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A Guide to Identifying What You Find Around our Island

by Brenna Lorenz

    How many of the following rocks and minerals can you find? Check them off as you find them, and keep track of where they come from.


Calcite: This is the easiest mineral to find on Guam. Limestone is made of calcite. It ranges from fine-grained and powdery to coarsely crystalline in rocks such as crystalline limestone and skarn. Calcite is so soft that it can be scratched by a penny. (Use a penny older than 1983.) Our calcite is mostly white to colorless, but we also have pale yellow, pink, brown, and gray calcite.  Calcite fizzes in dilute hydrochloric acid.

Chalcedony: A variety of microcrystalline quartz, this mineral is abundant around Guam's volcanic rocks. It results from the chemical weathering of basalt and other igneous rocks. Agate, a banded variety of microcrystalline quartz, is also common. This mineral is hard enough to scratch glass or the blade of a pocket knife. Chalcedony is colorless, and our agate is banded colorless and white. You can find lots of it along the trail leading from Mt. Alutom to Leo Palace.

Clay: Clays in a variety of colors, usually red, green, gray or brown, are widespread on Guam. They result from the chemical weathering of igneous rock. Commonly, the rocks weather in place to produce clays that preserve the textures of the original rocks. It is possible to distinguish banded tuffs and volcanic breccia that have converted entirely to clay. Clay is very soft, and when wet behaves in a plastic manner (you can shape it like putty). You can find lots of it along the trail leading from Mt. Alutom to Leo Palace.

Zeolites: The zeolites are a mineral family that include minerals such as chabazite and natrolite. They occur as secondary minerals filling in the vesicles in amygdaloidal basalt, and in seams running through basalt. They formed from the reaction of the hot basalt with sea water when Guam was underwater. These are easy to find in rocks from roadcuts between Agat and Umatac. Large crystals can be found at Facpi Point. They vary in color from white to colorless to pink. Some crystals are blocky and cubic, and others form radiating clusters of needles.

Magnetite: Magnetite is a primary igneous mineral that crystallized from basaltic magma. It is a component of basalt, but the easiest place to find it is where it has weathered out of the basalt and has accumulated as sand on beaches. Guam's magnetite occurs as tiny, fine-sand-sized black octahedral crystals. It is the easiest of all minerals to identify because it is attracted to a magnet. The best place to collect this is on Talofofo Beach.

Olivine: Olivine is a primary igneous mineral that crystallized from basaltic magma. It is a component of basalt, but the easiest place to find it as in beach sand. It occurs as tiny, fine-sand-sized, rounded green crystals. Under the microscope, it looks like green bottle glass. A good place to find it is on Talofofo Beach.

Limonite: Limonite is the mineral that gives red clay its red color. It also occurs in seams and lumps in limestone, especially in faulted areas. Limonite separates out from the limestone as the calcium carbonate recrystallizes and excludes iron from its crystal structure. This limonite is a dark orange-brown color, and is very hard. One of many places to find this mineral is Marine Lab Beach.

Pyrolusite: The sooty, purplish-gray stain on some of Guam's rocks is pyrolusite, which forms during chemical weathering. We also have manganese nodules composed of pyrolusite. These are lumpy black objects that can be found in stream-beds in southern Guam. You can find lots of nodules in the dry stream bed just north of the Sella Bay overlook.

Quartz: Guam does not have much quartz (except for microcrystalline varieties), except where microcrystalline quartz has recrystallized into coarser forms.  The best place to find it is in the Mount Alutom area. It is possible to find small, colorless, six-sided quartz prisms, usually stained with limonite.

Gypsum: Perfect gypsum crystals, some of them up to five centimeters long, crystallize from groundwater along fault zones during the dry season. This is not a common mineral on Guam. Gypsum is very soft, and can be scratched with the fingernail. The crystals are colorless to pale gray.

Halite: Small crystals of halite form from sea spray or high tidal pools during the dry season. The mineral is water-soluble and tastes salty.


Tips on identifying rocks:

Sedimentary Rocks:

Biolithite: This is a type of limestone that forms when corals fossilize in place, in their life positions. It is a common Guam rock. It is easy to recognize because it looks like a coral head imbedded in the rest of the limestone outcrop. The rock is generally white unless stained orange by limonite. You can find it in any of our limestone cliffs.

Fossiliferous limestone: This is a limestone rich in fossils. It differs from biolithite in that the shells and coral fragments have been transported and are usually broken and rounded. It generally has a chalk, sand or micrite matrix surrounding a mixture of whole and broken fossils. The color is generally white, although clay-rich limestones may be yellowish, pale brown, or pale pink.

Chalk: Guam's chalk is soft, white, granular and powdery. It is generally coarser and grittier than blackboard chalk. It is good for making afok, which is produced by burning the chalk to make calcium oxide. The calcium oxide picks up water from the air to make calcium hydroxide. This material is consumed with betel nut to raise the betel's pH and hence its potency. Most of our chalk comes from the Barrigada area.

