Granite Gallery 📷 EarthCache

This earthcache is a geologic exploration set against the historical backdrop of the National Underground Railroad Freedom Center. Just steps from the Ohio River, you’ll find a striking outdoor installation made up of nine distinct granite slabs arranged in a circular pattern. Each slab varies subtly in color, grain size, and mineral composition—inviting close observation and comparison.

Granite is a coarse-grained igneous rock composed primarily of quartz, feldspar, and mica, and it forms deep underground through the slow cooling of magma. The unique appearance of each slab here reflects its specific mineral content and the geologic conditions under which it formed. These differences tell a broader story of Earth’s dynamic interior processes.

This EarthCache challenges you to examine each granite slab carefully and discover how no two are exactly alike—geologically or aesthetically.

What’s in Granite? A Guide to Its Components

Granite is made up of a few key mineral components, each contributing to its color, texture, and origin story. Below is a breakdown of the main minerals found in granite, with detailed observations to help you identify them in the field.

1. Quartz

• Color: Usually clear, gray, white, or smoky; sometimes slightly pinkish.
• Typical Size: 1–5 mm in granite; may appear glassy or cloudy.
• Texture & Shape: Irregular, rounded crystals with a glassy or greasy luster.
• Hardness: 7 on Mohs scale (can scratch glass).
• Fun Fact: Quartz is very resistant to weathering, making it a lasting part of granite even as the rock breaks down.
• Where to Look: Often fills spaces between feldspar and mica, giving granite a sparkly or milky look.

2. Orthoclase Feldspar

• Color: Pink, salmon, or reddish hues.
• Typical Size: 2–10 mm; can be larger in some granite varieties.
• Texture & Shape: Blocky, rectangular crystals that often reflect light on flat surfaces.
• Hardness: 6 on Mohs scale.
• Fun Fact: Orthoclase gives pink granite its warm tone and is rich in potassium (K-spar).
• Where to Look: Often the dominant mineral in light-colored granites.

3. Plagioclase Feldspar

• Color: White to light gray, sometimes with a blue tint.
• Typical Size: 2–6 mm.
• Texture & Shape: Blocky crystals often with striations (fine parallel lines).
• Hardness: 6–6.5 on Mohs scale.
• Fun Fact: Rich in calcium and sodium, plagioclase can weather into clay minerals like kaolinite.
• Where to Look: Appears in almost all granites; may look like quartz until you spot the fine lines.

4. Biotite Mica

• Color: Black or dark brown.
• Typical Size: 1–3 mm flakes, but can be larger.
• Texture & Shape: Thin, flexible, flaky sheets that shine in the light.
• Hardness: 2.5–3 on Mohs scale.
• Fun Fact: Biotite gives granite its “salt-and-pepper” look and breaks into sheets (perfect basal cleavage).
• Where to Look: Usually scattered through the matrix, often next to feldspar or quartz.

5. Muscovite Mica

• Color: Silvery, pale gold, or translucent.
• Typical Size: 1–3 mm, sometimes forming larger shiny flakes.
• Texture & Shape: Like biotite, it forms flakes, but it’s lighter and pearly.
• Hardness: 2–2.5 on Mohs scale.
• Fun Fact: Muscovite was once used as “window glass” in stoves.
• Where to Look: Less common than biotite in granite, but it adds shimmer.

6. Hornblende (an amphibole mineral, sometimes found in granite)

• Color: Dark green to black.
• Typical Size: 1–4 mm.
• Texture & Shape: Elongated, blade- or needle-shaped crystals.
• Hardness: 5–6 on Mohs scale.
• Fun Fact: Hornblende can be mistaken for biotite but has more of a rod-like shape and a duller luster.
• Where to Look: Often present in darker-colored or more mafic granite varieties.

Earthcache Goal:

The goal of this EarthCache is to explore and identify the mineral components that make up granite by closely examining nine unique granite slabs on display on this sidewalk. Each slab varies in color, texture, and composition, offering a hands-on opportunity to learn how minerals like quartz, orthoclase feldspar, plagioclase feldspar, biotite, muscovite, and hornblende contribute to granite’s diverse appearance. By observing these differences, visitors will gain a deeper understanding of igneous rock formation, mineral identification, and the natural variability within a single rock type.