, prismatic crystals with striations parallel to the length, or granular aggregates. In basalt, it can also be anhedral (without well-formed crystal faces) when disseminated.

Cleavage

Epidote has perfect cleavage on {001} and good cleavage on {100}. The basaltic matrix, being fine-grained, typically does not show macroscopic cleavage.

Geological Environment

Hydrothermally altered basaltic rocks, commonly found in oceanic crust, ophiolite complexes, mid-ocean ridge systems, subduction zones, and areas of regional low-grade metamorphism (greenschist facies).

Key Facts

• Hardness: 6-7 on Mohs scale (epidote), 5-6 (basalt matrix).
• Specific Gravity: 3.3-3.5 (epidote), 2.8-3.0 (basalt).
• Crystal System: Monoclinic (epidote).
• Color: Pistachio-green, yellowish-green, olive-green (epidote).
• Luster: Vitreous to resinous (epidote).
• Transparency: Transparent to translucent (epidote).
• Fracture: Uneven to conchoidal (epidote).
• Cleavage: Perfect on {001}, good on {100} (epidote).
• Composition: Ca2(Al,Fe)3(SiO4)3(OH) (epidote), primarily plagioclase, pyroxene, olivine (basalt).

Quick Check

• Color: Pistachio-green to yellowish-green (epidote) within a dark, often greenish-tinged matrix (basalt).
• Luster: Vitreous to resinous (epidote), dull to sub-vitreous (basalt).
• Streak: White to grayish-white (epidote).

Physical Characteristics

• Crystal Habit: Prismatic, acicular, granular, massive, radiating aggregates (epidote).
• Cleavage Type: Perfect basal cleavage, good pinacoidal cleavage (epidote).
• Fracture Type: Uneven to conchoidal (epidote).
• Tenacity: Brittle (epidote).
• Luster Type: Vitreous to resinous (epidote).

Formation

Epidote in basalt forms through the hydrothermal alteration of basaltic rocks. This process, often termed propylitization or greenschist facies metamorphism, involves the interaction of hot, chemically active fluids (hydrothermal fluids) with the primary minerals of basalt. During this alteration, calcium-rich plagioclase feldspars, pyroxenes, and olivine within the basalt react with these fluids, leading to the formation of new minerals, including epidote, chlorite, albite, and quartz. The iron and aluminum from the original basaltic minerals, along with calcium from plagioclase, are incorporated into the epidote structure. This typically occurs in environments such as mid-ocean ridges, subduction zones (forearc and backarc basins), and continental rift zones where basaltic volcanism and associated hydrothermal activity are prevalent. The presence of epidote indicates conditions of relatively low to moderate temperature (200-400 C) and moderate pressure.

Usage

Epidote in basalt itself is not typically used as a primary resource. However, the presence of epidote can be an indicator of hydrothermal alteration zones, which are often associated with economic mineral deposits (e.g., copper, gold, silver). In some cases, highly epidotized basalt, sometimes referred to as 'greenstone', can be used as aggregate or decorative stone, though this is not specific to epidote in basalt but rather to altered basaltic rocks in general. From a scientific perspective, it is crucial for understanding metamorphic processes, fluid-rock interaction, and the thermal history of crustal rocks.

Age Distribution

Can occur in basalts of any age that have undergone appropriate hydrothermal alteration, from Precambrian to Cenozoic.

Where to Find

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Scientific Classification

Mineral Class

Sorosilicate (epidote)

Group

Epidote Group (epidote)

Crystal System

Monoclinic (epidote)

Chemical Formula

Ca2(Al2Fe)(SiO4)3(OH) (idealized epidote formula, with Fe3+ substituting for Al)

Composition

Calcium aluminum iron sorosilicate (epidote). Basalt is an igneous rock composed primarily of plagioclase feldspar, pyroxene, and often olivine, with epidote forming as a secondary alteration product.

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