Crystalline limestone: Limestone of any kind recrystallizes easily at surficial temperatures and pressures to produce this coarse-grained rock. Original textures are usually destroyed, but in some samples, traces of coral and other fossil remains may still be visible. The color may be white or colorless, but is most often a pale golden yellow.

Travertine: Travertine is "cave rock." In its depositional form, it is coarse-grained and banded, commonly in shades of brown and white. It forms stalagmites, stalactites and columns, as well as sheets on the walls of caves. In its erosional form, it takes on a highly porous, moth-eaten look. You can find a lot of travertine on Tagachang Beach in Yona.

Carbonate breccia: This rock is common, especially in fault zones. In many cases, the matrix of the breccia is rich in limonite, resulting in an attractive rock with angular chunks of white limestone in a red matrix. Common everywhere, you can find it easily at Marine Lab Beach.

Carbonate conglomerate: This rock is less common than carbonate breccia, and looks a lot like concrete. The rock consists of large, rounded pebbles and cobbles of limestone or coral usually in a sandy carbonate matrix. It forms when beach material is cemented together. You can find this on Marine Lab Beach and on some of the northernmost beaches.

Carbonate sandstone: The rock consists of loosely cemented carbonate sand. The sand is rich in forams, shell fragments and halimeda flakes. This is easiest to find in northernmost Guam.

Micrite: Guam's micrite is a hard, fine-grained yellowish, white or tan rock.

Argillaceous limestone: This is a light gray, clay-rich, layered limestone. It forms from a mixture of volcanic ash and lime mud. You can find this near Adelup and in southern Guam.

Lithic sandstone: On Guam, this rock is generally dark brown to greenish black. It forms from the product of mechanical weathering of Guam's igneous rocks. You can find this in road cuts around Adelup.

Conglomerate: Guam has conglomerates with either chert or volcanic rock pebbles, generally matrix-supported with a sandstone matrix. You can find this in the Nimitz Hill area.

Igneous Rocks:

Vesicular basalt: This basalt comes from lava flows. It is a black rock filled with round holes. The holes come from trapped volcanic gases. It tends to weather easily to a green or gray rock, not as dark as fresh basalt. You can find lots of this in the road cuts between Agat and Umatac.

Basalt: Massive black basalt, generally porphyritic, is less common here than is vesicular basalt. Most of it comes from dikes. You can find it on Mount Santa Rosa. You can also find nice pieces of basalt in the bed of the Madog River in Umatac. Many of these samples have good chilled margins, a glassy layer formed when the hot basalt cooled rapidly in contact with cooler rocks. The porphyritic varieties contain visible crystals of pyroxene (black) and plagioclase feldspar (white).

Amygdaloidal basalt: This is vesicular basalt with its holes filled with calcite and zeolites. These minerals formed in the vesicles as a result of the reaction of fresh basalt with seawater. You can find this in the road cuts between Agat and Umatac.

Silicified basalt: This is basalt that has been permeated and largely replaced with silica. It is green to black, slightly grainy-looking, with a conchoidal fracture. It is very hard, and can scratch glass or steel. You can find a lot of it on the trail between Mt. Alutom and Leo Palace, and on Mount Santa Rosa.

Volcanic breccia: This rock on Guam consists of chunks of amygdaloidal basalt in a calcite-rich matrix. It is generally green and crumbly due to weathering. You can find a lot of this in the road cuts just north of Umatac.

Tuff: Tuff is similar to volcanic breccia, except that the fragments are smaller, and the matrix consists of fine volcanic material. Grain size is variable; color is generally green or off-white. You can find this in road cuts from Inarajan to Umatac, and as large chunks in Madog River in Umatac.

Tuffaceous Mudstone: This rock consists of volcanic ash cemented together, in some cases with calcite. Colors are most commonly white, pale tan or light green. It may stick to the tongue. You can find it around Leo Palace.

Pumice: Pumice is foamy volcanic glass. It has such a low density that it floats in water. The pumice found around here is generally white to gray in color. Pumice did not form on Guam, but floats in from other volcanic islands. It can be found on the beach, especially after typhoons.

Man-made rocks:

Concrete: Concrete is a man-made rock resembling carbonate breccia or carbonate conglomerate. Generally, it has a cleaner, fresher appearance than the natural rocks, and a lighter color, containing less limonite. Asphalt has a black tar matrix. Some of the concrete found on Tagachang Beach has a striking red matrix.  Concrete is commonly found on Guam's beaches.

Ancient Chamoru pottery: This man-made material resembles red brick. It is common on Guam's beaches and in some areas forms a significant component of the beach sediment.

Published 6/28/2000.

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