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State Symbols
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Official state symbols represent the cultural heritage
and natural treasures of each state or the entire United States |
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Delaware Symbols
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Delaware State Mineral
Sillimanite
Adopted in 1977
Sillimanite is widespread throughout the schists of the Delaware Piedmont and occurs as large masses and steam-rounded boulders at the Brandywine Springs State Park. Sillimanite is suitable for lapidary work and under the name Fibrolite, it was recognized by geologists in Delaware prior to 1830.
In 1977, the Delaware General Assembly, acting on a proposal by the Delaware Mineralogical Society, established sillimanite as the Delaware State Mineral. This act recognizes the geological and mineralogical significance of the large masses of this mineral found as boulders at Brandywine Springs, an occurrence that was recognized as important in the 6th (1892) edition of Dana's System of Mineralogy. The Brandywine Springs boulders are remarkable for their size and purity. The sillimanite has a fiberous texture reminiscent of wood and could potentially be cut into cabochon gems showing a chatoyant ("cat's eye") effect. Sillimanite has no industrial value and is not mined as an ore or raw material.
Sillimanite is a polymorph with two other minerals; kyanite and andalusite. A polymorph is a mineral that shares the same chemistry but a different crystal structure with another, or other, minerals. Sillimanite is the rarest of the three trimorphs. A variety of sillimanite is called "fibrolite". It is a common fiberous massive form.
Sillimanite forms at tempreatures greater than 550oC, and its coarse grain size at Brandywine Springs indicates a prolonged period of high-temperature metamorphism of the rocks. These conditions are confirmed by the absence of muscovite and the occurrence of the pair sillimanite + K-feldspar (second sillimanite zone) in the schists/gneisses and by the presence of micropegmatites in the fold noses of the schists
Physical Characteristics:
- Color is white, brown and green.
- Luster is silky when fiberous or vitreous otherwise.
- Transparency crystals are transparent to translucent.
- Crystal System is Orthorhombic; 2/m2/m2/m
- Crystal Habits include rare prismatic crystals but mostly fiberous masses.
- Cleavage is good in one direction lengthwise.
- Fracture is splintery.
- Hardness is 7.5 in large crystals but more fiberous forms are softer.
- Specific Gravity is approximately 3.2+ (above average)
- Streak is white.
- Associated Minerals are garnets, biotite, feldspars, quartz, kyanite and andalusite.
- Other Characteristics: fibers are brittle distinguishing them from asbestos.
- Notable Occurrences include Brazil, New England area and many European sites.
- Best Field Indicators are crystal habit, color, brittlness and hardness if not fiberous.
Delaware Code Title 29 § 320
The official state mineral is sillimanite. (61 Del. Laws, c. 21, § 1.)
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State Symbols
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State symbols represent things that are special to a
particular state.
symbol \ˈsim-bəl\
noun
Etymology:
in sense 1, from Late Latin symbolum, from Late Greek symbolon, from Greek, token, sign; in
other senses from Latin symbolum token, sign, symbol, from Greek symbolon, literally, token
of identity verified by comparing its other half, from symballein to throw together,
compare, from syn- + ballein to throw — more at devil
Date: 15th century
1: Something that represents something else by association, resemblance, or
convention, especially a material object used to represent something invisible.
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