Quartz with Hedenbergite Inclusions

Silicon Dioxide, SiO2
Crystal System: Hexagonal
Hardness: 7.0
Density: 2.65

Quartz is by far the most abundant of the polymorphic forms of silica and the most widespread and abundant mineral, making up 12% of the earth's crust by volume. It occurs in igneous, metamorphic and sedimentary rocks as well as hydrothermal veins, metasomatic and hot spring deposits. Quartz crystals are found as a common gangue material in veins of many minerals and in cavities in granite porphyries and pegmatites.

Quartz is transparent and colorless when pure, and such clear material is known as rock crystal. When impurities are present in the crystals, a very wide range of colors is exhibited by quartz, which include purple (amethyst), white (milk quartz), pink (rose quartz), yellow (citrine), and brown to black (smoky quartz). Included materials such as hedenbergite, rutile, tourmaline, or zoizite can cause quartz to appear green, blue, red or yellow. Impurities and inclusions cause quartz crystals to vary from transparent to opaque.

Quartz is also extremely variable in form, and may occur as well formed, elongated, hexagonal, prismatic, and sometimes enormous crystals; in compact and concretionary masses; and in microcrystalline and cryptocrystalline forms.

The faces of prismatic crystals may exhibit horizontal striations with randomly corroded edges. Prismatic crystals are often terminated by two rhombohedra of opposite polarity. Quartz crystals are often twinned, with dauphine twins being the most common twinned form, followed by Brazil and Japan Law twins in occurrence. Scepter crystals, where a new quartz crystal forms over and caps a previously formed prismatic crystal, are rarer. Quartz crystals are rotary polar and occur as left or right handed crystals, as determined by the spiral structure of linked silicon-oxygen tetrahedra in the crystal. Bubbles of gas or liquid are often present and evident at the microscopic and macroscopic levels in quartz crystals.

Chalcedony is a microcrystalline variety of quartz that is of interest to may rockhounds who do lapidary work. Most chalcedony occurs as precipitates of silica bearing solutions and forms in cavity linings, veins and precipitated masses in a variety of rocks. Chalcedony also occurs as a dehydration product of opal. Types of chalcedony include banded agate, moss agate, jasper, onyx, carnelian, chrysoprase, chert, flint, heliotrope (blood stone), plasma and sard.

Quartz is a very important industrial material and many useful applications exist for it. It's piezoelectric properties are widely used in electronics as pressure sensors and oscillators. Quartz is the raw material used in the manufacture of silicon carbide, a widely used industrial abrasive. Quartz also finds many applications in optics. Quartz crystals have the ability to rotate the plane of polarization of light and it's transparency finds applications in heat lamps, prisms, lenses and optical windows and flats.

Calcium Iron Silicate, CaFeSi2O6
Crystal System: Monoclinic
Hardness: 5.0-6.0
Density: 3.58-3.60

Hedenbergite occurs in contact metamorphic rocks bearing iron minerals. It is associated with ilvaite, garnets, sulfides, epidote and calcite. Hedenbergite is a member of the pyroxene group, which is an important and widely distributed group of rock-forming silicates. Calcium poor pyroxenes crystallize in the orthorhombic system and are known as orthopyroxenes. Hedenbergite is a member of the clinopyroxenes, which crystallize in the monoclinic system and contain calcium, iron, aluminum, sodium, or lithium.

Hedenbergite crystals commonly occur as radiating fibrous aggregates, with stubby, prismatic crystals of nearly square cross section being rarer. They are almost opaque except when slivers viewed on the edge. Fibrous forms are often greenish-brown in color.

Rocks from Ron Zeilstra's Collection

Index of Specimen Images

Table of Contents

bkeller@rockhounds.com 4/12/96