<esri:FeatureLayer.Clusterer>
<esri:FlareClusterer Gradient="{StaticResource GradientBrush}" FlareBackground="#f5cce6" FlareForeground="#3c3c3c" MaximumFlareCount="15" />
</esri:FeatureLayer.Clusterer>
<LinearGradientBrush x:Key="GradientBrush" MappingMode="RelativeToBoundingBox" >
<GradientStop Color="#990011FF" Offset="0"/>
<GradientStop Color="#990055FF" Offset="0.25"/>
<GradientStop Color="#990099FF" Offset="0.5"/>
<GradientStop Color="#9900CCFF" Offset="0.75"/>
<GradientStop Color="#9900FFFF" Offset="1"/>
</LinearGradientBrush>