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  <xs:element id="mg_LitigationSettlementAmountAwardedFromOtherPartyNumberOfInstallments" abstract="false" name="LitigationSettlementAmountAwardedFromOtherPartyNumberOfInstallments" nillable="true" xbrli:periodType="instant" substitutionGroup="xbrli:item" type="xbrli:decimalItemType"/>
  <xs:element id="mg_CertainAcquiredCompaniesMember" abstract="true" name="CertainAcquiredCompaniesMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="mg_AssetsAndLiabilitiesLesseeTableTextBlock" abstract="false" name="AssetsAndLiabilitiesLesseeTableTextBlock" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:textBlockItemType"/>
  <xs:element id="mg_OilGasMember" abstract="true" name="OilGasMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="mg_RevenuePracticalExpedientIncrementalCostOfObtainingContractMaximumPeriod" abstract="false" name="RevenuePracticalExpedientIncrementalCostOfObtainingContractMaximumPeriod" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:durationItemType"/>
  <xs:element id="mg_LongTermIncentivePlan2016Member" abstract="true" name="LongTermIncentivePlan2016Member" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="mg_AccruedLiabilitiesDeferredRevenuesAndContingentConsiderationLiabilitiesCurrentDisclosureTextBlock" abstract="false" name="AccruedLiabilitiesDeferredRevenuesAndContingentConsiderationLiabilitiesCurrentDisclosureTextBlock" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:textBlockItemType"/>
  <xs:element id="mg_PetrochemicalMember" abstract="true" name="PetrochemicalMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="mg_DebtInstrumentCovenantComplianceTotalConsolidatedDebtLeverageRatio" abstract="false" name="DebtInstrumentCovenantComplianceTotalConsolidatedDebtLeverageRatio" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="xbrli:pureItemType"/>
  <xs:element id="mg_OtherAmericasMember" abstract="true" name="OtherAmericasMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="mg_OtherLongTermDebtMember" abstract="true" name="OtherLongTermDebtMember" nillable="true" xbrli:periodType="duration" substitutionGroup="xbrli:item" type="dtr-types:domainItemType"/>
  <xs:element id="mg_LineOfCreditFacilityMaximumBorrowingCapacityInNonUSCurrencies" abstract="false" name="LineOfCreditFacilityMaximumBorrowingCapacityInNonUSCurrencies" nillable="true" xbrli:periodType="instant" xbrli:balance="credit" substitutionGroup="xbrli:item" type="xbrli:monetaryItemType"/>
</xs:schema>
