Natural History Lost Valley Online Natural History Natural History

ROCKS AND MINERALS

(adapted from The Lost Valley Nature Guide)
Larry November
(1971)

Over the ages, and beginning long before life started on the earth, our geology has been developing. The rocks and minerals we see today are the outcome of that geological history, and so by studying them, the history of the earth can be pieced together.

This section looks into the types of rocks and minerals of the Lost Valley area, how they are identified, how they were formed, and interesting facts about them. Basically the section is divided into three parts -- “Geological Formations” which describes the processes that formed the earth and the rocks we see today, “Minerals” which points out the pure chemical crystals and geological forms that are found in the Lost Valley area, and “Rocks” which describes the commonly found mixtures of minerals we call rocks that are in the area.

Geological Formations

Basically three types of rock formations have occurred during the history of the earth. Rocks are said to be “igneous”, “sedimentary”, or “metamorphic”. Each type of rock has its origin as an igneous form and has been changed by various natural forces to become one of our present day types of rocks.

Igneous Rocks -- Early in the history of the earth and even now in many parts of the world there are pools of magma (liquid rock) a few miles beneath the surface. By various methods (volcanoes, dikes, etc.) some of this magma has reached the surface and cooled forming some of our common rocks and minerals. In some places this magma never reached the surface and cooled to form large granite mounts.

All rocks began as igneous, yet today only those which have been unchanged by history remain as igneous. Those minerals found in dikes (tourmaline, cassiterite, and some types of mica) were formed a hot gases that escaped through cracks in the earth from the magma below. In most places today, that magma has cooled to form granitic rock and leaving dikes which lead to veins of certain minerals.

Sedimentary Rocks -- The action of water and air carry certain rocks from place to place and have broken up other forms of rocks. These rocks and minerals often form layer by layer in lakes and pools. Pressure from the earth often crushes rock particles at the bottom of these sediments and creates new types of rocks (sandstone in our area).

Metamorphic Rocks -- Today and always metamorphic rocks are in the process of being made. Pressure, heat, and chemical action are constantly changing rock forms. Granitic rocks, deep beneath the surface, are changing to granite boulders and mountains by earthquake pressure. Deep beneath the earth, garnet and epidote are forming under heat and pressure. Quartz is slowly crystallizing out of water carrying traces of sand. All common rocks affected by one of these basic processes in their formation are metamorphic.

Minerals

Tourmaline -- This relatively common (in our area) semi-precious stone occurs in three-side crystals. Its hardness is 7 (cannot be scratched by quartz) on a scale of 10. The complex chemical structure of this mineral gives rise to variations in color. Specimens are sometimes colorless, or red, green, brown, or black (the most common), and always lustrous. Tourmaline is produced in dikes. Narrow cracks in the crust of the earth where hot vapors are allowed to escape from molten magma are what form these dikes. Tourmaline is found in quantity in only seven places in the world, the Lost Valley area being one.

Cassiterite -- Although found only in small quantities this mineral is relatively simple to locate (if you know what you are looking for) and simple to identify. It is relatively hard (6.5 on a scale of 10) but can be scratched by quartz. The mineral is dark-brown or black and very dense. Like tourmaline, cassiterite is formed in dikes and veins, which are mineral deposits from gases escaping from lower volcanic magma. Due to its density, cassiterite will often be found as grains and pebbles in stream beds near the original dikes. These are dark, often very small pyramid-shaped crystals with light streaking.

Spodumene -- This rare mineral is found in only two spots on earth in any quantity, San Diego County and Madagascar. It occurs usually as very large crystals (several feet across) and in granite rocks. Its color may be grayish or greenish-white to emerald green, lilac or light blue and glossy in appearance, depending upon its chemistry. Spodumene is of medium hardness (6.5).

