Marble of church of four Martyrs EarthCache

Introduction

You're in front of a rose window composed of several types of traditional marble used in Mediterranean sacred art. These metamorphic rocks, formed from limestone transformed under pressure and temperature, display a variety of colors and structures.

The goal of this EarthCache is to learn how to recognize the main structures of these marbles, observe their minerals, and understand how they were formed.

Reminder about EarthCaches: There is no container to find or logbook to fill out. You simply need to visit the location, answer the questions, and send me your answers.

Access: Access to the church appears to be unrestricted during the day. I cannot guarantee that this will be the case during ceremonies.

To validate the Earthcache

To validate this EarthCache, you must correctly answer the following questions and include in your log a photo of yourself, your GPS, or your username at the cache waypoint. This photo will help confirm the “found” status in case of incorrect answers. Important: do not show your answers in the photo.

Please send your answers via my profile or through the geocaching.com messaging system—do not include them in your log. Logs submitted without answers will be deleted.

You may log the cache as “found” without waiting for my confirmation; I will contact you if there’s an issue.

Feel free to read the description text. You’ll likely need it to answer some of the questions.

Question 1 : What is the dominant color of the marble under the blue form?

Question 2 : Which marble contains mainly white calcite? Can you tell me where is this marble on the rose window ?

Question 3 : Explain why green marble contains veins.

Question 4 : Describe serpentinization in a few words.

Question 5 : If you observe this zone in details, you are going to see a micro-tectonic fault. Can you tell me the name of this fault (normal, reverse, …)? Size of the displacement? How this fault appear?

🧭  The different types of marble to observe:

✅ 1. Verde Alpi

• Color: Intense green with dark green veins.
• Structure: Metamorphic rock, massive texture with veining.
• Main crystals: Mainly serpentine (hydrated magnesium silicates), with sometimes veins of calcite and traces of magnesite and chlorite.
• Color origin: Presence of serpentine (hydrated magnesium silicates).
• Probable origin: Formed from ultrabasic rocks (ophiolites) from the oceanic crust, metamorphosed during the Africa-Europe collision. Extracted in the Aosta Valley (Italian Alps)
• Period of formation: These rocks were formed during the Mesozoic era, then metamorphosed during the Alpine orogeny (≈ 65 to 30 million years ago)

✅ 2. Carrare marble

• Color: Pure white to white veined with gray.
• Structure: Metamorphic rock, homogeneous granoblast.
• Main crystals: Almost exclusively calcite (CaCO₃), sometimes with impurities (mica, quartz, iron oxides) responsible for the gray veining.
• Origin of color: Purity of calcite (white); veining due to impurities.
• Probable origin: Metamorphism of very pure marine limestone (Apuan Alps, Tuscany).
• Period of formation: Metamorphism of limestone dating from the late Triassic to early Jurassic periods (≈ 220 to 190 million years ago).

✅ 3. Botticino

• Color: Light beige to pinkish beige.
• Structure: Compact sedimentary rock, sometimes presence of stylolites and fossil inclusions.
• Main crystals: Compact limestone rock dominated by calcite. Beige color due to iron oxides and organic matter
• Color origin: Iron oxides and organic matter.
• Probable origin: Marine deposits in a tropical lagoon. Extracted near Brescia (Lombardy, Italy).
• Formation period: Mesozoic, more specifically Lias (Jurassic)

✅ 4. Rosso Levanto

• Color: Purple-red with white veins.
• Structure: Metamorphic rock (ophicalcite), heterogeneous texture.
• Main crystals: Mixture of ophicalcite (calcite + serpentine), with veins of white calcite, sometimes quartz and traces of hematite (responsible for the red color).
• Color origin: Hematite (iron oxide).
• Probable origin: Metamorphic rock derived from serpentinites and limestones, mined in Liguria (Italy).
• Period of formation: Initial formation in the Mesozoic era, metamorphism during the Alpine orogeny (≈ 65 to 30 million years ago).

 🧬 Serpentinization

Serpentinization is a metamorphic and hydrothermal process that transforms magnesium-rich rocks, such as peridotites (mantle rocks) or certain dolomites, into serpentine. This phenomenon occurs when hot fluids rich in water and silica circulate through the rock under high pressure and temperature.

• Typical reaction:

olivine + water → serpentine + magnetite + heat

This reaction releases heat and sometimes hydrogen.

• Visual aspect:

Characteristic green color, sometimes fibrous texture, and sometimes presence of white calcite veins or other serpentine.

• Geological context:

         Subduction zones, mountain ranges (e.g., the Alps), or ocean floors.

General description

Serpentine results from the hydrothermal alteration of mantle peridotites. During this transformation, CO₂-rich fluids circulate in the fractures. As they cool or change pressure, these fluids precipitate secondary minerals, including calcite (CaCO₃) or serpentine, which forms the veins visible in the rock.

Geological history

1. Deep origine:

It all begins in the Earth's mantle, with olivine-rich peridotites.

When an ocean opens up or in a subduction context, these rocks rise to the surface.

2. Transformation:

They are then altered by hot fluids: this is serpentinization, a process that changes the chemistry of rocks and fluids, and sometimes releases hydrogen.

3. Circulation of late fluids:

Later, tectonic fractures allowed fluids rich in calcium and CO₂ to circulate. These fluids can have other composition like serpentine.

As these fluids cooled, they precipitated calcite or serpentine, filling the cracks and forming the veins that can be seen today.

These veins are markers of ancient hydrothermal systems and bear witness to past tectonic dynamics.

🪨  Formation of calcite in veins

The white veins visible in marble originate from cracks opened by tectonic forces, which are then filled with fluids rich in calcium and carbonate ions.

• Precipitation: Ca⊃2;⁺ + CO₃⊃2;⁻ → CaCO₃ (calcite).
• Results: white veins contrasting with the color of the host rock.
• Geological indication: fluid circulation and fracturing episodes after initial recrystallization.

🔄  Micro-tectonic fault

A micro-tectonic fault is a small fracture in rock caused by tectonic stresses (compression, extension, or shearing).

The patterns caused by deformation regimes:

Here are the key points:

✅ Caracteristics

1. Size: A few millimeters to a few centimeters of displacement.
2. Offset: The veins or patterns in the rock appear “broken” and offset on either side of the fracture.
3. Filling: Often filled with precipitated minerals (calcite, quartz), creating a clear vein.

✅ Formation

1. The rock undergoes tectonic stress (pressure related to plate movements).
2. It fractures locally.
3. Mineral-rich fluids flow into the fissure and deposit crystals (e.g., calcite).
4. Result: a mineral vein.

✅ Why in Verde Alpi?

Ophicalcites (such as Verde Alpi) originate from subduction zones and ophiolites, which are subject to significant deformation. They are therefore often traversed by recemented microfaults.

Sources :

• Stone Forensics – Carrara Marble: Geology, Extraction, Processing
• Wikipedia (FR) – Serpentinisation
• Passchier & Trouw (2005) – Microtectonics
• Lacombe et al. (2023) – Faults and fractures in rocks: mechanics, occurrence, dating, stress history and fluid flow
• Zucchetti et al. (1988) – Serpentinization and metamorphism in ophiolitic ultramafics from the Alps