<p>Rutilated quartz from Brazil.
</p><p>A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties.  At its simplest, a mineral is a naturally-occurring solid chemical.  Currently, there are over 5200 named and described minerals - about 200 of them are common and about 20 of them are very common.  Mineral classification is based on anion chemistry.  Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.
</p><p>The oxide minerals all contain one or more oxide anions (O-2).  The oxide minerals include species that are hydroxy-oxides.  The hydroxide minerals (those with one or more OH-) are usually considered together with the oxides.  Many sulfide minerals are not stable in Earth-surface conditions.  In the presence of oxygen and moisture, sulfide minerals tend to tarnish or alter to oxides and hydroxy-oxides.  All except the most inert elements (such as the platinum-group elements and gold and noble gases) readily form oxides.  Gold oxide forms only under special conditions.
</p><p>Rutile is a titanium oxide mineral (TiO2).  It is an important titanium ore mineral.  Rutile's most common crystal form consists of long, striated needles (acicular crystals).  Rutile has an intense submetallic luster, a pale yellow to golden-brown to yellowish-brown to red to almost black color, and is fairly hard (H=6.5).  Many twinned crystals have a bent or kinked appearance ("elbow twins").
</p><p>Rutile occurs in igneous, sedimentary, and metamorphic rocks.  It usually occurs as long, slender needles within quartz crystals (rutilated quartz) and in placer beach sands.
</p><p>The rutilated quartz shown here is unusual in that the rutile inclusions are not scattered individual needles (the latter is typical in rutilated quartz).  Instead, this quartz has a rutile trilling, which refers to the presence of three, centrally-twinned rutile crystals forming a hexaradiate structure.
</p><p>Locality: unrecorded/undisclosed site at or near Novo Horizonte, Bahia State, southern Brazil
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<hr><p>Photo gallery of rutile &amp; rutilated quartz:
&lt;a href="<a rel="nofollow" class="external free" href="https://www.mindat.org/gallery.php?min=3486">https://www.mindat.org/gallery.php?min=3486</a>" rel="nofollow"&gt;www.mindat.org/gallery.php?min=3486&lt;/a&gt;
and
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&lt;a href="<a rel="nofollow" class="external free" href="https://www.mindat.org/gallery.php?min=3485">https://www.mindat.org/gallery.php?min=3485</a>" rel="nofollow"&gt;www.mindat.org/gallery.php?min=3485&lt;/a&gt;
<a rel="nofollow" class="external text" href="https://www.flickr.com/people/47445767@N05">James St. John</a>
CC BY 2.0
<a rel="nofollow" class="external text" href="https://www.flickr.com/photos/jsjgeology/34479905202/">Rutilated quartz (Novo Horizonte, Bahia, Brazil) 4</a>