Lepidolite -- This is a form of mica, the only type found in the Lost Valley area. This soft mineral (2.5 in hardness) comes in extremely thin and glossy sheets, sometimes less than a 1000th of an inch thick. It splits readily into thinner and thinner pieces until it is thin and plastic-like in appearance. Micas often occur in granite and are formed in dikes as are tourmalines and cassiterite. It is extremely common and easily seen at a distance by its very glossy surface.

Garnet -- This rare gem-mineral has been found in the area around Bucksnort Mountain, northwest of Lost Valley. It varies in coloration depending upon its chemistry from colorless to yellow, violet, brown-red, or green, and always glossy. It is very hard (8) and will scratch quartz. Garnet it formed deep within the crust of the earth by action of heat and pressure. Like all these types of metamorphic rocks it is only brought to the surface by volcanic or earthquake activity.

Epidote -- This hard, yellow-green mineral (hardness 6.5) has been found in small quantities in the Bucksnort Mountain area northwest of Lost Valley. It is glossy and needle like. This is a type of metamorphic rock formed near igneous rock areas. It is formed by heat and pressure.

Quartz -- This a very common, hard (7) rock that appears colorless and glossy in its most perfect form. It is a fairly common, light stone, and is easily identified by its hexagonal (six-sided) crystals or glossy grains. Quartz occurs in granite with mica and feldspar. It grows as a crystal in the granite as water with dissolved silica deposits it. Some common forms are:
Rock crystal -- clear and colorless in its pure form. Milky quartz -- white colored due to imperfections in the crystal (bubbles and cracks). Smoky quartz -- cloudy brown colored due to slight amounts of organic matter in the crystal. Citrine or false topaz -- yellow colored due to slight amounts of organic matter. Amethyst -- violet colored due to the presence of a trace of manganese. Rose quartz -- pale red due to small amounts of titanium. Adventurine -- spangled appearance due to small amounts of mica in the crystal.

Feldspar -- This extreme common material (making up 60% of the earth’s crust) is a major part of granite. This mineral is of medium hardness (6) and comes in three varieties, each chemically different. Colors among these crystals vary from white to pink, or blue to green, depending on which combination of minerals are present in the crystal. The crystal is metamorphic and formed by a process similar to that of quartz. Feldspar can be easily distinguished from quartz since it can be scratched by quartz.

Rocks

Granite -- A combination of orthoclase feldspar, quartz, mica or pyroxene (spodumene or augite) and other minor minerals produces granite. Usually these minerals are present in such small crystals that a coarse, light-colored rock is its appearance. Often within granite there are veins and dikes which result from deep cracks in the rock. Settling within the dike are often other minerals which are darker and contrast the original granite. This is known as a pegmatite dike. Other rocks that are forms of granite include sand and gravel (decomposed granite, a metamorphic form).

Sandstone -- This is a sedimentary rock made up of fine sand, mica flakes, and grains of feldspar and quartz. It is yellowish-brown, brown, red, gray, or white in color; often soft, and usually easily crushed. Sandstone takes up and holds tremendous amounts of water, holding it in the spaces between its particles. The material that holds these particles together determines the characteristics and type of sandstone. Some of these types are: Siliceous sandstone -- white, often very hard (cemented with silica). Ferruginous sandstone -- red in color and soft (cemented with iron oxides).

Mica schist -- This rock is not banded and is thickly layered with shiny, silvery flakes of mica. It can be easily split into slabs or crushed. This rock contains granite and sheets of mica.

Quartzite -- This reddish, brownish, or greenish rock is made of quartz, sand grains, and some mica. It is a sedimentary rock that is hard and lacks feldspar, which separates it from granite. If broken the rock will break entirely into fine quartz particles.

Gneiss -- This rock looks very much like granite but has distinct bands of white (mica) and black (quartz and feldspar).

Pegmatites -- Very large crystals of quartz with orthoclase feldspar and mica, often containing tourmaline and cassiterite form pegmatites. This is the general rock name applied to igneous and metamorphic minerals that are found in dikes


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