abstract elements and xsi:type substitution

Is the following valid? Why or why not?

 

Thanks,

Stan Kitsis

 

******* XSD **************

 

<?xml version="1.0" encoding="utf-8" ?> 

<xs:schema targetNamespace="foo"

                  elementFormDefault="qualified"

                  xmlns="foo"

                  xmlns:xs="http://www.w3.org/2001/XMLSchema">

  

  <xs:element name="Item" type="xs:string" abstract="true"/>

</xs:schema>

 

******* XML **************

<?xml version="1.0" encoding="utf-8"?>

<Item xmlns="foo" 

      xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" 

      xmlns:xs="http://www.w3.org/2001/XMLSchema"

          xsi:type="xs:string">text</Item>