Identifying the Greenish Mineral Found in Basaltic Rocks

Let's analyze the given characteristics to identify the mineral in question. We are looking for a mineral that is typically greenish, often found in basaltic rocks, and primarily composed of magnesium, iron, and silica.

Analysis of Characteristic I: Greenish Crystal

The fact that the mineral is usually a greenish crystal helps narrow down the possibilities. Several minerals can exhibit green colors, but this is a key visual clue.

Analysis of Characteristic II: Found in Basaltic Rocks

The information that the mineral is often found in basaltic rocks is very significant. Basalt is an extrusive igneous rock that cools quickly from lava. Certain minerals are commonly associated with mafic (magnesium and iron-rich) igneous rocks like basalt.

Analysis of Characteristic III: Major Elements - Magnesium, Iron, and Silica

The chemical composition of the mineral, with magnesium (Mg), iron (Fe), and silica (SiO₂) as major elements, is a crucial identifier. This composition points towards ferromagnesian silicate minerals.

Identifying the Mineral

Considering all three characteristics together:

• A greenish color.
• Occurrence in basaltic rocks.
• Major elements being magnesium, iron, and silica.

This combination strongly suggests the mineral Olivine.

Let's look at why the other options are less likely:

• Amphibolite: Amphibolite is a metamorphic rock, not typically found as crystals directly within basalt. While it contains magnesium, iron, and silica, its color can vary (dark green to black), and it's not primarily known as a greenish crystal found within basalt.
• Feldspar: Feldspar is a very common mineral group in igneous rocks, including basalt. However, feldspars are typically light-colored (white, pink, gray) rather than greenish, and their chemical composition is dominated by silica, aluminum, and either potassium, sodium, or calcium, not primarily magnesium and iron.
• Pyroxene: Pyroxenes are also ferromagnesian silicates found in basalt and can be greenish to black. However, the description "usually a greenish crystal" aligns more strongly with olivine, which often exhibits a characteristic olive-green color in its crystalline form. While some pyroxenes are green, it's not as consistently their defining color as it is for olivine.

Therefore, based on the provided characteristics, the mineral is most likely Olivine.

Why Other Options are Incorrect in Detail

• Amphibolite: While amphibole minerals (which make up amphibolite rock) contain magnesium, iron, and silica, amphibolite itself is a metamorphic rock formed under high pressure and temperature conditions, not typically found as distinct greenish crystals within basalt. The color of amphibole can also vary more widely than the consistent greenish hue often associated with olivine crystals found in basalt.
• Feldspar: Feldspar is a group of silicate minerals that are major constituents of many igneous and metamorphic rocks, including basalt. However, their dominant chemical composition includes aluminum and alkali or alkaline earth metals (like potassium, sodium, or calcium) along with silica. They generally lack significant amounts of iron and magnesium, and their color is typically white, pink, gray, or other light shades, not predominantly green.
• Pyroxene: Pyroxenes are a group of silicate minerals that do contain magnesium, iron, and silica and are commonly found in basaltic rocks. Some pyroxenes can be green (e.g., augite can have a greenish tint). However, the description "usually a greenish crystal" is more characteristic of olivine. Pyroxene crystals in basalt often appear dark green to black and have different crystal habits (e.g., short prismatic) compared to the more consistently olive-green, glassy crystals often seen with olivine.

Revision Table: Mineral Characteristics

Additional Information about Olivine

Olivine is a nesosilicate mineral with a chemical formula that can be represented as (Mg,Fe)₂SiO₄. This indicates a solid solution series between two end-members: forsterite (Mg₂SiO₄) and fayalite (Fe₂SiO₄). The proportion of magnesium and iron varies, leading to a range of compositions and slight variations in properties, including color.

• Color: Olivine typically exhibits a characteristic olive-green color, which is due to the presence of iron. The intensity of the green color can vary depending on the iron content, ranging from a pale yellowish-green in magnesium-rich varieties to a brownish-green in iron-rich varieties. High-quality, gem-grade olivine is often a vibrant green.
• Occurrence in Basalt: Olivine is a common early-crystallizing mineral in mafic igneous rocks like basalt and gabbro. It can occur as distinct, often euhedral (well-formed) crystals called phenocrysts within the finer-grained groundmass of basalt, indicating that it crystallized from the magma before the rest of the rock. It can also be found as smaller grains within the groundmass itself.
• Composition and Structure: The isolated SiO₄ tetrahedra in its structure (nesosilicate) are bonded by magnesium and/or iron ions. This structure contributes to its relatively high melting point and density.
• Weathering: Olivine is relatively unstable at the Earth's surface and weathers easily through processes like hydrolysis and oxidation. Its weathering often produces iron oxides (which can contribute to reddish-brown soils) and other secondary minerals.
• Other Occurrences: Besides basalt, olivine is a major component of ultramafic rocks like peridotite, which makes up a significant portion of the Earth's upper mantle. It is also found in some metamorphic rocks and even in meteorites.
• Industrial and Gem Use: While not as hard as some other gemstones, gem-quality olivine (peridot) is used in jewelry. In industrial applications, olivine sand is used in foundries as a molding material due to its high heat resistance.

In conclusion, the combination of a usually greenish crystal, its common occurrence in basaltic rocks, and its major elemental composition of magnesium, iron, and silica uniquely points to the mineral Olivine.